Download 3. BIOMOLECULES I. CARBOHYDRATES

Applied biochemistry
Kincses, Sándorné
Balláne Kovács, Andrea
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Applied biochemistry
írta Kincses, Sándorné és Balláne Kovács, Andrea
TÁMOP-4.1.2.A/1-11/1-2011-0009
University of Debrecen, Service Sciences Methodology Centre
Debrecen, 2013.
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Tartalom
Tárgymutató ....................................................................................................................................... 1
1. 1. INTRODUCTION ...................................................................................................................... 2
1. 1. 1. The object of biochemistry, its relationship with other sciences ................................... 2
2. 1. 2. Relationship between biochemistry and other sciences ................................................ 2
2. 2. THE LIVING SYSTEMS ........................................................................................................... 3
1. 2. 1. Characterization of living systems ................................................................................ 3
2. 2. 2. The composition of living matter .................................................................................. 3
3. 3. BIOMOLECULES I. CARBOHYDRATES .............................................................................. 8
1. 3. 1. Monosaccharides ........................................................................................................... 8
1.1. 3. 1. 1. Formation of cyclic monosaccharides .......................................................... 9
1.2. 3. 1. 2. Chemical reactions of monosaccharides ..................................................... 10
1.2.1. 3. 1. 2. 1. Redox reactions of monosaccharides ......................................... 10
1.2.2. 3. 1. 2. 2. Transformations of monosaccharides into each other ................ 11
2. 3. 2. Disaccharides .............................................................................................................. 12
2.1. 3. 2. 1. Reducing disaccharides .............................................................................. 13
2.2. 3. 2. 2. Non-reducing disaccharides ........................................................................ 13
3. 3. 3. Polysaccharides ........................................................................................................... 13
3.1. 3. 3. 1. Classification of polysaccharides ................................................................ 14
4. 4. BIOMOLECULES II. PROTEINS .......................................................................................... 16
1. 4. 1. Proteins can be classified ............................................................................................ 17
5. 5. BIOMOLECULES III. THE LIPIDS ....................................................................................... 19
1. 5.1. Classification of lipids: ................................................................................................ 19
1.1. 5. 1. 1. Saponifiable lipids ...................................................................................... 19
1.1.1. 5.1.1.1. Vaxes ............................................................................................. 20
1.1.2. 5.1.1.2. Neutral fats and oils (triglycerides) ............................................... 20
1.1.3. 5.1.1.3. Phosphoglycerides ........................................................................ 21
1.1.4. 5.1.1.4. Sphingolipids ................................................................................. 22
1.1.5. 5.1.1.5. Glycolipides .................................................................................. 22
1.2. 5.1.2. Insaponifiable lipids ...................................................................................... 22
1.2.1. 5.1.2.1. Steroids .......................................................................................... 22
1.2.2. 5.1.2.2. Carotenoids ................................................................................... 22
1.2.3. 5.1.2.3. Lipid soluble vitamins .................................................................. 23
6. 6. BIOMOLECULES IV. THE NUCLEIC ACIDS .................................................................... 24
1. 6.1. The deoxyribonucleic acid (DNA) ............................................................................... 25
1.1. 6. 1.1. The primary structure of deoxyribonucleic acid (DNA) .............................. 25
1.2. 6.1.2. The secondary structure of DNA .................................................................. 25
1.3. 6.1.3. The biological function of DNA
........................................................... 26
2. 6. 2. The ribonucleic acids (RNA-s) ................................................................................... 26
2.1. 6. 2.1. The messenger RNA-s ................................................................................. 26
2.2. 6. 2. 2. Transfer RNA-s .......................................................................................... 27
2.3. 6. 2. 3. Ribosomal RNA-s ...................................................................................... 27
3. 6. 3. Nucleoside triphosphates ............................................................................................. 28
7. 7. BIOACTIVE COMPOUNDS I. VITAMINS ........................................................................ 29
1. 7.1. Physiological effects of vitamins: ................................................................................ 29
2. 7. 2. Lipid soluble vitamins ................................................................................................. 29
3. 7. 3. Water-soluble vitamins ............................................................................................... 32
8. 8. BIOACTIVE COMPOUNDS II. HORMONS ..................................................................... 37
1. 8.1. Classification of hormones: .......................................................................................... 38
1.1. 8.1.1. Hormones of hypophysis ............................................................................. 38
1.1.1. 8.1.1.1. The anterior pituitary (adenohypophysis)The defect or removal of anterior
pituitary reflect in the operation of all organs to a smaller or larger extent. ........ 38
1.1.2. 8.1.1.2. Hormones of intermediate lobe (pars intermedia) ........................ 39
1.1.3. 8.1.1.3. Posterior lobe (neurohypophysis) hormones ................................. 39
1.2. 8.1.2. Hormones of pineal gland ............................................................................. 39
1.3. 8.1.3. Hormones of the thyroid glands .................................................................... 40
1.4. 8.1.4. The parathyroid gland ................................................................................... 40
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Applied biochemistry
1.5. 8.1 5. Hormones of the adrenal cortex ....................................................................
1.6. 8.1.6. Hormones of adrenal medulla (catecholamines) ...........................................
1.7. 8.1.7. Hormones of pancreas ................................................................................
1.8. 8.1.8. Hormones of the ovary .................................................................................
1.9. 8.1.9. The testicular hormones (androgens) ............................................................
2. 8. 2. Tissue hormones ..........................................................................................................
3. 8. 3. Plant growth hormones (Phytohormones) ...................................................................
9. 9. BIOACTIVE COMPOUNDS III. ENZYMES (BIOCATALIZATORS) ..............................
1. 9. 1. Structure of the enzymes .............................................................................................
2. 9. 2. The function mechanism of the enzymes ....................................................................
3. 9. 3. The specificity of the enzymes ....................................................................................
4. 9. 4. Classification of enzymes ............................................................................................
4.1. 9.4.1. Oxidoreductases ............................................................................................
4.2. 9.4.2. Transferases ..................................................................................................
4.3. 9.4.3. Hydrolases ....................................................................................................
4.4. 9.4.4. Lyases (Synthases) ........................................................................................
4.5. 9.4.5. Isomerases ....................................................................................................
4.6. 9.4.6. Ligases (synthetases) ....................................................................................
5. 9. 5. Factors influencing the function of enzymes ..............................................................
10. 10. THE METABOLIC PROCESSES I. CARBOHYDRATE METABOLISM .......................
1. 10. 1. Carbohydrate biosynthesis in photosynthetic organisms ...........................................
1.1. 10.1.1. The light dependent phase (Hill reaction) ...................................................
1.2. 10.1.2 The light independent phase of photosynthesis (Calvin cycle) ....................
1.3. 10.1.3. The sucrose synthesis .................................................................................
1.4. 10.1.4. The starch synthesis ....................................................................................
2. 10. 2. Catabolic processes of carbohydrates ........................................................................
2.1. 10.2.1. Cellular respiration .....................................................................................
2.1.1. 10.2.1.1. Glycolysis ....................................................................................
2.1.2. 10.2.1.2. Pyruvate decarboxylation ............................................................
2.1.3. 10.2.1.3. Citric acid cycle ...........................................................................
2.1.4. 10.2.1.4. The terminal oxidation and oxidative phosphorylation ...............
2.2. 10.2.2. The pentose phosphate pathway ................................................................
2.3. 10.2.3. Fermentation processes ...............................................................................
2.3.1. 10.2.3.1. The fermentation processes in the rumen of ruminants ...............
2.3.2. 10.2.3.2. The fermentation processes in silo .............................................
3. 10.3. Gluconeogenesis (Glucose-resynthesis) .....................................................................
4. 10.4. Glycogen metabolism .................................................................................................
4.1. 10.4.1. Glycogen synthesis .....................................................................................
4.2. 10.4.2. Glycogen mobilization, catabolism ............................................................
11. 11. THE METABOLIC PROCESSES II. LIPID METABOLISM ............................................
1. 11.1. Biosynthesis of lipids .................................................................................................
1.1. 11. 1. 1. Biosynthesis of triglicerides ......................................................................
1.1.1. 11.1.1.1. Biosynthesis of fatty acids ...........................................................
1.1.2. 11.1.1.2. The synthesis of glycerol ............................................................
1.2. 11.1. 2. Biosynthesis of phospholipids ..................................................................
1.3. 11. 1. 3. The biosynthesis of carotenoids and steroid skeleton lipids .....................
1.3.1. 11.1.3.1. The synthesis of steroids .............................................................
2. 11. 2. The breakdown of lipids ............................................................................................
2.1. 11. 2.1. The β-oxidation of saturated fatty acids ....................................................
2.2. 11.2.2. The catabolism of steroids ..........................................................................
3. 11. 3. The formation of ketone bodies (ketogenesis) ..........................................................
4. 11. 4. Glyoxylic acid cycle (Kornberg Krebs cycle) ...........................................................
12. 12. THE METABOLIC PROCESSES III. PROTEIN METABOLISM ....................................
1. 12.1. The nitrogen fixation ..................................................................................................
2. 12.2. The synthesis of essential amino acids .......................................................................
2.1. 12.2.1. The methionine and threonine biosynthesis ................................................
2.1.1. 12.2.2.1. Methionine formation from homoserin ......................................
2.2. 12.2.2. Lysine biosynthesis .....................................................................................
2.3. 12.2.3. Arginine biosynthesis .................................................................................
2.4. 12.2.4. Leucine, isoleucine and valine synthesis ....................................................
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Applied biochemistry
2.5. 12.2.5. The phenylalanine and tryptophan biosynthesis ......................................... 78
3. 12.3. Protein Synthesis ........................................................................................................ 78
3.1. 12.3.1. The transcription ......................................................................................... 79
3.2. 12.3. 2. The translation .......................................................................................... 79
3.2.1. 12.3.2.1. Initiation ...................................................................................... 81
3.2.2. 12.3.2.2. Elongation ................................................................................... 81
3.2.3. 12.3.2.3. Termination ................................................................................. 82
4. 12.4. The fate of dietary proteins in heterotrophic organisms ............................................. 82
4.1. 12.4.1. The quality of proteins ................................................................................ 82
4.2. 12.4.2. The protein balance of the organism .......................................................... 84
4.3. 12.4.3. The digestion of proteins ............................................................................ 84
4.3.1. 12.4.3.1. Proteases occur in each cell. Their role is wide ranged. .............. 84
4.3.2. 12.4.3.2. The common features of amino acid degradation pathways ....... 85
4.3.3. 12.4.3.3. The catabolism of carbon skeleton of amino acids in the tricarboxylic
acid cycle ............................................................................................................. 86
4.4. 12.4.4. Protein turnover .......................................................................................... 87
4.5. 12.4.5. Nitrogen excretion ...................................................................................... 88
4.5.1. 12.4.5.1. Nitrogen excretion in mammals, synthesis of urea (carbamide) . 88
4.5.2. 12.4.5.2. Nitrogen excretion of birds and reptiles. Synthesis of uric acid .. 89
4.6. 12.4.6. Disturbances of amino acid metabolism .................................................... 89
13. 13. OTHER BIOCHEMICAL PATHWAYS ............................................................................. 91
1. 13.1. The biochemical bases of the function of skeletal muscle ......................................... 91
2. 13. 2. Factors influencing the quantity and quality of the urine .......................................... 91
3. 13. 3. The gastric juice and its separation ........................................................................... 92
3.1. 13. 3. 1. The mechanism of the hydrochloric acid production of the stomach ....... 92
4. 13. 4. The control of metabolic processes ........................................................................... 93
4.1. 13. 4. 1. The control of lipid metabolism ................................................................ 93
4.2. 13. 4. 2. The function of adenylate cyclase - cAMP system .................................. 94
4.2.1. 13.4. 2. 1. The presentation of adenylate-cyclase system operation through the
mobilization of glycogen ..................................................................................... 95
4.2.2. 13. 4. 2. 2. Hormone control of carbohydrate metabolism ......................... 95
5. 13. 5. The role of liver in the intermediate metabolism ...................................................... 97
14. 14. BIOCHEMICAL PATHWAYS IN THE FOOD INDUSTRY ............................................. 99
1. 14. 1. The application of the fermentation in the food industry .......................................... 99
2. 14. 2. The biochemical processes of cereals germination .................................................. 99
3. 14. 3. Respiration during storage ...................................................................................... 100
3.1. 14. 3. 1. Respiration of grain during storage
................................................ 100
3.2. 14.3.2. The respiration of fruits and vegetables ................................................... 100
3.3. 14. 3. 3. The ripening of fruits .............................................................................. 101
4. 14. 4. The biochemistry of meat ripening ......................................................................... 102
5. 14. 5. Changes of colour through the meat processing ...................................................... 103
15. 15. RECOMMENDED REFERENCES ................................................................................... 105
16. Questions .................................................................................................................................. 106
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Az ábrák listája
2.1. Table 1: The frequency of elements in the earth's crust and in the human body ......................... 3
2.2. Table 2. The chemical composition of Escherichia coli bacterium ............................................. 4
2.3. Figure 1: Molecular organizations in cells ................................................................................... 6
2.4. Table 3: Classification of organisms based on their mass sources .............................................. 7
3.1. Figure 2: The most important monosaccharides .......................................................................... 8
3.2. Figure 3: Formation of cyclic monosaccharides ........................................................................ 10
3.3. Figure 4: Redox reactions of monosaccharides, of D -glucose .................................................. 10
3.4. Figure 5: Transformations of monosaccharides ......................................................................... 11
3.5. Figure 6: Hexose-pentose transformation .................................................................................. 12
3.6. Figure 7: Disaccharides ............................................................................................................. 13
3.7. Figure 8: Polysaccharides .......................................................................................................... 15
4.1. Figure 9: Peptide bond ............................................................................................................... 16
4.2. Figure 10: Conformation of proteins ......................................................................................... 17
5.1. the core of gonane ...................................................................................................................... 22
6.1. Figure 11: Nucleobases .............................................................................................................. 24
6.2. Figure 12: From nucleotide monomers connected to polynucleotide ........................................ 25
6.3. Figure 13: The deoxyribonucleic acid. ...................................................................................... 26
6.4. Figure 14: The mRNA and the tRNA ........................................................................................ 27
7.1. Figure 15: Vitamin A ................................................................................................................. 30
7.2. Figure 16: Vitamin D, E, K. ...................................................................................................... 31
7.3. Figure 17: Vitamins (B1, B2, B3) .............................................................................................. 33
7.4. Figure 18: Water-soluble vitamins ............................................................................................ 34
7.5. Table 4: The role of vitamins in the function of enzymes ......................................................... 35
8.1. Figure 19: Regulation of hormone production ........................................................................... 37
8.2. Figure 20: The synthesis of melatonin ....................................................................................... 39
8.3. indole-acetic acid ....................................................................................................................... 43
8.4. gibberellic acid .......................................................................................................................... 44
8.5. zeatin .......................................................................................................................................... 44
9.1. Figure 21: Enzymes and activation energy ................................................................................ 45
9.2. Figure 22: The specificity of the enzymes ................................................................................. 46
9.3. Table 5: Coenzymes of transferases and transmitted chemical groups ...................................... 48
10.1. Figure 23: The carbon, hydrogen and oxygen biological cycle ............................................... 51
10.2. Figure 24: The nitrogen biological cycle ................................................................................. 51
10.3. Figure 25: Hill reaction ............................................................................................................ 53
10.4. Figure 26: Calvin cycle ............................................................................................................ 54
10.5. Figure 27: Catabolic processes of carbohydrates ..................................................................... 55
10.6. Figure 28: Glycolysis ............................................................................................................... 56
10.7. Figure 29: Pyruvate decarboxylation ....................................................................................... 57
10.8. Figure 30: Citric acid cycle and terminal oxidation ................................................................. 57
10.9. Figure 31: The pentose phosphate pathway ............................................................................. 59
10.10. Figure 32: Fermentation processes ........................................................................................ 60
10.11. Figure 33: Alcoholic- and lactic acid fermentation ............................................................... 60
10.12. Figure 34: Propionic acid and butyric acid fermentation ...................................................... 62
10.13. Table 6: The fermentation processes in silo ........................................................................... 63
10.14. Figure 35: Gluconeogenesis .................................................................................................. 64
11.1. Figure 36: Biosynthesis of fatty acids ...................................................................................... 66
11.2. Figure 37: The synthesis of triglycerid .................................................................................... 67
11.3. Figure 38: The synthesis of isoprene ....................................................................................... 68
11.4. Figure 39: The β-oxidation ...................................................................................................... 70
11.5. Figure 40: Breakdown fatty acids with odd-numbered carbon ................................................ 71
11.6. Figure 41: Ketogenesis ............................................................................................................ 73
11.7. Figure 42: Glyoxylic acid cycle ............................................................................................... 74
12.1. Figure 43: The nitrogen fixation .............................................................................................. 75
12.2. Figure 44: The synthesis of essential amino acids ................................................................... 77
12.3. Figure 45: Transcription .......................................................................................................... 79
12.4. Figure 46: tRNA ...................................................................................................................... 80
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12.5. Figure 47: The steps of initiation complex formation .............................................................. 81
12.6. Table 7: Biogenic amines ........................................................................................................ 85
12.7. Figure 48: Oxidative deamination (amino acids) ..................................................................... 86
12.8. Figure 49: Entering of carbon skeleton of amino acids the citric acid cycl ............................. 87
12.9. Figure 50: Synthesis of carbamide ........................................................................................... 89
13.1. Figure 51: Steps of gastric acid secretio .................................................................................. 93
13.2. Figure 52: cAMP system ......................................................................................................... 95
13.3. Figure 53: The outline of the neourohormonal control of the carbohydrate metabolism and the bloodsugar level ......................................................................................................................................... 96
13.4. Figure 54: The glucose transport between organs and its hormonal regulation ....................... 97
14.1. Figure 55: The biochemistry of meat ripening ....................................................................... 103
14.2. Figure 56: Changes of colour through the meat processing ................................................... 104
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Tárgymutató
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1. fejezet - 1. INTRODUCTION
1. 1. 1. The object of biochemistry, its relationship
with other sciences
Biochemistry is a life science between biology and chemistry. Its development is connected to these two
disciplines tightly. The new results of chemistry and biology also appear in biochemical research, thus they
enhance the development of this discipline. Considering the content of biochemistry it is closer to biology, while
based on its methods it is closer to chemical sciences.
Biochemistry deals with the physiology of living nature, and of the organisms in molecular terms. Emphasizing
the unit of living world the aim of its discipline is to recognize chemical processes taking place in all living
organisms in a multi-faceted way.
Biochemistry examines the following areas:
• The structures, organizations and functions of living matter in molecular terms,
• The chemical structures of molecules constructing the living matter,
• Interactions taking place during the formation of supramolecular structures, cells multicellular tissues and
organisms.
• Material and energy transport between living matter and its surroundings,
• The storage and transmission of information that is needed for self reproduction in cells
• Chemical changes accompanying the reproduction, aging and death of cells and organisms,
• Self controlling processes of chemical reactions in living cells, influencing factors of the direction of these
chemical reactions
2. 1. 2. Relationship between biochemistry and other
sciences
Biochemistry is a distinct discipline . The structures of biomolecules, the directions of metabolic pathways, and
their regulatory mechanisms by enzymes can be understood based on the chemical laws only. Biochemistry is an
interdisciplinary science. It is tightly connected in animal and plant physiology and organic chemistry. It applies
the results of mathematics, physics, physical-chemistry and colloid chemistry. The results of biochemistry
provide basis for more applied sciences such as medical science, pharmacology and toxicology. Dietetics, food
industry and forage doctrine technologies, plant production, animal husbandry and environment protection can
also use the results of biochemistry.
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2. fejezet - 2. THE LIVING SYSTEMS
1. 2. 1. Characterization of living systems
Living systems differ from lifeless ones qualitatively. Lifeless systems tend to an intense disorder (entropy is
growing), by the combination of various elements. Lifeless systems are characterized by the sum of compounds
with small molecular mass. Living systems tend to maintain dynamic equilibrium with their environment,
maintaining order as an independent unit.
Metabolic processes are important processes of life. They can be divided into two parts: Catabolism is the
decomposition of organic matter and anabolism is the construction of organic compounds. Living beings take up
and lose substances and energy continuously from their environment to ensure their metabolism and their
survival. They are characterized by constant renewal. Their macromolecules are formed by various
combinations of a few elements.
Major features of living organisms:
• They are self maintaining,. They keep their inner stability (homeostasis), peculiarities and individuality in
spite of the change of their environment; they are in dynamic balance with their environment.
• They follow the change of their environment by modifying their metabolism.
The velocity and direction of biochemical processes can be changed by enzymes.
• They are open systems →there is an energy and matter replacement between living matter and its
environment.
• They are economical (end product→intermedier→precursor). Catabolism and anabolism are in relationship,
the intermedier or end product of a certain process can be the precursor of another process.
• They are capable of self-reproduction (DNA). Due to their capability of forming successors similar to them,
life can be maintained. They have information carrying,
• Storing, reading, and copying systems. Information can be passed to their successors (DNA, RNA).
• There is uniform material construction (ATP). The same biochemical processes take place in living beings
independently on their state of development. (e.g.: ATP)
• They have been continuously developed by evolution.
2. 2. 2. The composition of living matter
The composition of living organisms (both in the quality and in the quantity of the elements) differ from that of
lifeless environment and the earth’s crust. Table 1. shows the frequency of elements in the earth's crust and in
the human body.
2.1. ábra - Table 1: The frequency of elements in the earth's crust and in the human
body
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2. THE LIVING SYSTEMS
Among the elements in the earth's crust, oxygen, silicon and metals occur in the largest quantity. Living matter
consists of four elements in 99%, these are hydrogen, oxygen, carbon and nitrogen. These four elements are
called organogenic elements. Organogenic elements rank among non-metallic elements. There is a wide variety
of their molecules formed by covalent bond. The existence of the numerous varieties of their molecules can be
explained by the specific properties of carbon atom (it can form single and double bond as well with itself,
nitrogen and oxygen)
Phosphorus and sulphur are also essential in the construction of the living matters. Phosphorus can be found
mostly in ester ulinkage, while sulphur is attached to the carbon atom by covalent bond. Sulphur and
phosphorus together with organogenic elements are called biogenic elements
To the normal life function of living organisms of other elements are also essential, but these occur in a much
smaller quantity in them. Na, K, Ca, Mg belong to macro elements as they are present in plant in quantities more
than 0.1% on a dry matter weight basis, and in humans and animals more than 0,005%. Cl, I, Fe, Zn, Mn, Co,
Cu, Mo, Se, B are micro elements while their amount in human organisms is smaller than the above-mentioned
quantities. Apart from the listed ones living beings contain other elements but these are not essential.
Their biological role is not known yet. The composition of molecular constituents that can be found in the living
organisms are represented through the example of Escherichia coli bacterium (Table 2.).
2.2. ábra - Table 2. The chemical composition of Escherichia coli bacterium
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2. THE LIVING SYSTEMS
Living organisms contain water in the largest quantities. Among their organic compounds lining organisms
contain proteins, carbohydrates, nucleic acids and lipids in a significant quantity. The amount of the other types
of organic matter is negligible compared to the mass of these four biomolecule groups.
These biomolecules (carbohydrates, lipids, proteins, nucleic acids) are well-separable structurally and
functionally, though they have common properties as well. Their common feature is, that
they consist of monomer units. These monomer units are connected to each other by water loss reactions
(condensation) creating macromolecules.
The monomer units of carbohydrates are monosaccharides that of nucleic acids are nucleotides. Proteins formed
by the attachment of a lot of amino acids (monomers), while most of the lipids consist of the fatty acid
monomers.
They are the main characters of metabolism processes. There are wide variety of proteins and nucleic acids.
Carbohydrates and lipids do not have so many variants, the number of their monomer units and the variations of
the ulinks are fewer. The features of biomolecules differ from inorganic molecules.
biomolecules:
molecules of lifeless matter:
complicated diverse construction,
simple construction, disordered mixtures
ordered structure
energy taken up from the environment ensures the energy taken up from the environment increases
maintenance of the organization
disorder
their structure is suitable for specific functions
specific function is not recognizable
contain information,
do not contain information,
are capable of self-reproduction
are not capable of self-reproduction
Water has numerous functions in living organisms, due to its specific properties. (V-shape, polar molecule,
hydrogen bonds between their molecules, amphoteric character, great specific heat and great vaporization heat.)
The role of water in biological systems:
• Water molecules hydrate macromolecules. Hydrogen bonds can be formed between water and
macromolecules or macromolecules can possess charges thus polar water molecules surround them forming a
hydration shell.
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2. THE LIVING SYSTEMS
• It is solvent. Water dissolves many kinds of substances such as salts. Among non dissociating substances,
water dissolves polar substances. Amphipathic molecules (containing polar and apolar parts) form micelles in
water.
• It is a transfer medium, due to the fact that it is a good solvent, it takes part in mass transport between cells,
tissues and organs
• It participates in many chemical reactions. It can be reactant or end product in biochemical reactions.
• It plays role in the regulation of heat balance. Due to its high specific heat and vaporization heat, water can
buffer the changes of temperature in environment, thus the cells of living organisms can maintain a relatively
constant temperature. Due to its high vaporization heat organisms can lose heat during sweating, protecting
themselves from warming
In living organisms, inorganic ions are present in a small amount, although they are of great importance also.
They have several functions.
The role of inorganic ions in biological systems:
• Enzyme activators: They influence the velocity of metabolic processes by activating or blocking catalysts of
biochemical processes,
• Components of enzymes: Their deficiencies can inhibit the metabolism processes, which cause the
accumulation of intermedier products,
• Participants in stimulus transfer,
• Regulators of osmotic potential, influencing water uptake and loss.
• Regulators of the acid-base equilibrium. Biochemical processes take part at certain pH values. Enzymes have
pH optimum. Maintaining the acid-base balance in the living organisms is an essential function.
• Components of the compounds taking part in the oxidation-reduction processes,
• Components of hormones, they can influence the effect of hormones on metabolism.
• Hormone regulators,
• Constituents of multi cellular tissues and organs. They are often attached to organic matters.
Living beings can construct their own organic substances during metabolism (Figure 1). Reactants of Anabolism
processes are always simple inorganic compounds (autrophic living beings) or organic compounds
(heterotrophic organisms). From precursors intermediate products are formed in biochemical processes, which
then transform to monomers. As monomers attach to
each other they form macromolecules that are the building blocks of the cells. During the interactions of
macromolecules cell constituents with special functions are formed. They are the so called supra molecular
systems, that are required for the special life processes of cells. Catabolism processes take place through similar
steps (supra molecular systems, precursors). Anabolism processes require energy, while catabolism processes
provide energy for the cell and the organism.
2.3. ábra - Figure 1: Molecular organizations in cells
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2. THE LIVING SYSTEMS
Living organisms provide energy and precursors for their anabolism processes from their environment. They can
be classified based on the source of matter and energy (Table 3.)
2.4. ábra - Table 3: Classification of organisms based on their mass sources
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3. fejezet - 3. BIOMOLECULES I.
CARBOHYDRATES
Carbohydrates are organic compounds that contain carbon, hydrogen, and oxygen. Oxygen can occur in the
groups of hydroxyl, ether or oxo-group. Hydrogen is usually in 2:1 ratio to oxygen.
Carbohydrates belong to the most important organic compounds from chemical and biological aspect too. They
can be found in flora and fauna as well. Among nutrients, the carbohydrates are important energy sources.
One part of carbohydrates with low molecular weight (e.g. glucose, fructose and beet sugar) is nutrient, while
carbohydrates with high molecular weight are either reserve nutrients (e.g. starch and glycogen), or support and
frame substances (e.g. cellulose).
Being attached to other substances, carbohydrates create various compounds: e.g. nucleotides, alkaloids,
heparin, and glycoprotein. Based on their structure carbohydrates are polyhydroxy-aldehydes or polyhydroxyketones, and their derivatives (e.g. their condensed products)
Classification of carbohydrates:
• monosaccharides (sometimes called simple sugars: glucose, fructose, etc.)
• di- and oligosaccharides (contain 2-8 monosaccharide units: sucrose, maltose, etc,)
• Polysaccharides (Carbohydrates with large molecular weight, containing hundreds or thousands of
monosaccaharide units. They are not sweet: starch, cellulose, glycogen, pectin, etc)
1. 3. 1. Monosaccharides
Monosaccharides are monomer molecules, the simplest carbohydrates. They cannot be hydrolyzed to smaller
carbohydrates. The monosaccharides are polyhydroxy-aldehydes or polyhydroxy-ketones without side chains.
The formers called aldoses and the latter ketoses.
According to the number of carbon atoms monosaccharides can be classified as triose, tetrose, pentose, hexose
and heptose.
They have sweet taste, they are crystalline substances solving well in water.
Except for dihydroxyacetone all of the m onosaccharides have at least one asymmetric carbon atom, thus m
onosaccharides have optical stereoisomers. Stereoisomers have the same molecular formula, they differ only in
their spatial arrangement, optical stereoisomers rotate the plane of polarized light in opposite directions.
Most of the naturally occurring monosaccharides have D configuration. D and L nomination refers to the
configuration of the asymmetric carbon atom that is farthest from carbonyl group. If the OH group (in Fisher
projection) attached to that asymmetric carbon atom points to the right the monosaccharide has D configuration,
if it points to the left it has L configuration.
Figure 2 shows the structural formula of the most important trioses, pentoses and hexoses.
3.1. ábra - Figure 2: The most important monosaccharides
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1.1. 3. 1. 1. Formation of cyclic monosaccharides
In aqueous solutions, the open-chain form of a monosaccharide often coexists with a cyclic (ring) form. The
ratio of open chain form is very small compared to cyclic form. For instance, more than 99% of D-glucose
molecules are in cyclic form, while less than 1% are in open chain form.
Formation of ring structure is a reversible, nucleophilic addition reaction. The aldehyde or ketone carbonyl
group carbon (-C=O) and the hydroxyl group (-OH) (the hydroxyl group is bound to the farthest chiral carbon
atom from the oxo-group) react forming a hemiacetal with a new C-O-C bridge. Figure 3 shows the ring
formation of D glucose. The hydroxyl group shown with red in the cyclic form called hemiacetal hydroxyl
group.
Two types of stable rings can be formed during the conversion from open-chain form to the cyclic form:
• the five membered ring structure is called furanose ,
• the six-membered ring is called pyranose .
Aldohexoses form six-membered ring, while ketohexoses form five membered hemiacetal formation. Figure 3
represents the formation of cyclic D-ribose.
The atoms of pyranose ring are not in a plane, they have a chair structure.
By forming the pyranose ring a new chiral centre (the C-1 carbon atom) appears, that was not there in the openchain structure.
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CARBOHYDRATES
The OH group on C-1 of the hemiacetal atom has two possible orientations: above or below the plane of the
ring, namely axial or equatorial. The axial group is perpendicular to the mean plane, while equatorial is parallel
to the mean plane (Fig. 3.).
3.2. ábra - Figure 3: Formation of cyclic monosaccharides
1.2. 3. 1. 2. Chemical reactions of monosaccharides
1.2.1. 3. 1. 2. 1. Redox reactions of monosaccharides
Compounds containing oxo-group or free hemiacetal-hydroxyl group are oxidized easily thus they are
reductants. Reductant monosaccharides give positive Fehling’s test result and silvering process.
Sorbite can be found in fruits /e.g. apple/. It has sweet flavour, and it is suitable for diabetic people as sweetener,
since its decomposition does not increase the blood-sugar level. In simple carbohydrates, as an effect of mild
oxidation (e.g. aldehyde) the hydroxyl group on the end of the chain oxidizes to COOH- group. With the
oxidation of the aldehyde group of the aldoses polyhydroxyacids form, while through oxidation of
primaryhydroxyl group to carboxyl group uronic acid forms.
Uronic acids contain formyl group beyond carboxyl and hydroxyl groups. When both C atoms on the end of the
chain transform to carboxyl group, glucaric acids will form.
Among uronic acids D-glucuronic acid, D-galacturonic acid, and D-mannuronic acid can be found in nature
(Fig. 4).
3.3. ábra - Figure 4: Redox reactions of monosaccharides, of D -glucose
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CARBOHYDRATES
1.2.2. 3. 1. 2. 2. Transformations of monosaccharides into each other
The transformations of monosaccharides into each other are very important steps of the carbohydrate
metabolism /e.g. anaerobic glycolysis, pentose phosphate cycle, Calvin cycle, etc./.
The main types of transformations catalysed by enzymes are the following:
• During the transformation, the number of the carbon atoms does not change.
• Epimerization: during this reaction, substituents restructure sterically around one single carbon atom (e.g.
transformation of D-ribose to D-xylose.
• Isomerization: the aldose-ketose transformation, where the carbonyl group shifts onto the adjacent carbon
atom (e.g. transformation of D-glucose to D-fructose.).
• The transfer of C3 unit /active dihydroxy-acetone/ or C2 unit /active glycol aldehyde / from one of the sugars
onto another one.
Through these reactions we get trioses, tetroses, pentoses and heptoses. The donor of C3 and C2 units is always
the ketose, while the acceptor is the aldose.
During the transformation, the sum of the carbon atoms in the monosaccharides does not change.
• Transaldolase reaction: in this process the enzyme splits off fructose or sedoheptulose and transfers the C3
unit onto the appropriate aldehyde.
(e.g.) C7 + C3 → C4 + C6
• Transketolase reaction:
C2 unit is transferred (The donor is the ketose) from one monosaccharide to another one.
(e.g. C5 + C5 → C3 + C7)
To the formation of C2 unit ketose phosphate is needed, which steric arrangement on C3 atom is equal to that
of fructose. In this way, it is possible, that ribulose -5-P epimerizes to xilulose-5-P and than becomes C2 unit
donor.
• Aldolase reaction: Hexoses are transformed to trioses or the reverse (Fig. 5).
3.4. ábra - Figure 5: Transformations of monosaccharides
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CARBOHYDRATES
• During the transformation, the chain is shortened with a carbon atom.
• Hexose-pentose transformation
During the oxidation, the aldehyde group is converted to carboxylic-group then the molecule was
decarboxylase (Fig. 6).
3.5. ábra - Figure 6: Hexose-pentose transformation
2. 3. 2. Disaccharides
In disaccharides, two monosaccharide units are attached together by splitting a water molecule off. The
glycosidic ulinkage can be split by acidic or enzymatic hydrolysis. In disaccharides one of the components is
always glucose. Disaccharides can be reducing or non-reducing. In reducing disaccharides, the hemiacetal
hydroxyl group of a monosaccharide unit reacts with the alcoholic OH group of the other monosaccharide unit,
with the loss of a water molecule. Thus, the molecule contains a hemiacetal hydroxyl group, which can reduce
Fehling solution.
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In non-reducing disaccharides both of the hemiacetal hydroxyl groups are involved in the glycosidic ulinkage,
therefore they can not reduce Fehling reagent.
2.1. 3. 2. 1. Reducing disaccharides
Maltose
In maltose the hemiacetal OH group of an a β-D glucose molecule reacts with the alcoholic OH group of
another β-D glucose molecule. α (1→ 4) glycosidic ulinkage is formed between the two monomer units, as
hemiacetal OH group has a position and it is on C-1 while the alcoholic OH group that is involved in the
reaction is on C-4. There is a sharp bend at the glycosidic ulinkage in maltose. Maltose is formed by enzymatic
decomposition of starch and glycogen. Maltose can be splitted to there monomers by maltase enzyme.
Cellobiose
In cellobiose the hemiacetal OH group of a b -D glucose molecule reacts with the alcoholic OH group of another
b -D glucose molecule. β (1→ 4) glycosidic ulinkage is formed between the two monomer units. In cellobiose
the second monomer unit is rotated 180o .
Lactose
In lactose a β-D galactose molecule combines with α-D- glucose through a β(1→ 4) ulinkage. The hemiacetal
OH group of the glucose molecule is retained; therefore lactose is a reducing sugar. Lactose can be found in the
milk of mammals (5,5- 8% in breast milk, 4,5-5,5% in cow milk. Lactose molecules are broken down by lactase
enzyme in the gut. Lactose is of great importance in the production of dairy products made with fermentation
(yoghurt, sour cream).
2.2. 3. 2. 2. Non-reducing disaccharides
Sucrose
One of the most important nutrients. It occurs in sugar beet or sugar cane in dissolved state, it can be extracted
from them.
In sucrose an α-D- glucose combines with a β-D- fructose molecule forming an α,β (1→ 2) ulinkage. This
molecule differs from the above-mentioned ones, as t he glycosidic bond is formed between the reducing ends
of both units , therefore it is a non-reducing sugar (Fig. 7).
3.6. ábra - Figure 7: Disaccharides
3. 3. 3. Polysaccharides
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CARBOHYDRATES
Polysaccharides are macromolecules of repeated monomers units (monosaccharide) joined together by
glycosidic bonds. Polysaccharides do not have sweet taste, and crystallic structure. They do not solve in water or
their aqueous solution result in a colloidic suspension.
3.1. 3. 3. 1. Classification of polysaccharides
Polysaccharides can be classified according to there chemical structure and there biological function.
• Chemical structure: Polysaccharides can be divided into two classes, based on how many C atoms their
monomer units have.
• Pentosans are composed of pentoses (e.g. xylan, araban)
• Hexosans are composed of hexoses (e.g. cellulose, starch)
• Biological function: Polysaccharides are of great importance as food reserves in plants or as structural
components of plants. Some examples for the most important polysaccharides
• food reserves polysaccharides: starch, glycogen, inulin,
• structural polysaccharides: cellulose, chitin, pectin.
Starch
Starch is a plant reserve nutrient that is stored in plant seeds and tubers. Potato contains cca. 2o % starch, while
grain seeds 55-7o %. Starch is the principal carbohydrate source for human nourishment and it is utilized by
decomposing to glucose. Starch does not dissolve in cold water, swells up in hot water, and then forms colloid
solution. Colloid solution turns into gel due to cooling.
Starch is a mixture of two types of molecules, the linear amylose (~ 20%) and the branched amylopectin (~
80%).
Amylose
It consists of 100-300 α-D-glucose subunits involving exclusively α(1→4) ulinks, as they are in maltose.
The α(1→4) structure promotes the formation of a helix structure. In the helix spiral six glucose units are in a
thread. The spiral is stabilized by intramolecular hydrogen-bonds. Amylose molecules consist of single
unbranched chains.
Amylopectin
Amylopectin has greater molecular weight than amylose, it consists of more than thousand glucose units. In
amylopectin α (1→4) bonds are dominant also, but 12-20 glucose units are always followed by α (1→6) acetal
ulinkage, that causes branches in the molecule.
Glycogen:
Glycogen is a reserve polysaccharide in animals. It is synthesized and stored in the cells of the liver and the
muscles. Glycogen has a similar chemical structure and size to amylopectin but it is more extensively branched
and compact than starch.
Inulin
Inulin is a fructose polymer and it naturally occurs in many types of plants. In an inulin molecule 30-35 fructose
units attached with 2→1 ulinks. There are some glucose molecules attached to the end of a fructose chain.
Cellulose
Cellulose is the structural component of the primary cell wall of green plants (trees, grasses). Trees comprise of
40-50% cellulose. The purest cellulose source in nature is cotton. Cellulose is a linear polymer of 1000-14000 ßD-glucose molecules that are connected by β (1→4) glycosidic bonds (cellobiose). Cellulose is a straight chain
polymer, no coiling or branching occurs.
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CARBOHYDRATES
In cellulose, the hydroxyl groups on the glucose from one chain form hydrogen bonds with oxygen atoms on the
same or on a neighbour chain, holding the chains firmly together side-by-side and forming microfibrils.
Chitin
Chitin constitutes a major structural material in the exoskeletons of many arthropods and mollusks. Chitin is a
homopolymer of N-acetyl-β-D-glucosamines that are connecting with ß-1→4 bonds.
Pectin
Pectin is a structural heteropolysaccharide, a constituent of the primary cell wall of plants. Pectin is a polymer of
D-galacturonic acid monomers with α-1→ 4 glycosidic bonds. Pectin is usually used as gelling agent in jams,
jellies or fruit juice. Certain fruits contain especially large amount of pectin (e.g. currant, gooseberry, quince)
(Fig. 8)
3.7. ábra - Figure 8: Polysaccharides
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4. fejezet - 4. BIOMOLECULES II.
PROTEINS
Proteins are one of the most important building blocks of living organisms. They have broad task. Their
monomeric units are amino acids. Amino acids contain acidic carboxyl group and basic amino group as well.
In proteins 20 kinds of α-L-amino acids can be found. In these molecules, the amino group is ulinked to that
carbon atom (α.) that is closest to the carboxyl group. Amino acids of proteins differ from each other in their
groups that are attached to the α-carbon atom. The α-carbon atoms are chiral. Among the enantiomers amino
acids with L-configuration occur in proteins. Amino acids (monomers) joined together by peptide bonds and
create a polypeptide chain (Fig. 9).
4.1. ábra - Figure 9: Peptide bond
Atoms in peptide bond (C, O, N, H) are coplanar. Due to the π-bond and the delocalization of the nitrogens nonbonding electron pair the rotation is inhibited, and there is trans-spacing. The trans-spacing is of great
importance in the stabilization of secondary protein structure
Proteins can be classified chemically and biologically as well. By chemical classification the nature of the R
groups are taken into account. Thus we can distinguish monoamino monocarboxylic acids with non-polar side
chains, etc.
Based on biological properties, essential amino acids can not be produced by our organism, thus we must use for
protein synthesis those amino acids that are produced by other organisms.
We are able to produce non-essential amino acids from precursors. The ones that can be produced only by adults
in an appropriate amount, from other essential amino acids belong to the semi-essential amino acids.
Polypeptide chains consisting of more than 100 amino acids are called proteins, and their structure is organized
in four stages.
The primary structure of proteins is the amino acid sequence in it. Primary structure determines the properties
of other structures as well.
Two types of secondary structure of the proteins are known: these are α-helices and beta-pleated structure.
Secondary structure is stabilized by hydrogen bonds formed among the peptide bonds (amide groups). The
formation of hydrogen bonds is facilitated by the trans spatial position of-peptide bonds.
The polypeptide chain’s three-dimensional structure (conformation) is called the tertiary structure of the
peptide. From the aspect of tertiary structure there are globular and fibrillar proteins. Fibrillar proteins can be
characterized by pure α-helix or beta-pleated structure. In general, fibrillar proteins contain only a few types of
amino acids. Tertiary structure is stabilized by primary - and secondary bonds between the R-groups of amino
acids. (Among primary bonds the ionic and the disulfide bonds can be found while among secondary bonds all
kinds of them can be found).
The quaternary structure of proteins is a complex structure formed by the interaction of more proteins. (For
example, enzyme complexes) Proteins are able to perform their biological function by formation of their
quaternary structure (Fig. 10).
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4.2. ábra - Figure 10: Conformation of proteins
1. 4. 1. Proteins can be classified
According to their shape and structure:
• Fibrillar (fiber, fibrous) structure is characteristic for the frame, the supporting connective tissue proteins (e.g.
proteins of the muscles, wool, hair).
• Globular (spherical) structure: is characteristic for the enzyme proteins, blood proteins (albumin, globulins).
Not only simple but complex proteins also belong to here (e. g. myoglobin, casein).
There are proteins that do not strictly fall into one of these groups, they are characterized by the transition
between the two types. There are also proteins that can be found both in globular and fibrillar forms.
According to their composition:
• Simple proteins only consist of amino acids
• Complex proteins also contain other components beside amino acids They can be classified based on their
non-protein part
The main groups of complex proteins:
GROUP
NON-PROTEIN PART
TASK
Hemproteins
iron-porfirin
oxygen and electron transport
Metalloproteins
Metallic ions
biocatalysts
Phosphoproteins
Phosphoric acid
Reserve nutrient
Lipoproteins
lipids
Lipid transport
Glycoproteins
Carbohydrates
protection
Nucleoproteins
Ribonucleic acids
protein synthesis
According to their solubility:
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GROUP
SOLVENT
Albumins
→
water, dilute salt solution
Globulins
→
dilute salt solution
Prolamins
→
dilute alcohol
Glutelins
→
dilute acids or alkalis
Histones
→
dilute acids
Frame proteins
→
insoluble
Albumins and globulins can be found in plants and animals as well. Prolamins and glutelins are characteristic
proteins for the cereal grains. Histones are nuclear constituents. Frame proteins can be found in animals.
According to their biological function
GROUP
TASK
Enzymes
Catalyze biochemical processes
Hormones
Regulate biochemical processes
Receptor proteins
Bind and transport stimuli
Protective proteins
Protect against injuries and infections
(thrombin, immunoglobulins)
Transport proteins
Material transport
(hemoglobin: oxygen transport, transferrin iron transport)
Structural proteins
Construct frame and connective tissue
(collagen, elastin, keratin)
Reserve proteins
Fundamental
ovalbumin)
sources
of
embryonic
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development
(zein,
casein,
5. fejezet - 5. BIOMOLECULES III. THE
LIPIDS
Lipids are organic compounds; they carry out important functions in the living organism. Some lipids are used
for energy storage, since the higher animals and oilseed plants store their reserve energy in fats, in oils. Large
fraction of lipids together with proteins are the components of membranes, so directly or indirectly can influence
some vital functions of the cells.
Lipids are chemically diverse group of compounds that are classified together because of their apolar structures,
which give them high solubility in apolar solvents. These compounds have very low solubility in the aqueous
environment of the cell.
1. 5.1. Classification of lipids:
1. Saponifiable lipids:
• Simple lipids: (Alcohol and carboxylic acid(s) are obtained by hydrolyzing them.)
• vaxes
• neutral fats and oils (triglycerides)
• Compound lipids: (Besides alcohol and carboxylic acid other compounds are also obtained by hydrolyzing
them.)
• phospholipids
• sphingolipids
• glycolipids
2. Insaponifiable lipids:
• Steroids
• sterols (cholesterol, ergosterol)
• bile acids
• hormons
• steroidal glycosids (digitoxine, strophantin)
• steroid alkaloids (tomatine, solanine)
• Carotenoids (tetraterpenoids and derivatives)
• Lipid soluble vitamins (A, D, E, K)
1.1. 5. 1. 1. Saponifiable lipids
Lipids treated with concentrated alkali such as NaOH or KOH, give sodium or potassium salt of the fatty acid,
which are soluble in water.
The fatty acids:
The fatty acids are the simplest lipids. These carboxylic acids are constituents of many complex lipids.
The classification of fatty acids:
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• saturated fatty acids
• lauric acid (C11H23 – COOH);
• myristic acid (C13H27 - COOH);
• palmitic acid (C15H31– COOH);
• stearic acid (C17H35– COOH);
• arachidic acid (C19H39– COOH);
• unsaturated fatty acids
• oleic acid (C17H33 – COOH);
The place of the double bond beyond carbon: C9
• linoleic acid (C17H31– COOH);
The place of the double bonds beyond carbons: C9, C12 ( Ώ 6)
• linolenic acid (C17H29– COOH);
The place of the double bonds beyond carbons: C9, C12, C15 ( Ώ 3)
• arachidonic acid (C19H31– COOH);
The place of the double bonds beyond carbons: C5, C8, C11, C14
Most of the naturally occurring fatty acids have a chain of an even number of carbon atoms, from 14 to 22.
However, in milk fatty acids with lower number of carbon atoms and with odd number of carbon atoms can be
found. In most of the naturally occurring unsaturated fatty acids the orientation around double bonds is cis (cis
configuration).
The linoleic acid and the arachidonic acid are essential fatty acids, while the linolenic acid is semi-essential fatty
acid, because the human body can synthesize it from linoleic acid.
1.1.1. 5.1.1.1. Vaxes
Waxes are esters of saturated or unsaturated long chain monocarboxylic acids (fatty acids) and long chain
monohydroxy alcohols (fatty alcohols).
1.1.2. 5.1.1.2. Neutral fats and oils (triglycerides)
The neutral fats and oils are the triesters of glycerol and three molecules of fatty acids.
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In fats the ratio of saturated fatty acids to unsaturated fatty acids are higher then in the oils. The higher the ratio
of saturated fatty acids the higher the melting point of the fat.
The triglycerides with similar esterified fatty acids are called simple triglycerides, w hile two or three fatty acids
are different in triglyceride, is called mixed triglyceride.
The melting point of the neutral fat of the animals living on the warmer climate is higher. The diet rich in
carbohydrate favours the synthesis of the fats containing saturated fatty acids and having higher melting point.
Our organism stores the big part of energies in fat depots in the form of neutral fats. The fat is the major energy
source in most cells. The metabolic oxidation of fat consumes more oxygen, (gram per gram) than oxidation of
carbohydrate, so a correspondingly higher amount of energy is released.
The lipids need smaller space in the cell than carbohydrates, because opposed to carbohydrates all lipids are
hydrophobic molecules and are not surrounded by a thin film of water.
1.1.3. 5.1.1.3. Phosphoglycerides
The phosphoglycerides are the major component of all cell membranes. These compounds are similar to the
triglycerides with one important difference, namely one of the three fatty acids is substituted by a phosphate
group. All these compounds can be considered to be derivatived of glycerol-3-phosphate. Compounds, which
contain hydroxyl group (cholamine, choline, serine) can react with the phosphate group of phospholipid by an
elimination of water molecule.
Cholamine (ethanol-amine): HO-(CH2)2-NH2
Choline: HO-(CH2)2-N+(CH3)
Serine: HO-CH2-(CH)(NH2)(COOH)
Inositol: C6H12O6
Phosphatides are amphipathic molecules. O ne end of the phospholipid molecule (the phosphate group with
bounding alcohols) is hydrophilic (lipophobic)
and the other end (the long carbon chain part) is hydrophobic (lipophilic). Because of the amphipatic nature,
they can form micelles, they are the components of membranes.
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Two common phospholipids are cephalin (phosphatidylethanolamine) and lecithin (phosphatidylcholine). These
two phosphpglycerides are the main components of membranes of the animal cells.
1.1.4. 5.1.1.4. Sphingolipids
The sphingolipids mainly occur in the brain, in the spleen, in the liver and in the blood. Sphingolipids contain
sphingosine (long chain unsaturated amino-diol).
Beside the fatty acid, a phosphoric acid can also join to sphingosine. A fatty acid is ulinked via an amide bond to
the amino group of sphingosine. The hydroxyl group is phosphorylated and this is esterified by other alcohol.
Sphingosine is converted into a variety of derivatives to form the family of sphingolipids. Sphingosine and the
long carbon chain of fatty acid are the apolar (hydrophobic) parts of the molecule, the other part is polar
(hydrophylic).
1.1.5. 5.1.1.5. Glycolipides
Glycolipids occur mainly in the nerve tissue. Glycolipids are carbohydrate-attached lipids. In glycolipids besides
the fatty acid, monosaccharide (often galactose), oligosaccharide molecule is connected by an O-glycosidic
bond to the sphyngosine.
1.2. 5.1.2. Insaponifiable lipids
Lipids treated with concentrated alkali such as NaOH or KOH, do not hydrolyze. If they hydrolize, the end
products are not soluble in the water.
1.2.1. 5.1.2.1. Steroids
The common feature of the steroids is the gonane skeleton (four cycloalkane rings join to each other). The rings
are not aromatic. The rings are non-planar, they exist in the chair or in boat conformation. Several substituents
can be attached to the carbon atoms of the ring, these substituents vary by the configuration. Considering their
chemical composition and their function exceptionally diverse molecules belong here.
5.1. ábra - the core of gonane
The sterols (cholesterol, ergosterol) are the precursor for the synthesis of the vitamin D. The cholesterol in a
free state or in an ester form with fatty acids is a component of animal fats. It is the constituent of the animal
fats. It appears in the bile and in the blood too.
The bile acids differ from each other in the substituents (containing carboxyl-group also) being attached to the
sterane skeleton. In the bile, they can be found in the form of their salts. The bile acids emulsifies lipids, so
facilitate their digestibility.
The steroid type hormones are the hormones of the adrenal cortex and the sexual hormones. These hormones
are originated from the progesterone. We will deal with their task in a later chapter.
The steroidal glycosides have effect on cardiac muscle (digitoxine, strophantin). They enhance the contraction
of the cardiac muscle. The steroidal glycosides are synthesized by plants. In these molecules, special sugars are
bound to the steroid skeleton via a glycosidic bond.
The steroidal alkaloids (solanine, tomatine) are vegetal origin. A strong physiological effect characterizes
them.
1.2.2. 5.1.2.2. Carotenoids
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LIPIDS
The carotenoids areproduced from isoprene molecules (five-carbon units). The tetraterpenoids (carotene) and
their derivatives (beside C and H they contain heteroatom, xanthophyll) are important carotenoids. They are
produced from 8 isoprene molecules and they contain 40 carbon atoms. The double bonds in these molecules are
conjugated. That is the reason of their colours, so they provide one part of the colour molecules in plants.
1.2.3. 5.1.2.3. Lipid soluble vitamins
We will deal with lipid soluble vitamins in a later chapter in particular .
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6. fejezet - 6. BIOMOLECULES
IV. THE NUCLEIC ACIDS
The nucleic acids (DNA, RNA) are macromolecules. Their monomer units are nucleotides. The nucleotides can
be hydrolysed into three parts: phosphoric acid, pentose, and N containing aromatic heterocyclic base
(nucleobase) (Fig.11). The nucleotides joined by phosphodiester bonds between the 3’ hydroxyl of one sugar
and the 5’ hydroxyl of the other.
DNA nucleotide
RNA nucleotide
phosphoric acid
phosphoric acid
pentose sugar
deoxy-D-ribose
D-ribose
nucleobase
purine base: adenine, guanine;
purine base: adenine, guanine;
pyrimidine base: cytosine, thymine. pyrimidine base: cytosine, uracil.
6.1. ábra - Figure 11: Nucleobases
Nucleobases (uracil, thymine, cytosine, and guanine) due to the migration of hydrogen atom and the double
bond may undergo amide-imidic acid tautomeric shifts,
Which yields the lactam tautomer (lactim ↔ lactam tautomers). Due to the tautomerization process, the bases
are capable to attach the 1’ carbon atom of the pentose by removal of water molecule. For the construction of
hydrogen bonds, which are a major factor of stabilizing the spatial conformation of polynucleotide chain, the
tautomerization process also is responsible.
The chemical structure and the connection of nucleotide monomers to polynucleotide chain is shown in Fig. 12.
In each nucleotide the nucleobase is attached to the 1’ carbon of the sugar via an N-glycosidic bond, while the
phosphate group is attached to the 5' carbon atom of the ribose sugar via ester bond.
The connection between nucleotide units in chain is through a phosphate group attached to the hydroxyl on the
5’ carbon of one unit and the 3’ hydroxyl of the next one (by elimination of water). This forms phosphodiester
bonds. In this way very long nucleic acid chain can be formed. At one end the nucleic acid always has free
hydroxyl group on the 3’ carbon atom of the sugar (3’ end) and the other end of the molecule always has
phosphoric acid connecting to the 5’ carbon atom of the sugar (5’ end), which is able to form other bonds.
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6.2. ábra - Figure 12: From nucleotide monomers connected to polynucleotide
1. 6.1. The deoxyribonucleic acid (DNA)
1.1. 6. 1.1. The primary structure of deoxyribonucleic acid (DNA)
The primary structure of nucleic acids (DNA, RNA) is determined by the sequence of connected nucleotides.
The nucleotides containing different N-bases can be attached in an optional order. The sequence of the bases
determines the information available for building the living material.
The DNA consists of two long antiparallel polynucleotide chains. The two strands form a spiral called a double
helix. The strands run in opposite directions and they are coiled in a right-handed manner about the same axis.
The DNA double helix is stabilized by hydrogen bonds between nucleobases of nucleotides opposite each other.
In the DNA the mole ratio of purine bases (adenine, guanine) and pyrimidine bases (cytosine, thymine) is equal
(1:1).
A + G = T + C, which is possible when opposite of the nucleotide containing purine base can settle only
nucleotide containing pyrimidine base. The consistent space filling may evolve in that way. The distance of the
two chains is constant throughout the entire DNA molecule. The diameter of DNA is 2 nm.
In DNA the number of nucleotides containing adenine and thymine and the number of nucleotides containing
guanine and cytosine are equal (A=T; G=C). This law can be explained by the secondary structure of
DNA.
1.2. 6.1.2. The secondary structure of DNA
In DNA the sequence of the nucleotides of one chain defines the sequence of the nucleotides in the other chain.
Opposite of the nucleotide containing adenine can settle only nucleotide containing thymine, while opposite of
nucleotide containing guanine can settle only nucleotide containing cytosine. Adenine and thymine are forming
two hydrogen bonds, while guanine and cytosine are forming three hydrogen bonds. The number of possible
hydrogen bond is increased by forming of lactam tautomer.
The two strands are described as complementary to one another, following from the rule of base pairing. Two
linear strands run in opposite direction to each other, the 5’ end of the one chain is in connection with the 3’ end
of the other chain.
The double strand of DNA twists together to helical form. The phosphodiester-ulinked sugar residues form the
backbone of the nucleic acid molecule and are on the outside of the helix.
The nucleobases are inside of the helix protected by the sugar phosphate groups (Fig. 13). In DNA ten basepairs are in each turn of the helix.
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6.3. ábra - Figure 13: The deoxyribonucleic acid.
The DNA in the nucleus of eucaryotes bounds to histone proteins forming the nucleosome (histon + a part of
DNA’s chain = nucleosome). The nucleosomes are protected and stabilized by additional histone proteins from
outside. The DNA inside the nucleus is organized by histone into DNA-histone nucleoprotein complex known
as chromatin.
1.3. 6.1.3. The biological function of DNA
DNA is responsible for self-reproduction of the living organism. DNA is able to produce an exact copy of itself
and at cell division (mitosis, meiosis) information is passed from an organism to its descendents. Through this
feature DNA is capable for transmitting of unique and racial characteristics of organism to successor
generations.
Genetic information in DNA is stored in „codons” (in the form of a coded sequence of bases). The sequence of
codons on a DNA tells the cell the sequence of amino acids in a protein. DNA directs the protein synthesis.
DNA determines the number and the connection sequence of amino acids of the proteins in living organisms.
2. 6. 2. The ribonucleic acids (RNA-s)
RNA-s consist of long, single-stranded, unbranched chain of nucleotides. RNA forms an A-form helix, but may
have some double-strand regions too, as a consequence of self-complementary. The RNA the double-strand
regions often folds RNA into three-dimensional structures. RNA-s take part in protein synthesis directly.
According to their biological tasks and their role in the protein synthesis, there are three types of RNA-s:
• messenger ribonucleic acids (mRNA),
• transfer ribonucleic acids (tRNA),
• ribosomal ribonucleic acids (rRNA).
RNA-s differ in molecular weight, in composition of nucleotides, and their three-dimensional structure may also
be different. The quantity of RNA-s in the cells are multiple (5-10 fold) compared to that of DNA.
2.1. 6. 2.1. The messenger RNA-s
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Messenger RNA-s (mRNA) carry coded genetic information about a protein sequence to the ribosomes for
synthesis of new proteins. In the cells hundreds of thousands of different proteins are synthesized, and it requires
the same kinds of mRNA-s also. The mRNA (like other RNA-s) is produced in the nucleus in the process of
transcription. (Transcription: is the process of creating an RNA copy of a sequence of DNA.)
It is characteristic to mRNA-s that they can be synthesized quickly, but they can break down also quickly so
they have very short lifetime. The RNA molecule consists of 300-3000 nucleotide units. The sequence of
nucleotides (bases) of all mRNA-s is complementary with the sequence of nucleotides (bases) of the appropriate
section of DNA, so it determines the amino acid sequence of protein (translation). The RNA synthesized from
DNA may contain non-informal parts (pre-mRNA).
The pre mRNA during its migration in the cytoplasm (the pre-mRNA must get out of the nucleus to the place of
protein synthesis, being in the ribosomes in the cytoplasm) goes through a ripening process, when the noninformal parts are torn out of it.
2.2. 6. 2. 2. Transfer RNA-s
tRNA-s have the smallest molecular weight of all nucleic acids. They consist of 70-100 nucleotide units. tRNAs have highly ordered structure, the single strand is folded into a three-dimensional structure stabilized by
hydrogen bonds between base pairs. The secondary structure of tRNA usually is visualized as the cloverleaf
structure. At the developing of three-dimensional structure, the loops are created where no hydrogen bonds are
formed. Each loop or chain’s end has an important role. Free nucleotides (nucleotide triplet) remain in the loops,
which ones and also the chain’s ends have important role in the biological tasks of tRNA-s. The tertiary
structure is described as L-shaped.
The function of tRNA-s is to carry the activated amino acids to the site of protein synthesis. There are specific
tRNA-s for each amino acids. There are at least 20 kinds of tRNA-s needed to the transport of 20 kinds of amino
acids. (Some amino acids belong to diverse tRNA-s.)
For transportation to the ribosome, an amino acid is joined to its specific tRNA. This process is directed by a
three-nucleotide sequence in one end of tRNA (anticodon).
The common feature of tRNA-s is that all of them have cytosine-cytosine-adenine nuleotide triplet at the 3’ end
of the chain, where the amino acid is connected (binding site). (Fig. 14)
6.4. ábra - Figure 14: The mRNA and the tRNA
2.3. 6. 2. 3. Ribosomal RNA-s
Ribosomal RNA-s comprise up to 50-65% of the total ribonucleic acids in the cells. Several types of rRNA-s (5)
are possible to distinguish. The rRNA molecules generally have single-stranded polynucleotide chain, but may
contain unordered and double-strand regions too. The different rRNA types can be detached on the basis of their
sedimentation coefficient. The ribosomal RNA with proteins forms the subunits of the ribosome
(ribonucleoproteins). The ribosomes consist of two units, the large subunit and the small subunit. The two
subunits are associated in the presence of Mg ions.
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The most important task of ribosomes is to supply the corresponding structure for binding of mRNA and
binding of tRNA-s (carrying the amino acids) because the protein synthesis takes place on the surface of
ribosomes.
3. 6. 3. Nucleoside triphosphates
The nucleotides in the cells act not only as the monomers of the nucleic acid, but also as an energy source. In the
nucleotides more phosphate groups can be joined together by phosphoanhydride bonds. To the formation of
these bonds a great amount of energy is needed, which is produced in the metabolism. Energy stored in ATP is
released upon the hydrolysis of the anhydride bonds and this energy is consumed in the anabolic pathway and in
other energy-consuming processes (ATP ↔ ADP ↔ AMP).
Nucleotides: ATP (Adenosine-triphosphate, contains adenine), UTP (Uridine-triphosphate, contains uracil),
GTP (Guanosine-triphosphate, contains guanine), CTP (Cytidine-triphosphate, contains citosine).
In the important biochemical processes, the enzymes carry out their activities together with the cofactors
(NAD+, NADP+, FAD, CoA). These cofactors are analogous with the nucleotides or with the compounds
containing nucleotides. We will write about them in the following section.
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7. fejezet - 7. BIOACTIVE
COMPOUNDS I. VITAMINS
Vitamins are biologically active organic compounds that are essential to the biological processes of animal and
human beings. Humans and many other organisms can not synthesize these essential compounds or can
synthesize in small amounts and therefore must obtain from their diet, either the vitamin itself or a compound
from which the required vitamin can be synthesized.
Vitamins do not provide energy, but they are essential for the material and energy flows. Sometimes one
compound is essential vitamin for one organism, while another species is able to produce it. For example,
ascorbic acid (vitamin C) is a vitamin for humans, monkeys and guinea pigs, but is not vitamin for most other
animals. Other species studied to date can be synthesized vitamin C from glucose. Vitamins as a unified group
can be formed only biologically because their chemical structures are very diverse and different.
If a particular vitamin is permanently missing from the diet, the organism will suffer from a deficiency and
problems will appear in its functioning. Mostly growth disorders may occur. This phenomenon is named
hypovitaminosis.
The more severe version of this is when (usually) typical clinical picture develops, which can be cured by
dosage of vitamin. This phenomenon is named avitaminosis.
Dysfunction, hypervitaminosis may also occur if very large amount of vitamin is introduced into the body.
Food often does not contain the active form of vitamin, but also contains the precursor, from which the body is
able to produce the active form of vitamin by using the converter mechanism. These compounds are called
provitamins in the literature.
Organic compounds that inhibit the absorption or actions of vitamins are called antivitamins. The structures of
antivitamins are very similar to the structure of vitamin.
Classification of vitamins
1. 7.1. Physiological effects of vitamins:
The vitamins can be absorbed from the digestive system and as catalytic or regulating factors can join in the
vital processes.
One part of vitamins is ulinked to proteins and affect as a component of coenzyme or prosthetic group of an
enzyme. These vitamins are called prosthetic vitamins.
The inductive vitamins are identified as those of the vitamins that are essential for living organisms but their
physiological role is not fully understood.
The vitamins can be divided into two classes based on their solubility. Thus, we distinguish lipid-soluble
vitamins (vitamins A, D, E and K) and water soluble vitamins (B vitamins, vitamins H, C, P, pantothenic acid,
folic acid).
2. 7. 2. Lipid soluble vitamins
Physiological effects of lipid-soluble vitamins have been known, but their function in metabolic processes of the
cells is only poorly understood.
In their absence, the activity of enzymes changes in certain animal tissues. It is believed that the fat-soluble
vitamins regulate the biosynthesis of certain proteins. The lipid-soluble vitamins can be stored by the organism,
so their lack develops later or takes shape more rarely. Hypervitaminosis also occurs, especially in the case of
vitamin A and D.
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Today people often try to cover their vitamin needs by synthetically produced vitamin products. In this case an
overdose may often occur.
At the time of the natural foods consumption, the appearance of the hypervitaminosis can hardly be imagined.
Vitamin A (retinol)
Vitamin A or retinol is a primary, unsaturated alcohol of molecular formula C 20H30O. They consist of β-ionone
ring and the connected side chains of isoprene unit. An alcoholic hydroxyl group is attached to side chain.
Vitamin A aldehyde or carboxylic acid may be formed by the chemical oxidation of this group (-OH), which are
also bioactive compounds. The hydroxyl group (-OH) of vitamin A can form an ester with fatty acids. The
resulting compounds often outweigh the biological activity of vitamin A.
Vitamin A occurs only in the animal world, while in plants the precursor or provitamin is found in the pigments
called carotenes. The vitamin A1 may be formed from 12 different carotenoid compounds (for example: α-, β-,
γ-carotene, criptoxantine) in animals by the enzyme of carotinase. The conversion is the most efficient from βcarotene.
The recommended daily intake of vitamin A is ranged from 0.8 to 1.5 mg, or 5-9 mg of β-carotene per day.
Vitamin A is necessary for cell growth and differentiation, skin and cellular health. With the protection of
mucosa of respiratory organs protects the body against invading pathogens. Vitamin A affects the development
of bones and teeth and is responsible for their protection.
One of the most obvious consequences of deficiency of vitamin A is the night-blindness. This is the inability to
see in dim light. The effect of vitamin A in this process is accurately known.
The rhodopsin in photoreceptor cells of the retina is decomposed to opsin and 11-trans-retinal. This change
generates nerve impulses in the central nervous system, which develops a sense of vision. In order to the
resynthesis of rhodopsin the 11-trans –retinal have to be retransformed into 11-cis retinal. This process is multistage, first 11-trans retinol is formed by reduction, than in the isomeration process 11-cis retinol is issued. In the
following oxidation process, 11-cis-retinal is re-produced again by the retinol-dehydrogenase enzyme and with
connecting to opsin forms rhodopsin. This reaction sequence does not proceed quantitatively; the resulting loss
can be replaced from the 11-cis-retinal derived from vitamin A of the blood (Fig. 15).
The best sources of vitamin A are cod-liver oil and other fish-liver oils, animal liver and dairy products (milk,
butter), egg-yolk. The fruits and vegetables (carrot, spinach, pumpkin, cantaloupe) contain large quantities of
carotenoids.
The overdose of vitamin A may cause hair loss, skin inflammation, fatigability beside pain in limb, malaise,
upset stomach, lips and skin dryness, cracking.
7.1. ábra - Figure 15: Vitamin A
Vitamin D (Calciferols)
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Calciferol is a collective noun. Several compounds with the same biological effects but with different chemical
structure belong to the group of calciferols.
Vitamin D1 is called calciferol and lumisterol, vitamin D2 is called ergocalciferol, while vitamin D3 is called
cholecalciferol.
Vitamin D controls the absorption and incorporation of calcium and phosphorous into the bones. In the absence
of vitamin D „rachitis” symptom may occur. The patient's bones become soft, bent under the weight of the
body. The joints become thick, the teeth break off.
Sources of vitamin D are fish liver oils, egg-yolk, yeast, wheat germ, champignon, oat-flake, where mainly
provitamins of vitamin D can be found in large quantities.
Vitamin D is produced by the action of ultraviolet light from ergosterine (with plant origin) and from 7-dehydrocholesterol (with animal origin), therefore vitamin D can be said to be produced from its provitamins (also in the
skin).
The recommended daily intake of vitamin D is 10 μg (up to 20 years) and for adults 5μg.
The overdose of vitamin D may cause brittle bones, calcium level become higher in the blood, or may cause
atherosclerosis.
Vitamin E (tocopherols)
In the body Vitamin E functions as an antioxidant, inhibits the oxidation of fatty acids, membrane lipides,
provitamins and vitamins (vitamins A, C, carotene).
It plays a role in neurological functions. Vitamin E is needed for race preservation, because it promotes the
fertilizing ability and fecundity, promotes the germ cell proliferation, growth and development of the fetus.
Vitamin E has anti-inflammatory effect, reduces the permeability of capillary blood vessels, and influences
collagen formation as well.
Tocopherols consist of a chromanol ring and a hydrophobic side chain, which is a phytil (allows for penetration
into biological membranes). Some versions differ in the groups (their number and location) connecting to
chromanol ring.
Tocotrienols have the same structure as the tocopherol, but there is a double bond in the hydrophobic side chain,
which consists of three isoprene units.
Vitamin E occurs only in the plant world. Tocopherols are synthesized in the chloroplasts of green plants and
are transported to the seeds, to the lipid storage part of plant.
The recommended daily intake of vitamin E is 5-15 mg. The main sources of vitamin E are wheat germ oil,
sunflower oil, safflower oil, nuts, nut oils (almond, hazelnuts), leafy green vegetables (spinach, turnip, beet), and
avocados. (Fig. 16)
7.2. ábra - Figure 16: Vitamin D, E, K.
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Vitamin K (Phylloquinones)
Vitamin K deficiency can cause severe bleeding, it is essential for blood clotting. Vitamin K helps the synthesis
of the coagulation factors II., VII., IX. and X.
The Vitamin K1 and K2 are 2-methyl-naphthoquinone derivatives. In the vitamin K1 there is a phytil, while in
the vitamin K2 there is a prenyl side chain.
The recommended daily intake of an adult of vitamin K is 1-4 mg, what the diet and the synthesized quantity
(vitamin K2) by the intestinal flora living in the digestive system is usually able to cover. Vitamin K1 mainly
occurs in the plants, whereas the vitamin K2 is found in animal food. The main sources of vitamin K are green
leafy vegetables, cabbages and liver.
The fat-soluble vitamins have different sensitivity to the environmental effects. We have to pay attention to the
reactivity of the vitamins during the storage and processing of food in order to get appropriate amount of
vitamins into our body.
3. 7. 3. Water-soluble vitamins
The group of prostethic vitamins belongs to water-soluble vitamins. Binding to proteins, they can influence the
biological processes in the organism as the component of enzymes, coenzymes or prosthetic groups. They can
not be stored in the body, so the vitamin deficiency can develop quickly, but for this reason, symptoms of
hypervitaminosis do not occur. The water-soluble vitamins do not have provitamins, therefore we have to put on
the active compounds in our food.
Vitamin B1 (thiamine)
Thiamine was the first of the B vitamins to be identified and that is why it is called vitamin B1. Thiamine
contains a substituted pyrimidine ring as well as a five-membered ring containing nitrogen and sulphur. It plays
important role in carbohydrate metabolism; in the absence of vitamin B1 intermediate metabolites (lactic acid,
pyruvic acid) accumulate in the tissues and in the blood. Vitamin B1 is a constituent or prosthetic group of
several enzymes such as pyruvate decarboxylase and transketolases as thiamine-pyrophosphate (TPP). Vitamin
B1 enhances nervous system functions, and has an important role in normal functions of muscles and heart.
Deficiency of vitamin B1 may cause “Beriberi” disease (disease of the nervous system include weight loss,
emotional disturbances, peripheral neuritis, muscular dystrophy).
The recommended daily intake of vitamin B1 of an adult is 1.5-2 mg.
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The main source of vitamin B1 are yeast, whole grains, vegetables, fruits, meat (pork, fish) liver, milk, egg, and
butter.
Vitamin B2 (riboflavin)
Vitamin B2 as a central component of the flavin enzymes (cofactors) influences the metabolic processes.
Vitamin B2 acts as a hydrogen transfer in the oxidation processes. Vitamin B2 is an isoalloxazine derivative,
contains a fused three-ring molecule called flavin. One of the nitrogen atoms of this ring system is substituted
with a five-carbon sugar alcohol, ribitol.
Vitamin B2 can be found as riboflavin-5’-phosphate (FMN: flavin mononucleotide or as FAD: flavin adenine
dinucleotide) in the prosthetic group of flavin enzymes classified in the dehydrogenases.
The FAD is formed with a connection of two nucleotide units through anhydride bond.
The functional part of FAD is the isoalloxazine ring, which serves a two-electron acceptor (reversible process).
The recommended daily intake of vitamin B2 of an adult is 1.5-2 mg. Source of vitamin B2 are venison, yogurt,
soybean, milk, mushroom and spinach. The lack of vitamin B2 may cause cracked and red lips, inflammation of
the lining of mouth and tongue; the eyes may become bloodshot, itchy and sensitive to bright light.
Vitamin B3 (nicotinic acid amid, niacinamid, vitamin PP)
Natural materials include nicotinic acid amide. Vitamin B3 expounds its biological effect as a component of
oxidoreductase enzymes. It is a part of NAD+ and NADP+. The uptake and release of electrons and protons
happens at the nicotinic acid amide part of the molecule.
The NAD + and NADP+ as a coenzyme of many dehydrogenase enzymes take part in the oxidation or reduction
processes of a wide variety of compounds (Fig. 17).
The recommended daily intake of vitamin B3 is 10-20 mg. The lack of vitamin B3 besides pellagra disease may
cause stomach and intestinal disorders.
7.3. ábra - Figure 17: Vitamins (B1, B2, B3)
Vitamin B6 (pyridoxine group)
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The pyridoxol, the pyridoxal and the pyridoxamin are three natural forms of vitamin B6. All of these forms can
be found in phosphoester form in the body and can be easily transformed into each other. They are converted in
the human body into a single biologically active form, pyridoxal 5-phosphate.
The phosphoric acid is bound to the CH2-OH group of the 5’ carbonic atom of the ring trough ester bond.
Vitamin B6 has biological effect on the amino acid-protein metabolism and carbohydrate metabolism. It is a part
of many enzymes (amino transferase, amino acid decarboxylase, phosphorylase).
The recommended daily intake of vitamin B6 of an adult is 2-3 mg. The lack of vitamin B6 may cause
dermatologic and neurologic changes (seborrhoeic dermatitis-like eruption, ulceration, conjunctivitis,
somnolence, confusion, neuropathy).
Vitamin B5 (pantothenic acid),
Pantothenic acid is the amide between pantoate and β-alanine.
Vitamin B5 has biological effect as the component of transacylase (CoA-SH). The CoA-SH is also a type of
dinucleotide coenzyme. In the CoA-SH the pantothenic acid is connected as panthetein-4-phosphate to
adenosine-3, 5-diphosphate.
The functional part of the coenzyme is the –SH group, which reacts with one or other carboxyl group of organic
acid and produces thioester and water. In the absence of vitamin B5 fatigue, restlessness and muscle spasms
may occur.
Vitamin B9, Folic acid (pteroil-glutamic acid)
The biologically active form of folic acid is tetrahydrofolic acid (FH 4), which is a multi-reduced derivative. The
biochemical role of tetrahydrofolate coenzymes is in the methylation processes.
Vitamin B9 (together with vitamin B12) has effect on forming of red - white blood cells, platelets. In the
absence of vitamin B9 anemia may occur. This vitamin plays an important role in the forming of mucosa of the
digestive system and in the synthesis of nucleic acids.
Vitamin B12 (cyanocobalamin)
Vitamin B12 has biological effect (often with folic acid) as the coenzyme component of enzymes (e.g.: methylmalonyl-CoA mutase). In the absence of vitamin B12 complaints of the nervous system, pernicious anemia, lack
of appetite, weakness and digestive complaints may occur.
Vitamin –H (Biotin)
Biotin is composed of a sulphur-containing ring part and valeric acid side chain. In nature, its amide (formed
with lysine), biocytin occurs.
Biotin takes part in carboxyl-transfer as a prosthetic group of enzymes, which catalyse the transfer of CO 2 and
HCO3-. These enzymes require energy (ATP) and Mg2+-ions during their operation, thus CO2 temporarily
connects to the N atom of the biotin. The activated carbon-dioxide connects to the proper acceptor in the form of
carboxyl group as a result of the function of carboxylases.
This vitamin influences the protein, carbohydrate and lipid metabolism as well.
The recommended daily intake of vitamin H is 100-300 μg. The lack of vitamin H may cause hair loss,
conjunctivitis, dermatitis, neurological symptoms, like depression, lethargy.
7.4. ábra - Figure 18: Water-soluble vitamins
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Vitamin C (Ascorbic acid)
Vitamin C is a carbohydrate derivative. Dienol group can be found in the molecule, which causes the acidic
properties of the compound. It can be found in oxidized and reduced form in cells and it forms redox system.
The oxidized form is the dehydroascorbic acid, the reduced form is ascorbic acid.
Vitamin C plays an important role in cell respiration. It is essential for the repair and maintenance of connective
and bone tissues, for recovery of wounds and fractures (vascular function).
Its biological effect is related to its oxidation-reduction ability. Vitamin C promotes the absorption of iron and
calcium in the digestive tract, acts as hydrogen donor in the biochemical processes, is responsible for
maintaining the reduced state.
Vitamin C plays a role in synthesis of collagen, adrenal hormones, serotonin and at the breakdown of tyrosine.
Vitamin C requirement is 80 mg, according to literature data, while fivefold quantity is recommended by others.
In case of lack of vitamin C chronic pain in the limbs or joints may occur. Deficiency will tend to bruise easily,
have a negative impact on general healing of wounds. Deficiency can cause deterioration of the gums, bleeding
gums may occur.
The main sources of vitamin C are vegetables, fruits, some animal products, plucks (liver, muscle).
7.5. ábra - Table 4: The role of vitamins in the function of enzymes
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8. fejezet - 8. BIOACTIVE
COMPOUNDS II. HORMONS
In general hormones are organic compounds produced in endocrine glands, which regulate the chemical
processes in the organism. The endocrine glands release their secretion directly into the bloodstream or
lymphatic system, thus hormones can also affect other exterior parts of the organisms. They regulate the
function, development, mass transport and growth of other organs. It is characteristic of hormone-producing
glands that their secretions enhance or inhibit one another, thus they can be synergists or antagonists to each
other.
The maintenance of inner balance (homeostasis) is ensured by the integrated system of nervous system and
hormone-producing glands (neuroendocrine system) by the following process:
stimulus → excitation → regulator compound → enzyme → biochemical reactions
The hormone production is usually a result of nerve impulses, and it starts in the endocrine glands. The
produced hormones get to the target cells via bloodstream, which receptors are capable of recognizing and
binding them. The hormone-producing glands are also under hormonal control. This control is carried out by a
three-level hierarchical system.
The operation of endocrine system is controlled by the diencephalon. Nerve impulse causes the secretion of
neosecretum by hypothalamus (the part of diencephalon) that stimulates hormone production of the pituitary
gland. According to it glandotrop hormones produced in anterior pituitary induce and stimulate the hormone
production of other endocrine glands. The excess hormones in the bloodstream have effect on the hormone
production of hypothalamus and pituitary as well (negative feedback). (Fig.19)
8.1. ábra - Figure 19: Regulation of hormone production
Endocrine glands
• Pituitary gland (hypophysis)
• Pineal gland
• Thyroid gland
• Parathyroid glands
• Adrenal glands
• Pancreas (islets of langerhans)
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• Ovary (female)
• Testes (male)
1. 8.1. Classification of hormones:
Hormones produced by endocrine glands can be classified based on their chemical structure, the type of
metabolism affected by them or their mechanisms.
According to their chemical structure hormones can be
• Derivatives of the amino acids (adrenaline, melatonin, tiroxine)
• Peptides, polipeptides, protein (hormones o pituitary)
• Steroids (adrenal cortex, hormones of testicle, ovary)
According to the type of metabolism
• lipid metabolism
• carbohydrate metabolism
• protein metabolism
According to their mechanism
• Amino acid and peptide type hormones: bind to the cell membrane and start up the synthesis of compounds.
These materials stimulate enzymes in the cell.
• Hormones that after entering the cell react with molecules inhibiting DNA in the nucleus. The transcription
from DNA and the enzyme protein synthesis can begin (usually steroid hormones).
1.1. 8.1.1. Hormones of hypophysis
Hypophysis consists of three lobes: the anterior pituitary (adenohypophysis), posterior pituitary
(neurohypophyseal) and the middle (pars intermedia) pituitary.
1.1.1. 8.1.1.1. The anterior pituitary (adenohypophysis)The defect or removal of
anterior pituitary reflect in the operation of all organs to a smaller or larger
extent.
Their Hormones:
• Growth hormone (somatotropic hormone, STH), that is a long- polypeptide chain consisting of 191 amino
acids. It is responsible for the formation of genetically determined body size and body shape. It has an effect
on protein transfer too. It enhances the uptake of amino acids by tissues, decreases the blood sugar level. It
mobilizes the breakdown of fat tissue, thus fatty acid concentration increases in the plasma. Heat production
enhances also.
• Adrenocorticotropic hormone (Corticotropin, ACTH) consists of 39 amino acids
It stimulates the hormone production of adrenal gland, thus it influences the amount glucocorticoids (cortisol,
corticosterone)
• Thyroid-stimulating hormone (thyrotropin, TSH)
It belongs to glycoproteins. It has large molecular weight and it is sensitive thermally.
It regulates the hormone production of thyroid (T3 and T4). It affects the growth of the thyroid gland and
iodine metabolism. It also influences the metabolism of connective tissue (eye socket).
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II. HORMONS
• Gonadotropic hormones (hormones affect sex glands)They have central role in the regulation of
reproduction. They organize and control structural and functional changes in female and male body. .
Glycoproteins
• This group includes Follicle-stimulating hormone (FSH). It is responsible for the stimulation of follicular
development and the maintenance of spermohistogenezis.
• Luteinizing Hormone (LH) affects the hormone production of ovary and testicule
• Prolactin (mammotrope hormone PRL) has an effect on mammary glands, it can increase milk supply.
• Lipotrop hormone (LPH) consists of 90-amino acid peptide and affects lipid metabolism.
1.1.2. 8.1.1.2. Hormones of intermediate lobe (pars intermedia)
The color adjustment of some animals is a protective response in the fight for survival. It is important for them
whether their skin is light or dark, which is affected by hormone of the intermediate lobe.
The melanocyte stimulating hormone (MSH) is a polypeptide that affects the pigmentation of the skin.
1.1.3. 8.1.1.3. Posterior lobe (neurohypophysis) hormones
In the case of destruction of the posterior lobe urination increases. Posterior lobe is in close structural and
functional relationship with the hypothalamus. The lobe stores hormones of the hypothalamus. These hormones
are cyclic peptides constructed by 9 amino acids.
• Vasopressin (antidiuretin, ADH) maintains the water balance of the body, it increases water retention in the
collecting ducts of the kidney nephron.
• Under pathological conditions it also plays role in the regulation of blood circulation. Oxytocin increases the
contraction of uterine smooth muscle and the outflow of the milk from mammary glands. It helps the sperm to
enter the uterus. Its effect on uterine musculature is enhanced by estrogens while decreased by progesterone.
1.2. 8.1.2. Hormones of pineal gland
The function of pineal gland declines from sex maturity.
Melatonin is its hormone (acetylamino metoxitriptamin). It is formed by serotonin (Figure 20). The synthesis of
melatonin and the activity of enzymes required to it fluctuate with the daily changes of light and darkness.
Melatonin has an effect on skin color and its change.
8.2. ábra - Figure 20: The synthesis of melatonin
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II. HORMONS
1.3. 8.1.3. Hormones of the thyroid glands
Thyroid plays important role in regulation of the organism energy transfer and in iodine household. It is the only
endocrine gland that can store its secretion. Its hormones are T3 (triiodothyronine) and T4 (thyroxine).
• Thyroxine is formed in much larger quantities, but triiodothyronine has greater efficiency. When thyroid has
an increased hormone production (hyper tireoidism) due to the increased basal metabolism the organism loses
weight, the velocity of blood flow increases, the heart rate increases. Thyroid gland also increases.
Thyroid hormones affect the growth and the development of organs and organ systems. They enhance oxygen
consumption and their basal metabolism.
They increase the breakdown of stored carbohydrates and glucose consumption of the cells. Protein
degradation is also increased by them. They facilitate the conversion of amino acids to glucose (process of so
called gluconeogenesis), which is reflected in the raise of blood sugar level. They affect the salt and water
balance of the organism as well. In case of decreased thyroid activity water and salt content of the organism
are increased.
They increase fat mobilization in fat metabolism, and enhance the construction of fatty acids from
carbohydrates. They also stimulate the synthesis of cholesterol in liver.
• Calcitonin (tireokalcitonin, TCH) is a peptide hormone that consists of 32-amino acids. It regulates the level
of calcium. It reduces the blood calcium level, thus assimilation of calcium into bones increases. The
calcitonin is the antagonist of parathyroid hormone.
1.4. 8.1.4. The parathyroid gland
Parathyroid gland plays important role in the regulation of the body Ca and P household. It affects the
mobilization of calcium ions and the selection of phosphate ions. Hormone production of the gland is influenced
by blood calcium level.
• Its hormone is parathyroid hormone (PTH) that is a peptide consisting of 84-amino acids. The sequence of
the first 34 amino acids in the peptide is almost identical to the amino acid sequence of calcitonin.
As parathyroid hormone is the antagonist of calcitonin, it increases blood calcium level and effects ossification
of bones because it enhances the mobilization of Ca from bones together with phosphate ions.
The increased production of PTH can increase calcification of certain vital organs, as calcium phosphate is
deposited due to the increased calcium and phosphate level in blood (e.g. in kidney). Bones become brittle as a
result of calcium and phosphorus mobilization.
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1.5. 8.1 5. Hormones of the adrenal cortex
The adrenal plays important role in the metabolism and regulation of salt and water balance in the organism.
Steroidal hormones of the gland protect cells and tissues against external harmful stimuli. Corticoids are the
hormones of adrenal, which are classified based on their physiological effect.
• Hormones influencing
deoxycorticosterone)
salt
and
water
household
(mineralocorticoids)
(e.g.
aldosterone,
They exert their effect on tubular cells of kidney. They increase the retention of sodium ion, together with
chloride ion. They enhance the secretion of potassium and hydrogencarbonate ions. Thus they affect the ratio
of Na/K-in urine. The maintenance of the desired osmotic concentrations can be achieved by them.
• Hormones that affect carbohydrate metabolism (glucocorticoids) (e.g. cortisol, corticosterone)
They are the key hormones of gluconeogenesis, they enhance the transformation of amino acids, lactic acid,
propionic acid, glycerol and oxaloacetic to glucose and to glycogen. They stimulate protein breakdown and
fat mobilization. They increase blood sugar level.
• Hormones that affect sexual behavior (sexual steroids)
These are androgens and estrogens that are responsible for the development of secondary sex characteristics.
In addition to it adrenal hormones also affect lymphatic tissues and blood cellular elements and the
connective tissue. They have anti-inflammatory effects. Stress increases there quantities.
1.6. 8.1.6. Hormones of adrenal medulla (catecholamines)
In general they have a sympathetic effect on the organism. As a result of their effect blood pressure and the heart
rate increase and the coronary and muscle veins loosen.
The red blood cells stored in the smooth muscles of the honeycomb get into the bloodstream by means of the
contraction of that. Breathing also intensifies.
Blood sugar level rises, as sugar formation is also increased by glyconeogenesis.
These hormones also play a role in transmission of the stimulus of sympathetic nerve fibers. The adrenal
medullary function is necessary for the Cannon's "emergency response". They are also responsible for increased
wakefulness.
1.7. 8.1.7. Hormones of pancreas
Insulin and glucagon are produced by means of the endocrine islets of Langerhans in pacreas.
• Insulin is formed by β-cells, while the glucagon by α-cells.
Insulin consists of two open polypeptide chains (from 51 amino acids) that is bound together by disulfide
bridges.
Glucose permeability of cell membranes is increased by insulin, which is reflected in the rise of glucose
utilization in peripheral tissues. It enhances glycogen synthesis and storage in liver and muscle. It inhibits
gluconeogenesis that is the formation of glucose from amino acids and other intermediate products. It
stimulates fatty acid synthesis from glucose in the liver. It also enhances protein synthesis, thus it inhibits its
breakdown to amino acids. As a result of these processes it decreases blood glucose level.
• Glucagon is a polypeptide consisting of 29 amino acids, which has an opposite effect on the organism to that
of insulin. It causes the rise of blood sugar level as it enhances the conversion of intermediates (e.g. amino
acids) into glucose. It enhances protein breakdown, stimulates fat mobilization, glucose formation from fat.
1.8. 8.1.8. Hormones of the ovary
Estrogen and progesterone are the ovarian hormones with steroid skeleton.
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• Estrogens are the collective names of hormones that are responsible for the healthy development of female
characteristics. These include estradiol, estrone and estriol. Their physiological effect decreases in the listed
order. Cholesterol is the starting material for their synthesis. They circulate in blood either bound to protein
(2/3) or in free form. They are inactivated in the liver. They are responsible for follicular and ovocitogenesis.
They stimulate the cyclic development of uterus and vagina. They are responsible for the development of
female sexual characteristics.
• Progesterone (corpus luteum or pregnancy hormone)
Progesterone provides appropriate conditions for inseminated egg in uterus. During pregnancy it inhibits the
uterine contractions. It prevents new ovum maturation.
1.9. 8.1.9. The testicular hormones (androgens)
These steroidal hormones are formed by Leydig's interstitial cells of testicles. Among them dihydrotestosterone
has the greatest, testosterone has lower and rosterone has the smallest physiological effects. They are
responsible for the development of male secondary sex characteristics. They affect metabolism, particularly
proteins synthesis. Cholesterol is the starting material for the biosynthesis.
2. 8. 2. Tissue hormones
Tissue hormones are compounds with some characteristics similar to the hormones produced in endocrine
secretion glands, but they are produced by tissue cells with other functions. Among tissue hormones there are
some hormones that are secreted by epithelial cells lining the lumen of the stomach and small intestine. The
hormones are important in controlling digestive function, they affect on function of stomach and other parts of
the digestive tube.
These peptide hormones are called gastrointestinal (GI) hormones.
Secretin is a gastrointestinal hormone and is secreted in the S cells of the duodenum. Secretin is a peptide
hormone, which is composed of 27 amino-acids, stimulates the formation and secretion of bile and pancreas.
Secretin enhances the secretion of water and bicarbonate production.
Gastrin is produced by G cells of the duodenum and in the pyloric antrum of the stomach. Gastrin is composed
by 17 amino acids. It effects on gastric juice secretion, such hydrochloric acid and pepsin production as well.
Cholecystokinin is arised in the duodenum as well. It consists of 33 amino acids and it stimulates the pancreatic
and gastric juice production. Cholecystokinin promotes excretion of bile from the gallbladder and it has a
positive effect on food intake.
In addition more GI hormones are known (enteroglucagon, motilin, somatostatin, etc), and it is possible to
discover new ones. Their effect is largely similar to the ones above.
Angiotensin I is a tissue peptide hormone that affects the vascular system and smooth musculature. This
protease decapeptide is produced in the kidney and is activated by renin from angiotensinogen. The
Angiotensin II. is formed from the Angiotensin I. by the cleavage of two amino acids. They have regulating
effect on the adrenal function and they increase the blood pressure.
Bradikinin consists of 9 amino acids, and have a strong vasodilatory effect. It stimulates the blood flow in the
brain, kidneys, coronary arteries and in the skin. Bradikinin lowers blood pressure. It increases the permeability
of the capillary wall, so helps the migration of white blood cells. It also influences the smooth muscle
contraction.
There are also tissue hormones that they expound their effects on the place of their formation. These are biogen
amines. Biogen amines are produced from amino acids.
Histamine is derived by decarboxylation of histidine. It can be found almost everywhere in the organism. It is
stored bound to the protein in the cells. It is released by mechanical, thermal, toxic and allergic stimuli. It
promotes the secretion of hydrochloric acid. It is a vasodilator, released locally in sites of inflammation or
allergic reaction. It increases capillary permeability, decreases the blood pressure.
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II. HORMONS
Serotonin (5-hydroxytryptamine) is derived by hydroxylation and then by decarboxylation of tryptophane.
Serotonin is also produced in the enterochromaffin cells in the alimentary canal. It has vasoconstrictor effects,
thus increases the blood pressure, increases the heart rate, causes the tracheal, bronchial stenosis. It stimulates
intestinal peristalsis. In case of the injury of vein walls serotonin is released, that causes local vasoconstriction.
Serotonin is a tissue hormone, but is also a neurotransmitter. It has effect on the central nervous system.
Acetylcholine acts as a neurotransmitter. The synthesis and the breakdown of acetylcholine are rapid
enzymatical processes. These reactions are catalysed by acetylcholine esterase (mainly in the brain and nerve
tissue) and cholinesterase (serum). The cholinesterase also catalyzes the hydrolysis of propionyl choline and
butyryl choline.
Gamma-aminobutyric acid (GABA) is derived from the decarboxylation of the glutamic acid. It can be found
in the brain. GABA is the major inhibitory neurotransmitter. It inhibits the ganglionic transmission.
3. 8. 3. Plant growth hormones (Phytohormones)
In higher plants, like in animal body, the connection between cells is achieved by chemical messengers. The
plant hormones are chemical mediators. They interact with specific proteins, or receptors. They can expound
their effect away from the place of their formation. They occur in low concentration in plants. Mostly, they are
effective together with other phytohormones. Their effects highly depend on their concentrations. Typically,
plant growth hormones have low molecular weight, they are not peptide hormones.
They can not be grouped clearly by their physiological function so their classification is based on their chemical
structure.
Auxins are indole skeleton hormones. Most important members of the auxin family are indole acetic acid (IAA)
and phenylacetic acid. They have effects on plant growth and development. The precursors for the synthesis of
auxin are the aromatic amino acids (tryptophan, phenilalanine). The auxin synthesis takes place in young leaves
and developing crops. The degradation of auxin is catalyzed by auxin-oxidases. The process is enhanced by the
light and ethylene. The auxins promote the effects of gibberellins and cytokinins on cells, and cooperate with
ethylene in the regulation of plant growth.
8.3. ábra - indole-acetic acid
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II. HORMONS
Gibberellins are derived from the ent-gibberellane skeleton. In higher plants their synthesis is intensive in
young, growing parts of the plant (in the youngest shoots). Their synthesis corresponds to the synthesis of
terpenoids. The speed of the reaction sequence is affected by the length of the days. In long days gibberellins,
while in short days abscisic acid is formed. Gibberellins promote flowering also.
8.4. ábra - gibberellic acid
Cytokinins are adenine derivatives containing isopentenyl side chain. The cytokinins stimulate cell division in
the presence of auxin, enhance the nutrient transport. They can be found in every plant and they are needed in
all developmental stages of plants. They help in maintaining the young status of plant.
8.5. ábra - zeatin
Ethylene stimulates the ripening of fruits and at the same time inhibits the growth, inhibits the auxin transport.
Abscisic acid is a terpenoid, and is an obstructive substance, which hurries the ageing, the defoliation. It plays a
role in the fall of leaves in autumn and in ensuring the winter dormancy.
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9. fejezet - 9. BIOACTIVE
COMPOUNDS III. ENZYMES
(BIOCATALIZATORS)
The enzymes are proteins that direct and accelerate the biochemical processes in the living organisms by
lowering the activation energy.
A + B → C + D,
pl.: 2 H2O2→ 2H2O+ O2
The activation energy of this process without catalyst is 75 kJ/mol, and with inorganic catalizator (Pt) is 49
kJ/mol, while with catalase enzyme is 23 kJ/mol (Fig. 21).
9.1. ábra - Figure 21: Enzymes and activation energy
The data also show that the activation energy is decreased to a greater extent by the influence of an enzyme than
by the effect of inorganic catalysts. The enzymes thus able to accelerate reaction rate dramatically.
1. 9. 1. Structure of the enzymes
Enzymes are proteins considered simple or complex proteins. In many cases, enzymes consist of a protein part
called apoenzyme and a combination of one or more parts (which may be a non-protein substance) called active
group. The apoenzyme carries the active group, which catalyzes the reaction. The two parts together, the
enzyme complex, is referred to as holoenzyme. An enzyme can perform its biological function only as a
holoenzyme.
Many enzymes are only proteins. These enzymes are known as simple enzymes (e g hydrolases). The active site
of an enzyme is created by functional groups of amino acid side chains. These groups have to be in proper
spatial position relative to each other, and they are stabilized by bonds. If the spatial position changes because of
alteration of the external circumstances, the enzyme loses its biological activity.
The active group (is bound to the protein part of an enzyme) is called coenzyme or prosthetic group.
The coenzyme is a non-protein active group, which is loosely attached to the apoenzyme. This group can be
detached by dialysis from the protein part of the enzyme. During the reaction the coenzyme can be released
from an apoenzyme and can get onto the other (NAD, ATP, UTP, CTP, CoA-SH).
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ENZYMES
(BIOCATALIZATORS)
Prostetic group is tightly bound (with primary or secondary bonds) to the apoenzyme. This group can not be
detached by dialysis. The prosthetic group is bound to the same apoenzyme in the process of catalysis (FAD,
Hem, vitamin H, vitamin B6, metal ions).
Multi-enzymes consist of a number of enzyme proteins (with different catalytic activities), which are held
together by secondary bonds. Many reactions take place with their help, where the specific sequence of
reactions is determined. The substrate is transferred from one of the reaction site to another one.
2. 9. 2. The function mechanism of the enzymes
The reactions catalysed by the enzymes are two-step processes. First, the enzyme (E) reacts with the substrate
(S) and a transient enzyme-substrate complex is formed (ES) at the active site of the enzyme. In the second step
there is a transformation process in the enzyme-substrate complex and the enzyme-product complex (EP) is
formed. Finally, the product is detached from the enzyme (Fig. 22)
In the catalytic processes, the task of apoenzymes is to keep the active group. Apoenzymes should provide the
appropriate spatial arrangement for running the reactions.
The catalytic process takes place in the active centre of the active group. The active centre contains the binding
site, where the substrate is bound and the catalytic site, where such groups may be found which react with the
substrate. It may even occur that the binding site is the same as the catalytic site.
The name of an enzyme can be derived from its substrate with the word ending in –ase (e.g. lactose-lactase), or
from the chemical reaction it catalyzes, also with the word ending in –ase (e.g. dehydrogenase). Enzymes often
have trivial names (for example: pepsin, papain), too.
The accurate, clear name of an enzyme has to refer to the substrate and the type of catalytic process as well.
For example: the substrate of glucose-1,6-transphosphorylase enzyme is the glucose-1 phosphate. This enzyme
cleaves the phosphate group from the first carbon atom of glucose-1 phosphate and transports it to the 6th
carbon atom.
3. 9. 3. The specificity of the enzymes
Most enzymes can catalyze only one reaction of one substrate. This feature is called specificity. There are two
specificities known of enzymes, the substrate specificity and the reaction specificity.
The substrate specificity means that at the binding site of the enzyme only its substrate can bind. For example:
the lactic acid can only bind to the binding site of the lactic acid dehydrogenase enzyme. The apoenzyme is
responsible for this substrate specificity.
The reaction specificity means that the enzyme catalyses one chemical reaction (Fig. 22).
9.2. ábra - Figure 22: The specificity of the enzymes
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ENZYMES
(BIOCATALIZATORS)
4. 9. 4. Classification of enzymes
Enzymes are principally classified according to the reaction they catalyse. Enzymes are generally classified into
six main family classes:
1. Oxidoreductases
2. Transferases
3. Hydrolases
4. Liases (synthases)
5. Isomerases
6. Ligases (synthetases)
More sub-groups can often be distinguished within the main groups.
4.1. 9.4.1. Oxidoreductases
Enzymes in this class catalyse oxidoreduction reactions, catalyse such a biochemical processes, where oxygen
uptake, proton and electron transfer happens. The systematic name of this class is based on donor: acceptor
oxidoreductase.
Their coenzymes are generally: NAD+, NADP+, FAD, Liponic acid, Coenzim Q.
The oxidoreductases can be divided into two sub-groups: dehydrogenases or oxidases. One group includes the
dehydrogenases, which oxidize the substrate by transferring hydrogen from the substrate (electrons and
protons) to an acceptor. The substrate is oxidized and the acceptor is reduced.
AH2 + B →A + BH2
Oxidases are enzymes involved when O2 acts as an acceptor of hydrogen or electrons. The donor is the
substrate. In this process, the substrate is oxidized, while reactive hydrogen peroxide is formed from the oxygen.
AH2 + O2 → A + H2O2
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ENZYMES
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Hydrogen peroxide is an easily degradable compound; it is regarded as a cytotoxic agent. During its
decomposition free radicals are formed, which attack the unsaturated bonds of lipid components of the
membrane, they have destroying effects.
In the defence mechanisms of living organisms, catalase and peroxidase enzymes catalyse the decomposition
of hydrogen peroxide.
Catalase enzyme catalyses the breakdown of hydrogen peroxide into water and oxygen.
2H2O2 → 2H2O + O2
Peroxidase also catalyzes the hydrogen peroxide decomposition. Peroxidase cleaves electrons and protons from
another compound, while this compound is oxidized. From the hydrogen peroxide two water molecules are
formed.
AH2 + H2O2 → A + 2H2O
4.2. 9.4.2. Transferases
Transferases catalyse the transfer of a functional group from the substrate (called the donor) to another
compound (called the acceptor).
AR + B → A + BR
R: chemical group
In the coenzyme part of transferases, water-soluble vitamins can often be found as we have discussed in a
previous section (Table 5).
9.3. ábra - Table 5: Coenzymes of transferases and transmitted chemical groups
4.3. 9.4.3. Hydrolases
Hydrolases are simple proteins that catalyze biochemical processes, where – with the incorporation of water –
decomposition occurs. Although these processes are reversible, they are strongly shifted towards degradation.
For example, the macromolecules in the organism are decomposed to their monomer units by hydrolysis.
AB + H2O → AH +BOH
Within the hydrolases more sub-groups can be distinguished.
Esterases (cleave ester ulinkages) include lipases and ribonucleases and phosphatases.
Glicosidases (catalyse the hydrolysis of the glycosidic ulinkage) include amylase, lactase, sacharase,
nucleosidase.
Proteases split peptide bonds of the polypeptide chains (carboxy, amino and dipeptidases). Proteases are pepsin,
trypsin, kimotrypsin and papain.
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ENZYMES
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4.4. 9.4.4. Lyases (Synthases)
Lyases catalyse chemical reactions without taking up or losing water, where cleaving of carbon-carbon, carbonoxygen and carbon-nitrogen bonds will appear, often forming a new double bound, a new ring structure. In the
coenzyme of lyases, there are (in general) vitamin B6- and B1. For example: the pyruvic acid decarboxylase
enzyme (as a lyase), is a central enzyme of the glucose anabolism.
4.5. 9.4.5. Isomerases
The isomerases catalyze reactions in which the configuration of one chiral carbon atom of the substrate is
changed and thus a new product is formed (epimerisation). In cis-trans isomerization processes enzymes modify
the spacing of molecules along the double bonds of a substrate. Enzymes catalyzing aldose-ketose
interconversion are also in this group. These enzymes modify the location of oxogroup and thus another
compound is created. For example, hexose-phosphate isomerase catalyzes the conversion of glucose-6phosphate to fructose-6-phosphate. Coenzymes of isomerases are UDP, vitamin B12.
4.6. 9.4.6. Ligases (synthetases)
The ligases catalyse the joining of two large molecules by forming a new chemical bond. The development of
the new compound needs energy. The energy required for the process is supplied by high-energy-ulinked
nucleotides (ATP, UTP, GTP).
The acetyl-CoA synthetase is also a member of the ligase family and catalyses the following process:
CH3-COOH + CoA-SH + ATP = CH3 - CO~SCoA + AMP + PPi
The ligases take part in the synthesis of nucleic acids and proteins.
5. 9. 5. Factors influencing the function of enzymes
The factors influencing the function and activity of enzymes also modify the velocity of reactions.
The enzyme activity is influenced by:
• enzyme concentration,
• substrate concentration,
• pH,
• temperature,
• activators,
• inhibitors.
Initially with the increasing of enzyme concentration, the velocity of reaction enhances, until all enzymes can
be in enzyme-substrate complex state.
With decreasing substrate concentration and with increasing product concentration the velocity of the reaction
begins to decrease. In this case, the substrate is present only in small concentration, so can not tie up the total
amount of enzyme. The decrease of reaction might be often caused by enriched products (final inhibition).
With increasing substrate concentration the velocity of reaction increases up to the saturation value, but at the
depletion of free enzyme molecules with increasing substrate concentration the velocity of the reaction will not
increase.
The excess of substrate often causes inhibition, because when too many substrate molecules are present, it does
not only attach to the binding site of the enzyme, but also modifies its configuration leading to inactivation.
With increasing temperature the velocity of reaction enhances up to a maximum then begins to decrease.
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ENZYMES
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The increase of temperature, as any chemical reaction including the enzyme-catalysed processes, enhances the
rate of reaction, because the energy of the reactant molecules increases. The temperature of maximum velocity
is called the optimum temperature of the enzyme. When the temperature is higher, the configuration of enzyme
changes and it will not catalyse the reaction any longer.
Above a given temperature, the conformation of the protein part is changed to such an extent, that the enzyme
loses its biological activity.
Enzymes have optimum pH. This is the pH range where enzymes can perform their biological tasks. Any
change in pH above or below the optimum will cause a change of conformation of protein part of the enzyme,
which influences its function negatively.
The redoxpotential of the medium also influences the functions of enzymes. Oxidoreductases are particularly
sensitive to this parameter.
The activators increase the catalysing ability of enzymes. The activators may be cations (K+, Na+, NH4+, Mg++,
Mn++, Zn++). Single charged cations expound their positive effect with pulling up the hydrate layer of the
enzyme, while double charged cations expound their positive effect with helping to bind either the substrate or
the coenzyme.
The inhibitors decrease the enzyme activity. The inhibition may be reversible or irreversible.
Reversible inhibitors bind to the enzymes with non-covalent interactions, so after their removal the enzyme may
become active. The reversible inhibitions might be competitive, non-competitive, uncompetitive and allosteric
inhibitions.
• Competitive (competing) inhibition means: The inhibitor is similar to the substrate or coenzymes chemically,
so either the substrate or the inhibitor competes for access to the enzyme's active site, or the inhibitor binds to
the apoenzyme instead of the coenzyme. In both cases the catalytic process is blocked.
• Non-competitive inhibition means: The inhibitor does not affect the binding of substrate, but prevents the
conversion of bound substrate. This happens in such a way that the inhibitor is attached to the catalytic site of
the enzyme's active centre.
• Uncompetitive inhibition means: The inhibitor cannot bound to a free enzyme, but is able to bind to the
enzyme-substrate complex only.
• Allosteric inhibition means: The inhibitor does not bind to an active site of the enzyme, but binds to other
parts of the protein. The inhibitor binds to an allosteric site and so a change in the shape of the active site will
occur. The activity of the enzyme will decrease. The substrate can not bind to the active site due to the fact
that the active site has changed shape and the substrate no longer fits. The accumulated end-product, in a
similar manner inhibits the activity of the enzyme.
At the reversible inhibition the denaturation of protein part of an enzyme is irreversible. The enzyme loses its
activity. Then the inhibitor is called destructor.
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10. fejezet - 10. THE METABOLIC
PROCESSES I. CARBOHYDRATE
METABOLISM
Living organisms are in dynamic relationship with their environment to maintain their life processes. They take
up nutrients and energy from their surroundings – from the lifeless nature, they transform it in their bodies and
empty the decomposition products that are useless to them, into their environment.
Material conversion processes taking place in living organisms are called intermediate metabolism. Intermediate
metabolic processes consist of constructing (anabolism) and decomposing (catabolism) processes. During
metabolism there are substance, energy and information flows. Living organisms acquire material and energy
for their constructing processes from different sources. It can be an aspect for the classification of living
organisms. There is close relationship between organisms and their metabolism. The autotrophic organisms
produce their own organic material from simple materials (precursors: CO2, H2O) taken up from their
environment by means of sunlight. The by-product of this process, oxygen is discharged into the environment.
The heterotrophic organisms construct their own materials from (organic) products of autotrophic organisms and
oxygen released by them into the environment.
The heterotrophic organisms first break down materials taken up from their environment, then during their
constructing processes they produce their own new materials from one part of the products (precursors) and
energy. They empty unnecessary materials formed during the metabolism. These materials can be precursors for
autotrophic organisms.Figure 23 show the turnover of the four main constituent elements (C, H, O and N) in the
biosphere.
10.1. ábra - Figure 23: The carbon, hydrogen and oxygen biological cycle
The carbon, hydrogen and oxygen biological cycle are self-sufficient considering the whole biosphere. Nitrogen
occurs in large quantities in the atmosphere, but the majority of the organisms can not utilize it in elemental
form. Only special bacterium species can build (fix) nitrogen from air in their bodies. From their dead organic
material after transformation processes in the soil (aminization, ammonification, nitrification) those forms of
nitrogen arise, which can be utilized by plants. Nitrogen entering the food chain in this way becomes part of the
biological cycle. Nitrogen can be found in precursors of constructing processes of higher ranking animals.
Animals excrete nitrogen during their metabolic processes. From the decomposition of their dead organic matter
such inorganic nitrogen forms are formed in the soil, which is used by plants in their constructing processes.
Those processes also take place in soil (denitrification), where elemental nitrogen is formed (nitrogen loss). This
nitrogen is released in the atmosphere and it can return to the biological cycle only through nitrogen fixing
bacteria (Fig. 24).
10.2. ábra - Figure 24: The nitrogen biological cycle
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1. 10. 1. Carbohydrate biosynthesis in photosynthetic
organisms
Carbohydrate synthesis of the photosynthetic organisms, is known as photosynthesis. Here carbohydrates are
formed from inorganic precursors by means of the energy of sun. Photosynthesis takes place in the chloroplasts
of photoautotrophic organisms. The gross equation of photosynthesis is the following:
6 CO2+ 6 H2O → (CH2O)6 (monosaccharide) + 6 O2
Photosynthesis can be divided into two phases (light dependent and independent phases). These are
characterized by different types of processes, and they take place in different places in the chloroplasts.
The light dependent phase takes place in the inner membrane system of the chloroplast. (thylakoid membrane).
In the light reactions photons are absorbed and electrons are transported. As a result of the processes ATP,
NADPH + H+ and O2 as by-products are formed at this stage. Light independent reactions take place in the
intermembrane space of chloroplasts (stroma), where using the by-products of light dependent phase CO2
fixation and carbohydrate synthesis occur.
1.1. 10.1.1. The light dependent phase (Hill reaction)
Light energy is absorbed by photoreceptors. These are chromoproteins consisting of protein components and
pigments.
Pigments such as chlorophyll -A and –B with porfirin frame, and carotenoids with isoprene frame are
conjugated double bond systems. Due to there structures the conjugated double bond systems can be easily
excited. During excitation their electrons enter into higher energy levels but they are still attracted by the atomic
core. The excited state exists for a short period, after which the electrons jump back to the ground level. The
difference of the energy level between the initial and the excited state is emitted or transferred to other systems.
Two kinds of pigment systems are involved in binding and concentrating light energy which are associated with
electron transfer and are called photochemical system I and II.
Pigment systems contain light-gathering pigments and reaction centers.
The center of photochemical system I is called P 700 and that of system II is called P680. The light absorption
optimum of these centers are in 700 and 680 nm wavelength range. The difference is resulted from the quality of
the pigments. While in photochemical system I carotene and chlorophyll-A dominate, in the system II
xanthophyll and chlorophyll-B are characteristics.
The two photochemical systems are connected by electron transport chain. In the photochemical systems,
electrons are transported from the more negative redox potential component towards the more positive one,
except for two points in photosynthetic electron transport.
(I.: +0,4 → - 0,6V; II.: + 0,8 →0,0V).
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In the process an electron is torn from photochemical system I by the absorbed and concentrated light energy
and it attaches to a primary electron acceptor (FRA) and from there to ferredoxin. From ferredoxin the electron
binds onto the oxidized NADP+ and reduces it.
The lack of electrons in photochemical system I is replaced by the electrons that leave photochemical system II
and enter the electron transport chain, thus it will be able to absorb new light quantums. Then photochemical
system II becomes electron-deficient, which is replaced by electrons derived from water hydrolysis. This form
of electron transport is called non-cyclic electron transport.
Thus at non-cyclic electron transport both photochemical systems work. In photochemical system I NADPH+ +
H+ is generated, in photochemical system II. water is split and oxygen gas is released. In the electron transport
chain, during photophosphorylation ATP is formed. At cyclic electron transport one photochemical system is
only in action. The electron from ferredoxin attaches not to oxidized NADP+, but to a member of the electron
transport chain that is cytochrome b6. From here it binds to the cytochrome f, and then through plastocianin it
returns to photochemical system I. The end product of cyclic electron transport is ATP. The cyclic path occurs
when there is lack of oxidized NADP+ and there is excess ADP (Fig. 25).
10.3. ábra - Figure 25: Hill reaction
Mitchell theory of photophosphorylation
One part of the energy of electrons involved in transport process (between the two photochemical systems) is
utilized for proton transfer. Plastoquinone has a central role in the proton transfer.
As a result of proton transfer their concentration increases on the inner surface of thylakoid membranes. Charge
and voltage difference result proton transport from the inner space to outside through proton channels of the
membrane. Proton channels are in connection with the ATPase enzyme. During proton transfer their energy is
applied to form ATP by means of the ATPase enzyme. Light energy is converted to chemical energy.
1.2. 10.1.2 The light independent phase of photosynthesis (Calvin
cycle)
In the process carbon dioxide is transformed to carbohydrate from the end products of the light dependent phase.
The cycle consists of three phases: these are carboxylation, reduction and regeneration phases. At carboxylation
phase carbon dioxide is bound by ribulose 1,5-bisphosphate, from which two mol glyceric acid-3-phosphate is
formed by the decomposition of the intermediate product. At the reductive stage first glyceric acid-1,3diphosphate is formed using ATP, that was generated in the light dependent phase, then glyceraldehyde-3phosphate is formed by means of NADPH + H+. This product is in equilibrium with dihydroxyacetone
phosphate. Trioses combine and form fructose-6-phosphate from which glucose-6- phosphate is built by
isomerization.
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At the regeneration phase ribulose 1,5-bisphosphate is formed, which is the starting compound of the
cycle. During transaldolase, transketolase reactions, epimerization and isomerization processes ribulose 1,5bisphosphate is reproduced by one part of the fructose-6-phosphate, glyceraldehyde-3-phosphate and
dihydroxyacetone phosphate (Fig. 26).
The equation of the cycle
(6 ribulose 1, 5- diphosphate) + 6CO2+ 18 ATP + 12 NADPH+H+ →
(6 ribulose 1, 5- diphosphate) + 1 hexose + 18 ADP + 18Pi + 12 NADP+
10.4. ábra - Figure 26: Calvin cycle
1.3. 10.1.3. The sucrose synthesis
Glucose is the end product of photosynthetic carbohydrate synthesis, but it does not accumulate in plants, as it is
transformed to sucrose or different polysaccharides. UTP provides energy for sucrose synthesis and ATP for
starch synthesis. The starting materials are sugar phosphates. During sucrose synthesis UTP ulinks to glucose-1phosphate with a split of inorganic pyrophosphate. The newly created, intermediate product with high energy is
uridine diphosphate-glucose that is capable of reacting with the fructose-6-phosphate. The process is catalyzed
by sucrose phosphate synthase. After cleaving inorganic phosphate from sucrose phosphate by phosphatase
enzyme sucrose is formed, while another product of the process is regenerated by UDP and ATP.
The reactions are the following:
glucose-1- phosphate + UTP → uridine-biphosphate-glucose +PPi;
uridine-biphosphate-glucose + fructose-6-phosphate ↔ sucrose-phosphate + UDP;
sucrose-phosphate → sucrose + Pi;
UDP + ATP→ UTP + ADP.
1.4. 10.1.4. The starch synthesis
During starch synthesis ATP attaches to glucose-1-phosphate that is of the starting compound. Adenosine
diphosphate-glucose that is formed by the cleavage of an inorganic pyrophosphate group ulinks to the acceptor
by transglycolase enzyme.
The process of the reaction is the following: glucose-1-phosphate → ATP + adenosine diphosphate-glucose +
PPi; adenosine diphosphate-glucose + acceptor → α-1 ,4-glycosyl acceptor + ADP.
2. 10. 2. Catabolic processes of carbohydrates
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Heterotrophic organisms use energy released during their catabolic processes for the constructing processes of
their cells. Intermediate products are starting materials (precursors) of anabolic processes.
Among catabolic processes the greatest quantity of energy releases when nutrient molecules burn and form
inorganic substances (CO2, H2O, NH3). The reaction sequence is called respiration (cellular
respiration). Organisms use atmospheric oxygen from their environment and release carbon dioxide.
Catabolic processes begin with the hydrolysis of the macromolecules. During hydrolysis, they break into their
monomeric units. The monomers decompose in individual reaction pathways forming the same intermediates
(pyruvic acid, acetyl-CoA). Intermediates transform through common reactions to simple inorganic substances,
and most of them are excreted by the body. The hydrolysis of macromolecules is catalyzed by hydrolases. The
decomposition of fats and oils are catalyzed by lipases, that of proteins by proteases and that of nucleic acids by
the nucleases. The hydrolysis of starch is enhanced by amylases, etc. The breakdown of starch and glycogen can
also take place through hydrolysis with phosphoric acid. The process is catalyzed by phosphorylase enzyme.
The end product of the reaction is glucose-1-phosphate (Fig. 27).
10.5. ábra - Figure 27: Catabolic processes of carbohydrates
In the following, the stages and steps of carbohydrate decomposition are described in detail, as the breakdown of
other organic molecules also ends in this reaction sequence although it starts in different routes.
2.1. 10.2.1. Cellular respiration
2.1.1. 10.2.1.1. Glycolysis
The first stage of glucose breakdown is glycolysis. The process takes place in the cytoplasm. The process does
not require oxygen. At the end of the reaction sequence 2 M pyruvic acid is formed by 1M glucose. Pyruvic acid
is decarboxylated at the second stage of the process. This process takes place already in the mitochondria. At the
end of the reaction acetyl-CoA is formed by pyruvic acid.
The acetyl group with two carbon atoms enters into the third stage of the process from acetyl-CoA, which is
called citric acid cycle. This takes place in the mitochondrial inner membrane within the matrix. In the process
carbon dioxide molecules and reduced coenzymes (NADH + H +, NADPH + H+, FADH2) are formed from the
acetyl group.
The last stage of complete breakdown is terminal oxidation that takes place in the mitochondrial inner
membrane.
In this process, a large amount of water and energy are generated from reduced coenzymes and inhaled oxygen
that is formed in the former stages of catabolic processes.
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Figure 28 shows the reaction sequence of glycolysis that is the first stage of cellular respiration, Starting from
glucose, two moles of ATP are used during the formation of fructose-1,6-diphosphate. The fructose-1,6diphosphate breaks into sugar molecule with two or three carbon atoms in aldolase reaction. The
glyceraldehyde-3-phosphate and dihydroxyacetone phosphate can be converted into each other. The subsequent
step of the process continues with glyceraldehyde-3-phosphate. The decrease of its concentration enhances the
transformation of dihydroxyacetone phosphate.
Glyceraldehyde-3-phosphate transforms to glyceric-1,3-diphosphate by incorporation of inorganic phosphate
and oxidation with the formation of reduced coenzyme (NADH + H +). This molecule stores its energy in ATP
and transforms to glycerin-3-phosphate. The next step in the reaction sequence is an isomerization process in
which glyceric-2-phosphate is produced. From this pyruvic acid is formed besides ATP molecules, which is the
end product of anaerobic glycolysis. In the process 2 moles of intermediates are formed from one mole of
glucose from glyceraldehyde-3-phosphate.
10.6. ábra - Figure 28: Glycolysis
2.1.2. 10.2.1.2. Pyruvate decarboxylation
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Pyruvate decarboxylation can be considered as the second stage of glucose combustion, which is closely related
to the third section, the citric acid cycle.
In this process pyruvic acid formed in the cytoplasm translocates to the mitochondria. The pyruvate is
transformed into acetyl-CoA by a process called oxidative decarboxylation. This process is catalysed by a multienzyme complex that is called pyruvate dehydrogenase complex (Fig. 29).
10.7. ábra - Figure 29: Pyruvate decarboxylation
The reaction is irreversible. Energy released during the oxidation reaction can be found in the ulinkage of
acetyl-CoA and in the reduced cofactor (NADH + H+). Enzyme complex consists of three different enzymes
and five cofactors.
These are:
• pyruvic acid decarboxylase enzyme with TPP (vitamin B1) coenzyme
• dihidrolipoil transacetylase enzyme with lipoic acid coenzyme
• the dihidrolipoil dehydrogenase enzyme with the FAD coemzyme
• CoA-SH,
• and NAD+.
Most of the produced acetyl-CoA molecules are utilized in citric acid cycle and the rest in fatty acid synthesis as
a precursor.
2.1.3. 10.2.1.3. Citric acid cycle
Citric acid cycle is the third stage of glucose breakdown. The process takes place in the mitochondria. Enzymes
catalyzing this reaction sequence can only be found in the mitochondria. In this cycle, the intermediate
metabolic products are burned completely.
The acetyl group of acetyl-CoA joins the oxaloacetic citric acid and gives citric acid with water and CoA-SH
loss. The process is catalyzed by the enzyme citrate synthase. Citric acid is transformed to isocitric acid by the
enzyme aconitase. The intermediate is cis aconitic acid. In the next step oxalic succinic acid is formed catalyzed
by dehydrogenase, which transforms to α keto glutaric acid through decarboxylation. CO 2 and NADH + H+
leave the cycle.
The α-ketoglutaric acid transforms to succinyl-CoA in the next step. Then, through decarboxylation CO2 leaves
the process and NADH + H+ are generated. Then Succinyl-CoA transforms to succinic acid, while the energy is
stored in GTP. From succinic acid fumaric acid and FADH2 are formed by means of succinate dehydrogenase
enzyme.
Fumaric acid is transformed to L-fumaric acid by fumarase enzyme, then in the final step of the cycle
oxaloacetic acid and NADH + H+ are formed by means of malate dehydrogenase (Fig. 30).
10.8. ábra - Figure 30: Citric acid cycle and terminal oxidation
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2.1.4. 10.2.1.4. The terminal oxidation and oxidative phosphorylation
This process is the fourth and final phase of glucose combustion. It takes place in the inner membrane system of
mitochondria.
In the previous catabolism sections protons and electrons are formed through the oxidation of reduced
coenzymes. They pass along redox enzyme complex systems in the mitochondrial inner membrane.
In the last step they react with the inhaled oxygen and form water. Proton transport results increased proton
concentration at the external part of the membrane (Fig. 30).
The equalization of the charge difference occurs across proton channels (from outside to inside), the released
energy transforms to ATP (oxidative phosphorylation).
Figure 30 also shows that while electrons and protons from NADP-and NAD- get to iron-sulfur proteins, from
the reduced FAD electrons get directly to ubiquinone. During oxidative phosphorylation process from 1 mole of
NAD and NADP 3 moles of ATP and from 1 mole reduced FAD 2 ATP molecules are generated.
The energy balance of the total breakdown of one mole glucose is the following.
Glycolysis: glucose → 2 pyruvate + 2 ATP + 2 NADH+H+ = 2 pyruvate + 8 ATP
Decarboxilation of pyruvate:
2 pyruvate → 2 acetyl-CoA +2 CO2 + 2 NADH+H+ = 2 acetyl-CoA + 6 ATP
Citric acid cycle:
2 acetyl-CoA → 4 CO2 + 4 NADH+H+ + 2 NADPH+H+ + 2 FADH2 + 2 GTP = 24 ATP
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Total: 38 ATP
During the breakdown of one mole glucose (CO2 and H2O) 38 ATP molecule
2.2. 10.2.2. The pentose phosphate pathway
An alternative pathway for glucose oxidation is the pentose phosphate pathway. The cycle enzymes can be
found in a variety of animals, plants and microbes so these reactions are common in the living world.
The penthose phosphate pathway operates to varying extents in different cells. Due to the high physical load,
with the aging of cells the breakdown of glucose may be shifted to this pathway.
The significance of the penthose phosphate pathway is that in addition to producing energy, also produces
NADPH + H+, which is formed only in a few reactions, but is required to other processes such as the synthesis
of fatty acid.
The major part of intermediate products of this process can be starting compounds of other processes.
The ribulose-5-phosphate is an important compound of the dark reaction of photosynthesis. The ribose-5phosphate is a precursor for the synthesis of nucleic acids. Intermediates of the transaldolase - and transketolase
reactions are involved in the synthesis of aromatic compounds through the shikimic acid pathway.
The pathway operates in the cytoplasm. The initial reaction is catalyzed by glucose-6-phosphate dehydrogenase.
The specific electron acceptor of this enzyme is coenzyme NADP. In one turn of the pentose-phosphate cycle
one mol CO2 is removed, thus six repeated turns result the complete oxidation of glucose-6-phosphate to CO2
and water. In six turns of the cycle besides the 6 moles CO 2, 12 moles NADPH+H+ are also generated, which
can be further oxidized in terminal oxidation and this generates 36 mol of ATP.
1 mol glucose (C6H12O6) → 6CO2 + 12 NADPH+H+→ 36 ATP
10.9. ábra - Figure 31: The pentose phosphate pathway
2.3. 10.2.3. Fermentation processes
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The role of fermentation is mainly important in the metabolism of microorganisms. Fermentation may also
occur in higher organisms. For example, the breakdown process of glycogen may occur in the muscle due to an
intense exercise where oxygen supply becomes limited. By the fermentation processes, the living cell obtains
energy through the breakdown of carbohydrates to simple organic acids without requiring oxygen and by using
hydrogen transfer coenzymes.
The end products are energy-rich compounds therefore little energy is released in this process. The end products
might be the precursors of biosynthesis of other organic matter.
This process is called anaerobic dissimilation, because its products do not enter the citric acid cycle, but in
additional anaerobic reactions, they are transformed into other compounds.
Figure 32 shows different fermentation pathways. The figure also shows that the pyruvic acid produced during
anaerobic glycolysis can be converted further in a variety of ways, so it has central role in the fermentation
processes as the starting compound.
10.10. ábra - Figure 32: Fermentation processes
The pyruvic acid can be decarboxylated to acetyl-CoA, which can be further converted to acetic acid or butyric
acid. The pyruvic acid can be converted to acetaldehyde while CO2 is released. The acetaldehyde can be
reduced to ethanol.
The pyruvic acid can also be transformed to lactic acid and propionic acid in the fermentation processes.
Bacteria are able to split or hydrolyse cellulose, hemicellulose and pectin too, therefore not only the starch can
be the starting compound of the fermentation processes in the rumen of ruminants or in silos.
Many of the fermentation pathways are named for their end products. For example there can be acetic acid,
butyric acid, lactic acid, propionic acid and mixed acid fermentation processes.
In alcoholic fermentation process, ethanol is formed from the pyruvic acid by reduction and after leaving
carbon dioxide.
In lactic acid fermentation, process lactic acid is formed from the pyruvic acid (Fig. 33).
10.11. ábra - Figure 33: Alcoholic- and lactic acid fermentation
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There are two pathways of propionic fermentation.
1. pathway: lactic acid → lactyl-CoA → acryloyl-CoA → propionyl-CoA → propionic acid.
2. pathway: oxalic acetic acid → malic acid → fumaric acid → succinic acid → succinyl-CoA → propionylCoA → propionic acid
The precursor of acetic acid fermentation is the acetyl-CoA. In this process, acetic acid is produced from
acetyl-CoA.
The butyric fermentation takes place in the following pathway: acetyl-CoA → acetoacetyl-CoA → ß-hydroxybutyryl-CoA → crotonyl-CoA → butyric acid.
In terms of ruminants the most important volatile acids are acetic acid, propionic acid and butyric acid. These
organic acids are formed in the rumen or in the silo by fermentation processes. The volatile acids contain the
most part of the energy of fodder, therefore plays a decisive contribution to the ruminant material and energy
consumption.
During the fermentation process, the terminal oxidation is stopped and thus the reduced coenzymes can only
transfer their electrons and protons to intermediates. In the process, the aforementioned volatile acids or in a bad
case hydrogen and methane are formed. The energy content of these two gases is lost for the animal.
2.3.1. 10.2.3.1. The fermentation processes in the rumen of ruminants
Rumen microbes ferment dietary carbohydrates to organic acids to obtain energy for their anabolic processes. In
rumen generally mixed acid fermentation takes place. Ruminants are able to cover their high percentage (6070%) of energy demands from acetic, propionic and butyric acids generated in the rumen during fermentation.
Besides volatile fatty acids methane and hydrogen gases are also generated during fermentation, which gases
leave with rumen and intestinal gases. The quality of fodder influences the rate of end products of fermentation.
During feeding acetic acid fermentation is promoted.
The ratio of volatile fatty acids, acetic acid:propionic acid:butyric acid are 6.5:2:1. The pH is close to neutral
(pH 6-7).
During feeding the volatile fatty acid content increases, acetic acid content decreases, while propionic acid
content increases. Due to the decrease of pH in the rumen the conditions are favourable for the synthesis of
butyric acid. Butyric acid has unfavourable influence onto the inner part of the first stomach (epithelium). The
absorption of fatty acids from the first stomach decreases, so the pH also decreases. If the pH reaches the 5 to
5.5 values, the lactic acid bacteria multiply and lactic acid fermentation is promoted. When the rumen pH
becomes more acidic, acidosis may occur. The production of volatile fatty acids is reduced so that the energy
supply of animals becomes insufficient, which can ultimately lead to the death of animals.
One part of volatile fatty acids is neutralized by the saliva (HCO 3-), while the higher part is absorbed across the
ruminal epithelium.
The volatile fatty acids passing through the epithelium can get directly or can get after transformation to the
bloodstream. Velocity of absorption of volatile fatty acids is determined by their polarity and pH of the medium.
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If the pH is low, the dissociation of volatile fatty acids decreases, thus the molecule become apolaric, which
helps their absorption.
15-20% of the acetic acid is able to be absorbed at pH=6; 30–35 % at pH=5; while 65–70 % at pH=4. Among
volatile fatty acids butyric acid has the highest absorptive capacity at the same pH followed by the values of
propionic and acetic acids. The fate of absorbed (and got to the blood) volatile fatty acids is different in the
organism.
One of the products of mixed acid fermentation in the rumen is acetic acid, so it is understandable that the acetic
acid concentration of ruminants’ blood is much higher than that of other animals. The absorbed acetic acid is
mainly utilized by muscle and fat cells.
Acetic acid found in the muscle cells is converted to acetyl-CoA in mitochondria and is oxidized to CO2 and
water in the citric acid cycle and in the terminal oxidation while energy (ATP) is generated.
Acetic acid found in the fat cells is converted to acetyl-CoA in the cytoplasm which compound is a precursor for
fatty acid synthesis.
Nearly 65% of propionic acid formed during fermentation can get directly into bloodstream. Propionic acid is
removed from blood by the liver, where it is formed to propionyl-CoA and after carboxylation transformed to
succinyl-CoA. This compound entering the citric acid cycle either is devoted to energy generation, or is formed
to glucose from the intermediate of the cycle (from oxaloacetic acid) by the process of gluconeogenesis. Nearly
half of the milk sugar is generated from propionic acid in this way.
The 35% of propionic acid formed in rumen is converted to lactic acid in epithelial cells and they can get into
the bloodstream in this form.
Nearly half of the butyric acid formed during butyric acid fermentation, gets directly to the bloodstream and is
transformed to acetyl-CoA in the liver. Acetyl-CoA is oxidized in the citric acid cycle and in the oxidative
phosphorylation and supplies energy to animals.
The other part of the butyric acid is transformed to β-hydroxy-butyric acid in epithelial cells, and is transformed
further to aceto-acetate by oxydation. In this process two moles of acetyl-CoA are generated, which produces
energy in the mitochondria of cells.
Energetically, the best form of fermentation process in the rumen is the propionic acid fermentation (Fig. 34).
10.12. ábra - Figure 34: Propionic acid and butyric acid fermentation
2.3.2. 10.2.3.2. The fermentation processes in silo
The fermentation processes taking place in silo show significant differences (besides many similarities)
compared to the processes taking place in rumen. These are presented in Table 6.
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During ensiling (in preservation process) the plant is still alive, respirating, so beside CO2 and H 2O heat is also
released. The CO2 has a higher density than air that is why it falls to the bottom of the silo and displaces the air.
In the oxygen-free condition the fermentation starts.
In preservation process the lactic acid fermentation is the best, the process is influenced by many factors. The
quality of product forming at the end of reaction chain is influenced by the quality and carbohydrate content of
fodder and also influenced by temperature, the moisture content and the anaerobic-aerobic conditions.
10.13. ábra - Table 6: The fermentation processes in silo
The lactic acid bacteria are able to ferment mono and disaccharides. The concentration of sugars must be so high
to let the bacteria to grow, meanwhile the pH has to fall below 4.5. At the beginning of the process
heterofermentation takes place, lactic acid, ethyl alcohol and acetic acid are formed, while the pH begins to
decrease. Because of the acidification the growth of proteolytic, putrefactive bacteria slow and fungi accumulate
(moulds). Moulds are not sensitive to low pH. Lactic acid generated previously is used by moulds.
In poorly compressed silages volatile fatty acids are formed by the acetic acid-butyric acid mixed fermentation
process.
Yeasts also grow at low pH and produce ethyl alcohol from pyruvic acid and beside this acetic acid may also be
formed. The slow decrease of pH leads to the formation of unfavourable esters. The esters are detrimental to the
quality of the silage.
If the pH ranges from 4.5 to 5 in silo, the lactic acid-producing Streptococci multiply. The decrease of pH (pH
<4.5) favours the growth of Lactobacilli, which ferment lactic and acetic acids. Formation of acetic acid is not
favourable because the fodder becomes quickly acidic.
The decrease in pH below 4 favours the growth of Clostridium species, producing mainly butyric acid (but butyl
alcohol, ethyl alcohol, isopropyl alcohol and acetone as well). The life activities of these species in silage reduce
the quality of silo. Their also harmful activities are that they hydrolyze proteins and with deamination of amino
acids produce iso-acids.
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The volatile fatty acids arising in mixed acid fermentation processes in large amounts are weaker acids than
lactic acid. This causes an increase in pH, whereupon the keeping quality of silage decreases. The ammonia
released by deamination of amino acids also increases the pH.
3. 10.3. Gluconeogenesis (Glucose-resynthesis)
One of the possible way of glucose formation is the gluconeogenesis, which is an opposite process to glucose
breakdown. The gluconeogenesis is defined as the biosynthesis of glucose (carbohydrate) from intermediates of
catabolic processes. The principal substrates for this process are lactic acid, produced from glycolysis in skeletal
muscle and amino acids, generated from dietary protein or from the breakdown of muscle protein during
starvation. The carbon framework of some amino acids entering the citric acid cycle can be transformed to an
intermediate product, which might be the precursor for resynthesis. These amino acids may be arginine, aspartic
acid, glutamic acid, histidine, methionine, proline, serine, threonine, tryptophan and valine. The resynthesis
takes place in the liver. The principal substrate of the process is phosphoenol pyruvate (Fig. 35).
10.14. ábra - Figure 35: Gluconeogenesis
Gluconeogenesis (resynthesis) occurs in glycogen mobilization or in intense muscle workout, when sugar is
generated from lactic acid. Cells also produce sugar from oxaloacetic acid during starvation, in the absence of
insulin, or even in carbohydrate-free diet. In such cases, the concentration of oxaloacetic acid may decrease in
cells, which may be the main cause of the formation of ketone bodies as well.
In the case of high input or input of bad quality protein, the organism is not able to build its amino acids and
proteins, therefore sugars will be released from intermediates of these proteins breakdown. Then the energy
efficiency of formation of glucose is not favourable, because energy is needed for deamination and for removing
ammonia resulting from this process (urea synthesis). Significant precursor of glucose in ruminants is the
propionic acid, from which the glucose can be formed with high efficiency. In this process propionyl-CoA is
synthesized first from propionic acid using ATP and CoA, then after its carboxylation methyl-malonyl-CoA is
synthesized. In the next step the succinyl-CoA is produced from methyl-malonyl-CoA by a mutase enzyme,
which is transformed to oxaloacetic acid in the citric acid cycle. The oxaloacetic acid may transform to glucose
as described above.
4. 10.4. Glycogen metabolism
4.1. 10.4.1. Glycogen synthesis
High amount of polysaccharide are stored in mammalian liver and muscle as glycogen. Mammals need to store
energy in this form because the breakdown of glycogen and the release of its energy are faster than that of fats.
The carbohydrates are absorbed in the form of monosaccharides. After meals the blood glucose level rises.
Glucose enters the liver and it is converted to glucose-6-phosphate, which is in equilibrium with glucose-1phosphate. Due to the high concentration of glucose-6-phosphate the synthesis of glycogen starts.
The reaction process is as follows:
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The energy of the glucose-1-phosphate is not enough to connect to the starter molecule (more connected glucose
units) by glycosidic bond, and therefore reacts with uridine triphosphate and thus takes up energy from UTP.
The uridine diphosphate-glucose (UDP-glucose) is an energy-rich precursor. This molecule is able to connect
with 1 → 4 ulinkage, while UDP is released.
Glycogen is composed of glucose molecules ulinked together linearly by 1→4 glycosidic bonds with branches
are created by 1→6-glycosidic bonds. Glycogen synthesis involves both polymerization of glucose molecules
and branching from 1→6 ulinkages. These branches can be introduced by branching enzyme called amylo(1,4→1,6)-transglycosylase.
Glycogen is synthesized mainly in the liver and in the muscles. In muscles, the precursor of glycogen synthesis
is glucose, while in the liver beside glucose other organic compounds can also be precursors (lactic acid,
oxaloacetic acid).
4.2. 10.4.2. Glycogen mobilization, catabolism
The stored carbohydrates, glycogen is broken down in the storage organs, in the liver and in muscle. The
mobilization of glycogen starts when the blood sugar level of the body is reduced after exercise or when stressed
state occurs.
The central enzyme of glycogen catabolism is the phosphorylase, which tears off monomers from the ends of
the polysaccharide chain, while transforms them phosphoric-ester form. The end product of the breakdown
process is the glucose-1-phosphate. Glycogen phosphorylase catalyses the removal of the terminal glucose of
glycogen when the bond is 1 → 4 ulinkage and stops glucose residues from a branch point (1 → 6 bond),
producing a limit-dextrin. The limit-dextrin is degraded by α-(1→ 6) glycosydase enzyme. The glucose-1phosphate originated from the breakdown is rapidly converted to glucose-6-phosphate by isomerisation
process.
The glucose-6-phosphatase enzyme can be found in the liver, which cleaves off phosphate group from glucose6-phosphate and thus glucose is produced. The membrane of the liver is permeable for the glucose resulted in
this process, so it can get into the bloodstream.
There are no free glucose molecules formed from muscle-glycogen in the muscle, because there is no
phosphatase enzyme in the muscle, which could catalyze the process of forming glucose from glucosephosphate. In the muscle, lactic acid is formed from the breakdown of glycogen. One part of the lactic acid can
be oxidized to carbon-dioxide and water, while the other enters into the liver via the bloodstream and is
converted to glucose and glycogen. The sugar formed in the liver gets into the muscles via the bloodstream,
where it is involved in energy supply. The circuit between the muscles and liver is called the Cori cycle.
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1. 11.1. Biosynthesis of lipids
1.1. 11. 1. 1. Biosynthesis of triglicerides
The biosynthesis of triglycerides can be divided into three inter-related reactions. In order to the formation of
triglycerides, fatty acids and glycerin should be synthesized first, and after that, they have to be ulinked.
1.1.1. 11.1.1.1. Biosynthesis of fatty acids
The biosynthesis of saturated straight-chain fatty acid takes place in the cytosol of the cells. The precursor of the
synthesis is the acetyl-CoA. This precursor can be derived from the degradation of fatty acids, it can also be
derived from the decarboxylation of pyruvate, which is the end product of glycolysis or it can be originated from
the degradation of amino acids (from their carbon chain). These processes take place in the mitochondria, thus
acetyl-CoA needs to get to the cytosol, which is an energy-intensive process.
Another important precursor of this process is malonyl-CoA, which is also formed from acetyl-CoA by
carboxylation. The reaction is catalyzed by acetyl-CoA carboxylase, which has a biotin prosthetic group. CO 2,
Mg2+ and ATP also need to run the process and for the enzyme to work.
The biosynthesis of fatty acids is catalyzed by fatty acid synthase multi-enzyme complex. This enzyme complex
contains six enzymes and an acyl carrier protein group, the ACP. The specific binding site of ACP is the SHgroup, which is able to bind the acyl group. The ACP are surrounded by six enzymes. The ACP holds the
molecules of the reaction by covalent bonds and forward them to one of the active sites of the enzyme to the
other.
As shown in Figure 36 the ACP first binds the acetyl and malonyl starting materials. In the next step malonyl
condenses with acetyl group to form acetoacetyl group. This condensation reaction is coupled with the loss of
carbon dioxide (decarboxylation). One SH-group of the ACP enzyme complex becomes free. In the first
reduction step acetoacetyl group forms β-hydroxy-butyryl group by NADPH+H+ and yields crotonyl group after
dehydration. This step is followed by reduction with a second molecule of NADPH+H + to form butyryl group.
Then the free SH-group of the ACP binds another malonyl group and the process steps described above are
repeated. One cycle results the lengthening of the chain with one C2 unit, and a total of seven such cycles leads
to the formation of a molecule of 16-carbon palmitic acid. When palmitic acid is formed, the synthesis is ended.
The longer chain fatty acids are formed from palmitic acids torn from the multienzyme in the mitochondria or
elongation of fatty acids may occur at edoplasmic reticulum by attaching other acyl groups.
11.1. ábra - Figure 36: Biosynthesis of fatty acids
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The equation of biosynthesis of 16-carbon palmitic acid:
The unsaturated fatty acids are formed from saturated fatty acids. The desaturation reaction is catalyzed by
microsomal oxygenase enzyme complex in the presence of oxygen. Its operation is membrane-bound. The
production of polyunsaturated fatty acids (linoleic acid, linolenic acid) in plants happens by further oxidation of
oleic acid. Animals are not able to synthesize these compounds, so they are essential for them.
1.1.2. 11.1.1.2. The synthesis of glycerol
The glycerol is originated from the dihydroxy-acetone-phosphate in the process of glycolysis. By reduction of
this compound glycerol-1-phosphate is formed, which is the active form of the glycerol. The synthesis of
triglycerides from glycerol-1-phosphate and activated fatty acids takes place in the liver and adipose tissues. The
fatty acids are activated by ATP, AMP is incorporated (R-CO-AMP), while pyrophosphate is released. The
energy-rich, activated fatty acid has ability to bound to CoA, thus acyl-CoA (R-CO-SCoA) is formed.
The activated fatty acids can react with the activated glycerol (glycerol-1-phosphate). In the first step of the
process, lysophosphatidic acid is formed from the reaction of glycerol-1-phosphate and fatty acid-CoA, while
CoA-SH is released. This process is catalyzed by glycerol-phosphate-acyl-transferase. Another acyl group is
attached to lysophosphatidic acid catalyzed by glycerol-phosphate-acyl transferase enzyme, while phosphatidic
acid is formed (Fig. 37).
The phosphate group of phosphatidic acid is hydrolyzed by phosphatidate-phosphatase. Another acyl group is
connected to the free hydroxyl group. This process is catalyzed by acyl transferase. The triglycerid is thus
formed.
11.2. ábra - Figure 37: The synthesis of triglycerid
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1.2. 11.1. 2. Biosynthesis of phospholipids
Phospholipids are the components of biological membranes and transport lypoproteids. Phospholipids are
formed by the esterification of phosphatidic acid. A compound with alcoholic hydroxyl group is bounded to the
phosphoric acid part of phosphatidic acid, while water is released.
The initial substance of the synthesis so is phosphatidic acid. Phosphatidic acid is activated by CTP. An energyrich intermediate, CDP-diacyl-glycerol is formed in this process, while pyrophosphate group is released. The
energy-rich CDP-diacyl-glycerol is able to bind kolamin, kolin, serine and inositol by ester ulinkage. The end
products of this process are phosphoglyceride and CMP. The phosphoglycerides can be formed not only
directly, but can also be transformed into each other by chemical processes.
Ethanolamine phosphatides (cephalins) may be formed by decarboxylation of phosphatidylserine, while from
ethanolamine phosphatides with bonding three methyl groups yield choline phosphatides (lecithin).
There are two biosynthetic routes known to synthesize phosphatidylcholine. One pathway is the previously
described process, when the ethanolamine part of phosphatidyl ethanolamine is directly methylated.
Phosphatidyl ethanolamine undergoes three successive methylations, where methyl groups are derived from
three S-adenosyl-methyonines. The reaction is catalyzed by phosphatidylethanolamine methyl transferase.
The other pathway is when choline is originated from food breakdown and is utilised. In the first step choline is
phosphorylated by ATP and the resultant phosphocholine (in high-energy state) undergoes a cytidyltransferase
reaction catalyzed by phosphocholine cytidyltransferase to give CDP-choline (beside pyrophosphate is
released). The phosphocholine group is transferred to diacylglycerol catalyzed by phosphocholine transferase,
yielding phosphatidylcholine and CMP as byproducts.
1.3. 11. 1. 3. The biosynthesis of carotenoids and steroid
skeleton lipids
The carotenoids and steroids are formed from isoprene molecules. The precursor of the isoprene units is the
acetyl-CoA.
11.3. ábra - Figure 38: The synthesis of isoprene
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In the first step of the reaction, two acetyl-CoA molecules are connected to yield aceto-acetyl-CoA. In the next
step, another acetyl-CoA is connected to aceto-acetyl-CoA to yield β-hydroxy-β-methyl-glutaryl-CoA. After
reduction this compound is transformed to mevalonic acid. Mevalonate is phosphorylated by pyrophosphate.
This will increase the energy of the compound, which is able to transform now to isopentenyl pyrophosphate in
a chemical process. Isopentenyl pyrophosphate (active isoprene) is a precursor of the synthesis of carotenoids
(Fig. 38).
1.3.1. 11.1.3.1. The synthesis of steroids
All the carbon atoms of cholesterol (the base compound of steroids) originate from acetate. The steroid skeleton
is formed in several steps with coupling of 30 active isoprene molecules (isopentenyl pyrophosphate).
One of the intermediates of cholesterol synthesis is squalene. Squalene is an open chain triterpene, which after
cyclization yields cholesterol.
The final stage of the cholesterol-synthesis is the sum of complicated processes:
• ring closing,
• migration of hydride,
• migration of methyl groups,
• saturation of unsaturated bonds.
Cholesterol serves as precursor to all of the steroid compounds, the corticoids, the sexhormones and vitamin D.
2. 11. 2. The breakdown of lipids
Lipids are the major energy source in most cells. Lipids are apolar compounds; they do not absorb water, so
their place claim is small. The metabolic oxidation of lipids yields large amount of metabolic energy. The
neutral fats are the most abundant class of lipids in terms of the energy storage. Energy released during their
metabolism gives half of the oxidative energy of liver, kidney, heart muscle, skeletal muscle. Under conditions
of starvation the fat is almost the only source of energy. The brain, though it has high lipid content, is unable to
use fatty acids as energy supply, therefore the brain gains energy from the oxidation of glucose.
The breakdown of lipids occurs through action of pancreatic lipase in the small intestine. These enzymes are
produced in an inactive form and become active in the intestine. The phospholipase A hydrolyses the
phospholipids, which is also produced in pancreas and in intestinal mucosa.
Bile emulsifies lipids, yields micelles and ensures a large surface area for the function of enzymes.
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The 1-3% of neutral fats as triglycerides, nearly half of them in the form of mono-and diglycerides, while the
other half is completely hydrolyzed as fatty acids and glycerol, are absorbed from the gastrointestinal tract.
Glycerol originated from the hydrolysis of triglycerides is phosphorylated by using ATP in the liver. This
process is catalyzed by glycerol kinase. The L-glycerol-3-phosphate yielded of this process may be the
precursor of the biosynthesis of triglycerides or phosphoglycerides. After dehydrogenation L-glycerol-3phosphate generates dihydroxyacetone phosphate, thus it can enter the glycolysis process.
2.1. 11. 2.1. The β-oxidation of saturated fatty acids
Fatty acids arise in the cytosol, but their breakdown takes place in the mitochondria. The fatty acids must be
activated to get through the mitochondrial membrane. It is an energy-intensive process (ATP). First, the fatty
acid reacts with coenzyme A to form acyl-CoA. This is an activation step because the fatty acyl-CoA is an
energy-rich compound and is more reactive. This reaction is coupled with the hydrolysis of ATP.
Since the mitochondrial membrane is impermeable to fatty acids or acyl-CoA-s, a specific transport system is
needed to move them into the mitochondrial matrix, where oxidation occurs. This involves transfer of the fatty
acyl moiety to a carrier called carnitine. The next step involves the formation of fatty acyl-carnitine, which can
traverse the membrane.
Inside the mitochondrial matrix the acyl group of acyl-carnitine is passed through the mitochondrial CoA-SH.
Fatty acid are oxidized in a series of repeating steps called β- oxidation. The steps of the β-oxidation of fatty
acids are marked with 1-4 numbers (as the four steps) in Figure 39.
The first (1) of four steps is a dehydrogenation process and is catalysed by acyl-CoA dehydrogenase. In this
process, the acyl thioester is oxidized by FAD to give an enoyl derivative. The dehydrogenation takes place
between the α- and β-carbons and yields a trans-isomer (trans-enoyl-CoA). The next step is a hydration process.
This reaction is catalyzed by enoyl-CoA hydratase. The product is 3-hydroxy-acyl-CoA, which contains a chiral
carbon atom. In this reaction L-isomer is formed. Step 3 is the second oxidation where dehydroganation of the
hydroxyl group takes place. In this process, 3-keto-acyl-CoA is formed from L-3-hydroxy-acyl-CoA. The
process is catalyzed by L-3-hydroxy-acyl-CoA dehydrogenase and it is a NAD-dependent, stereospecific
dehydrogenation. The enzyme can only carry out the conversion of L-isomers.
The last step (4) of the cycle is a cleavage by attack of a second molecule of coenzyme A on the β-carbon, to
release acetyl-CoA and thus acyl CoA will be two carbons (C2) shorter than the original substrate. This process
is catalysed by acetyl-CoA acyltransferase. Then the cycle starts again, and it will continue until the chain is
completely reduced.
C2 units formed in the oxidation process can enter in the citric acid cycle.
The oxidation of unsaturated fatty acids occurs similarly as in the case of saturated fatty acids, until the
breakdown is attained the unsaturated bond of chain. Here the reaction is stopped, because of the cisconfiguration. Enoyl CoA hydratase can not be acted on, because it acts only on trans-compounds.
11.4. ábra - Figure 39: The β-oxidation
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The position of double bond is also wrong. During the oxidation of unsaturated fatty acids the double bond
occurs between the C3-C4, while it has to be in C2-C3 position. To continue the process the isomerisation of
double bond and the change of double bond position are required.
The problem is solved by two auxiliary enzymes. The enoyl-CoA isomerase catalyzes the reversible
rearrangement of the double bond. The other auxiliary ezyme enoyl-CoA hydratase creates the L-3-hydroxyacyl-CoA, from which the reaction can be continued as already described.
In cells, fatty acids with odd-numbered carbon chain can also be found in small amounts. Their breakdown
occurs as the same method as the even-numbered ones. In the last step, propionyl-CoA (C3 units) is formed
instead of the acetyl-CoA. The propionyl-CoA has to be converted into a form, which is capable to enter one of
the sections of catabolic processes. The conversion steps are shown in Figure 40. In the first step, propionylCoA is transformed to methyl-malonyl CoA by carboxylation, which is an energy-intensive process. Then
succinyl-CoA is formed by carboxyl mutase enzyme, which can now enter the citric acid cycle. Its breakdown
will be continued there.
11.5. ábra - Figure 40: Breakdown fatty acids with odd-numbered carbon
The cells gain large amount of metabolic energy by breakdown of fatty acids. In the following, we can write
the total energy balance of oxidation of stearic acid (C18).
Stearic acid oxidation yields: 9 acetyl-CoA, 8 FADH2, 8 NADH+H+
The equation for catabolism of acetyl-CoA through the citric acid cycle:
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2 CO2, 1 NADPH+H+, 2 NADH+H+, 1 FADH2, 1 GTP
9 acetyl-CoA: 18 CO2, 9 NADPH+H+, 18 NADH+H+, 9 FADH2, 9 GTP
In the terminal oxidation:
9 NADPH+H+ → 3 ATP*9= 27 ATP
26 NADH+H+ → 3 ATP*26= 78 ATP
17 FADH2
→ 2 ATP*17= 34 ATP
Beside the 9 GTP-s (GTP=ATP) generated during the catabolism 139 ATP-s are also generated (the sum of
ATP is 148), from which two ATP-s are necessary to use for the active transport of fatty acid to the
mitochondrium.
As previously deduced the oxidation of one glucose (C6) molecule to carbon dioxide and water yields 38ATP-s.
On the basis of data, it can be stated that a correspondingly larger amount of metabolic energy is released in the
case of metabolic oxidation of fats than in the oxidation of carbohydrates.
2.2. 11.2.2. The catabolism of steroids
Cholesterol is one of the most important steroids. The largest amount of cholesterol is excreted in the bile, after
appropriate transformations it solves as salts of bile acids. It plays an important role in lipid metabolism, in the
absorption. Most of the bile acids are absorbed from the intestine and utilized for emulsifying lipids (with a split
from the chain substituted C17 atom).
90% of cholesterol taken up orally is converted to bile acids and a small amount of that is transformed to
coprostanol by bacteria. It passes out of the body as faeces. Sebaceous glands of skin can also secrete 100 to 300
mg of cholesterol per day.
In the skin cholesterol can transform to calciferol (vitamin D) through enzymatic or photochemical reactions.
In the endocrine glands cholesterol transforms to pregnenolone that is the precursor for the steroidal hormone
synthesis. Pregnenolone transforms to progesterone. It is a hormone compound and the precursor of the adrenal
glands hormones progesterone as well. Pregnenolone and progesterone transform to oestrogens and androgens
(sex hormones) in the adrenal cortex.
3. 11. 3. The formation of ketone bodies (ketogenesis)
Collectively, β- hydroxybutyric acid, acetoacetic acid and acetone are called ketone bodies. Ketone bodies are
created in the body in normal conditions. There are some amino acids (ketogenic amino acids; lysine; leucin)
that broke down and their carbon skeleton enters the process of ketogenesis. Large quantity of ketone bodies can
be formed in the metabolism of fats.
The acetil-CoA is produced from partial oxidation of fatty acids and transposted by the CoA-SH into the citric
acid cycle mainly and it will be transformed further. It could also be happened to acetyl-groups as well which
are derived from the catabolism of carbohydrates and some amino acids. The process requires large amounts of
oxaloacetic acid as the acetyl CoA connects to this compound at the first step in the citric acid cycle.
When there is not enough oxaloacetic acid, it is unable to accept the acetyl-group and the ketogenesis will start,
so the acetyl-CoA is then used in the synthesis of ketone bodies. The concentration of ketone bodies in the blood
increases.
If due to starvation or diabetes the glucose level in the body remains low, the body generates glucose from one
part of oxaloacetic acid to solve the problem. At the time of starvation or in case of diabetes the concentration of
ketone bodies may multiple and the tissues are not able to digest them (ketosis).
In this case the ketone bodies can also appear in the urine (ketonuria). This condition is dangerous, because
acetoacetic acid and acetone are harmful for the nervous system.
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In the first step acetoacetyl-CoA can be formed from 2 moles of acetyl-CoA by the CoASH transferase enzyme.
In the second step β- hydroxyl-β- methyl glutaryl-CoA forms from this compound. This process is catalyzed by
hydroxymethyl-glutamyl-CoA synthetase and during the process another acetyl group enters.
These compounds are synthesized in the liver and the circulatory system delivers them to peripheral tissues and
there they are oxidized.
The β- hydroxyl-β- methyl glutaryl-CoA is a precursor of steroid skeleton synthesis.
11.6. ábra - Figure 41: Ketogenesis
The β-hydroxybutyrate can be oxidized to acetoacetate and the opposite process can also occur.
β -hydroxybutyrate + NAD+ ↔ acetoacetate + NADH + H +
Acetoacetate can transform to acetone by decarboxylation. In peripheral tissues ketone bodies can join the
catabolism.
In the mitochondria acetoacetate is activated from succinyl-CoA by means of CoA-transfer:
succinyl-CoA + acetoacetate → succinate + acetoacetyl-CoA
Succinate transforms to further compounds in the citric acid cycle. Acetoacetyl-CoA is converted into two mols
of acetyl-CoAs with using of a CoA. These compounds then get into the citric acid cycle.
The brain converts large amounts of ketone bodies to gain energy if the glucose supply is inappropriate during
starvation (Fig. 41).
4. 11. 4. Glyoxylic acid cycle (Kornberg Krebs cycle)
This modified citric acid cycle occurs in some microorganisms and in germinating oilseeds. In this cycle
anabolism dominates, thus it is contrary to citric acid cycle, where the main goal is catabolism. In the process,
the starting amount of oxaloacetate is doubled in every turn of the cycle.
The balanced equation of the cycle:
2 acetyl-CoA + 3H2O + FAD + 2NAD+ → oxaloacetic acid + 2CoA + FADH2 + NADH+ +H+
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The oxidation of the intermediates differs from citric acid cycle in the trasformation of isocitric acid. In citric
acid cycle the citric acid is decarboxylised oxidatively. However, here isocitrate lyase cleaves isocitrate to
glyoxylic (C2) and succinic acid (C4). Glyoxylic acid reacts with the second acetyl CoA. The process is
catalyzed by the enzyme malate synthase and malic acid forms. Malic acid is oxidized to oxaloacetic acid.
From succinic acid cleaved by the enzyme isocitrate lyase another oxaloacetic acid forms through the already
known pathway: fumaric acid → malic acid → oxaloacetic acid. It is then ready to start another cycle again. The
enzymes of the process at this stage are the same as with that of citric acid cycle. The glyoxylic acid cycle
requires only one oxaloacetic acid, thus the other one is utilized for carbohydrates or amino acid synthesis (Fig.
42).
11.7. ábra - Figure 42: Glyoxylic acid cycle
Plants synthesize carbohydrates from fats by using glyoxylic acid cycle. In the process acetyl-CoA transforms
oxaloacetic acid through glyoxylic acid cycle. Oxaloacetic acid is transformed to phospho-enol-pyruvate (PEP).
The process is catalyzed by phospho-enol-pyruvate carboxylase enzyme. Glucose is built up from PEP via
multi-step process. During the sequential reactions the steps of glycolysis take place inversely.
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PROCESSES III. PROTEIN
METABOLISM
Nitrogen can get into the food chain through nitrogen uptake by plants.
Plants can take up nitrogen from the soil as ammonium and nitrate ions. Although air contains nitrogen in large
quantities, this form is not available for plants. Atmospheric nitrogen can be fixed by bacteria that live free in
the soil or in symbiosis with certain plants. These plants use atmospheric nitrogen taken up and converted by
bacteria. Nitrogen forms taken up by plants has to be reduced in plants.
1. 12.1. The nitrogen fixation
The symbiotic bacteria and host plants can fix and reduce nitrogen mutually. In there common metabolism
bacteria are the N-source, while host plants are the C- source.
The nitrogen fixing bacteria contains nitrogenase enzyme that plays a central role in the nitrogen fixation. The
enzyme consists of two subunits, which is a complex protein containing iron and molybdenum. One of the
subunits only contains iron. The nitrogenase enzyme reduces molecular nitrogen to ammonium ion.
The process requires electrons (reduced coenzymes) and large amounts of energy (ATP) which are provided by
the host plant. The transformation of 1 mole nitrogen into 2 moles ammonium ion requires 12 moles ATP.
Some parts of the reduced coenzymes from catabolic processes of the host plant enter the transformation
directly by providing electrons. The other part enters the terminal oxidation and provides energy for the process.
In the host plant the citric acid cycle works intensively, which requires oxygen-rich conditions. However
anaerobic environment is needed for the reduction. This contradiction is resolved by the tissue structure of the
root nodules.
Ammonium ion formed in the reduction process gets onto glutamic acid. The first product of the process is
glutamine. It is transported as aspartic acid that forms in the process of trans-amination. The fate of ammonium
ions taken up by the plant is the same as the previous one.
The plant must also reduce nitrate ions taken up from soil to ammonium ions. The process consists of two steps.
First, the enzyme nitrate reductase reduces nitrate ions to nitrite ions then nitrite reductase converts nitrite ions
to ammonium ions.
The process requires electrons that can originate directly from the light phase of photosynthesis. In this case the
electrons from ferredoxin do not get to oxidized NADP +, but to the nitrite ion. In the first stage of the reaction
sequence (nitrate ion → nitrite ion) electrons are indirectly from the reduced coenzyme.
The amino group of the amino acids forms by the incorporation of ammonium ions. With the direct uptake of
ammonium ion glutamic acid and glutamine can be synthesized. The reactions are shown in Figure 43.
Ammonium ion ulinks to α-keto-glutaric acid by reductive amination.
In the process that is catalyzed by glutamate dehydrogenase glutamic acid forms by means of, NADPH + H +.
Glutamic acid transforms to glutamine with energy input and ammonia bound. Glutamine synthetase catalyzes
the amide biosynthesis.
Glutamic acid synthase synthesizes two moles of glutamic acids from α-keto glutaric and glutamin.
12.1. ábra - Figure 43: The nitrogen fixation
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Glutamine stores ammonia in amide bond. During trans-amination reaction the stored ammonia get onto the
corresponding oxo acid (carbon skeleton of amino acids) and form other amino acids.
pyruvic acid (1) + glutamic acid (2) → alanine (1) + α-keto-glutaric acid (2)
oxaloacetic acid(1) + glutamic acid (2) → aspartic acid (1) + α-keto-glutaric acid (2)
The carbon skeleton of amino acids comes from the intermediates of different metabolic pathways.
The α-ketoglutaric acid is an intermediate of citric acid cycle and it can convert to glutamic acid and glutamine.
This compound is also the precursor of proline and arginine. These amino acids are also referred to as members
of the glutamine family.
Oxaloacetic acid can also exit the citric acid cycle. It is trans-aminated and converted to aspartic acid that
transforms to asparagine by binding a further ammonium ion with amide bond. The starting material of
methionine and lysine synthesis is also oxaloacetic acid. Threonine also forms from oxaloacetic acid, which
converts to isoleucine. The amino acids the synthesis of which starts with oxaloacetic acid are listed in aspartic
acid family.
The synthesis of alanine, valine and leucine starts with pyruvic acid that is the final product of glycolysis. They
are the members of pyruvic acid family.
Glyceric acid-3-phosphate can leave the process of glycolysis or photosynthesis and it can become the carbon
skeleton of cysteine and serine. Glycine forms through the conversion of serine. These amino acids belong to the
serine family.
Ribose-5-phosphate that is the intermediate product of the light independent phase of photosynthesis can be the
carbon skeleton of histidine. Phospho enol pyruvic acid and erythrose-4-phosphate are also members of the
reaction sequence of photosynthesis, and they together form the carbon skeleton of tyrosine, phenylalanine and
tryptophan. They are cyclic amino acids.
2. 12.2. The synthesis of essential amino acids
The human body and mammals can not synthesize some amino acids, they can only get them with nutrition. The
majority of plants and bacteria are able to synthesize the amino acids, the synthesis routes are similar, but much
more complicated than that of non-essential amino acids.
2.1. 12.2.1. The methionine and threonine biosynthesis
The common intermediate of methionine and threonine synthesis is homoserine. In mammals these amino acids
are essential, as they have lack of that reaction way section in which homoserine is formed from aspartic acid.
The steps of the process:
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Aspartic acid beta-aspartyl phosphate → aspartate - semialdehyde → ß-homoserine.
From homoserine threonine is formed through homoserine phosphate by threonine synthase.
2.1.1. 12.2.2.1. Methionine formation from homoserin
The formation of methionine from homoserin starts with the formation of O-succinate. In this step succinyl
group is attached to homoserine.
In the subsequent reaction cystathionine formed by means of the enzyme cystathionine γ-synthase. From this
compound cystathionine β-lyase cuts water, ammonia and pyruvic acid, creating homocysteine. Homocysteine
takes part in methylation process with methyl transferase in which the end product is methionine. Cystathionine
is the starting material of both cysteine and methionine, but in mammals the cysteine and in plants the
methionine can only be found, while in bacteria both of them are produced (Fig. 44).
12.2. ábra - Figure 44: The synthesis of essential amino acids
2.2. 12.2.2. Lysine biosynthesis
In bacteria and plants lysine biosynthesis passes through diaminopimelic acid, while in fungi through αaminoadipic acid.
For the formation of diaminopimelic acid aspartate semialdehyde and pyruvate are required. They react each
other during aldolcondensation then through a multi-stage reaction diaminopimelic acid is formed. After
decarboxylation of this compound lysine is synthesized.
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2.3. 12.2.3. Arginine biosynthesis
Arginine is generated in the liver of mammals in large quantities, but this is immediately decomposed by the
enzyme arginase. It follows that arginine produced by the liver can not be the arginine source of the body. The
precursor of arginine synthesis is ornithine. Ornithine is formed by glutamic acid.
2.4. 12.2.4. Leucine, isoleucine and valine synthesis
The three branched-chain amino acid biosynthesis takes place in a similar route.
Ketoacid → active acetaldehyde + acetohydroxy acid
The processes are catalyzed by enzymes containing thiamine pyrophosphate. After further reactions, such as
reduction, methyl-ethyl-migration, rearrangement, dehydration ketoanalog relevant to the amino acids is formed.
From ketoanalogs through transamination amino acids are formed.
During the biosynthesis of valine from two molecules of pyruvic acid acetolactate synthase form α-acetolactate
with carbon-dioxide release. In further steps α, β-dihydroxy isovalerate, and α-keto-isovalerate is formed. From
it valine transaminase forms valin using the amino group from glutamic acid.
2.5. 12.2.5. The phenylalanine and tryptophan biosynthesis
The basic condition for the formation of these amino acids is the formation of 6-membered aromatic ring. The
aromatic ring forms from aliphatic precursors. By plants the central intermediate in the formation of the
aromatic ring is shikiminic acid. The lignin, ubiquinone and plastoquinone also form from shikiminic acid. The
precursor of shikiminic acid is phospho-enol pyruvic acid and erythrose-4-phosphate.
The intermediate forms ring structure, then the process is followed by dehydration and reduction. After
phosphorylation the shikiminic acid transforms to korizminic acid that is found in the branches of aromatic
amino acid metabolism.
Korizminic acid is one of the products of the branches of metabolic pathways.
From it prephenic acid is generated, from which it is transformed to phenylalanine in a multi-stage process.
The korizminic acid can transform to antranilic acid as well, which synthesizes to tryptophan in additional
transformations. The formation of the histidine starts with the bond of phospho-ribosyl pyrophosphate and ATP
wherein the 5-phospho-ribosyl-glycosidic forms glycosidic ulinkage with first position nitrogen atom of the
adenine part of ATP while pyrophosphate exits.
The 2nd carbon atom of phospho-ribosyl-2-pyrophosphate integrates to carbon skeleton of imidazole ring,
whereas the 3rd carbon atom transforms to analin.
3. 12.3. Protein Synthesis
During the biosynthesis of proteins information stored in DNA is transcribed into protein with appropriate
amino acid sequence. The process takes place by means of the various types of RNA molecules.
The synthesis of proteins consists of two steps that can be divided to further sections.
1. During transcription information stored in the DNA is transcribed into RNA. The process takes place in the
nucleus. (DNA → RNA)
The products move to the place of protein synthesis through nuclear pores.
2. During translation process information transcribed to RNA is translated into amino acid sequence on
ribosomes. (RNA → protein)
The sequence of DNA nucleotides (N-bases) determines of the nucleotide sequence of the RNA. The sequence
of RNA nucleotide triplets (base triplet) is the information that is responsible for the polypeptide amino acid
sequence.
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3.1. 12.3.1. The transcription
During the process RNA molecules (mRNA, tRNA, rRNA) are synthesized based on the DNA sample. The
central enzyme complex of the process is the DNA-dependent RNA polymerase that consists of five subunits.
The mRNA is the messenger RNA, the amino acid sequence of which determines the nucleotide sequence of the
protein.
The tRNA is the transfer RNA, it transports the activated amino acids into the site of protein synthesis.
The rRNA is the ribosomal RNA, it is a part of the ribosome that is the center of the protein synthesis.
Transcription can be divided into three stages:
→ initiation is the start of the process
→ elongation is the prolongation of the chain
→ termination is the finish of the process.
Initiation begins with the recognition of promoter base composition on the initial section of the template DNA
molecule. In the promoter section the RNA polymerase unscrews the double helix DNA molecule. In this
section the two DNA strands are released. One of the strands of the DNA, the so called codic strand (starting
with 3') serves as template. On the other strand there is no transcription – this strand remains 'silent'. The RNA
synthesis occurs from the 5 'end to 3' end direction (Fig. 45).
12.3. ábra - Figure 45: Transcription
During elongation, monophosphate nucleosides attach themselves to the DNA in complementary order. The
monophosphate nucleosides are formed from nucleoside triphosphate with pyrophosphate cleavage. The
corresponding nucleotide monophosphate derivatives ulink to the appropriate 3'end.
At the final stage of chain construction (termination) the template DNA has palindromic structure. At this stage
the sequence of bases is repeated as a mirror image. The end of RNA chain transcribed from them can form a
hairpin loop. The RNA chain is cleaved by a protein from DNA. The site of termination is indicated with the
hairpin loop RNA. The process requires energy (ATP). The resulting RNA molecules (pre-RNAs) transform
during migration to the cytoplasm and they become mature. The non informal parts fall out. In the meantime the
reactive groups at the ends of the RNA chain are protected from undesirable side reactions by compounds
attached to them.
3.2. 12.3. 2. The translation
The second step of protein synthesis is the translation. The result of translation is the protein molecule. The
process required for mRNA, tRNA, rRNA, ATP, GTP, factors, Mg ions and amino acids.
The general steps of the synthesis are also characteristic for the synthesis of other macromolecules. Monomers
have to be activated. The activated units have to be connected to each other, than products have to break down
from the apparatus of synthesis, and they have to form appropriate spatial conformation. These steps follow
each other in the process of protein synthesis as well.
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The base sequence of messenger RNA (mRNA) determines the sequence of amino acids in the protein molecule.
In an mRNA molecule three nucleotid units (base triplet) signifies an amino acid in the polypeptide chain. The
majority of the amino acids are encoded by more than one base triplet.
There is a unique base triplet that is responsible for the start of the synthesis. AUG triplet is the start codon,
which is the code of methionine. The synthesis can be finished by several base triplets (stop codons).
The ribosomal RNAs (rRNAs) connected with proteins form ribosomes in the endoplasmic reticulum. The
protein synthesis takes place in ribosomes. Ribosomes consist of two subunits, one has small molecular weight
and the other has larger molecular weight. During protein synthesis they are associated with Mg ions. Their task
is to fix mRNAs, and the activated tRNAs that carry amino acids.
The biological role of transfer RNA (tRNA) is the transportation of activated amino acids into the place of
synthesis, and inserting them into the protein chain. Amino acids can be transported by different types of tRNA
molecules. (The 20 kinds of amino acids are transported by 60 kinds of tRNAs.) The tRNA molecules have a
special cloverleaf conformation. There are three loops in their and the rest of the molecule is arranged in a
double spiral.
The first loop (I) is the recognition site of the amino acids, i.e. that enzyme attaches here, which is responsible
for amino acid binding.
The second loop (II) is the anticodon part. Anticodon is a triplet the sequence of which is complementary to the
triplet of mRNA, thus they fit together. The anticodon part determines the amino acids binding to the tRNA
binding site.
The third loop of tRNA (III) provides the binding to the ribosome. The activated amino acid binds to the free
end of the molecule, which is the acceptor end. The end is the same (ACC) in all of the tRNA molecules. The
hydroxyl group located at the 3rd carbon atom of nucleotide unit containing adenine is freely available (Fig. 46).
12.4. ábra - Figure 46: tRNA
The attachment of amino acids to tRNA
There are CCA base triplets on the binding sites of each tRNA (3 'end). Amino acids attach with ester bond to
the 3’ carbon atom of ribose in the nucleotide containing adenine.
The nucleotide supplies the alcohol OH group, whereas the amino acid gives the carboxyl group for the ester
bond. The process involving water loss is catalyzed by aminoacyl-tRNA synthetase enzyme. The amino acid has
to be activated for the reaction. The activated amino acid is the aminoacyl-adenylate that ulinks to tRNA with a
loss of AMP.
Three phases of translation can be separated: these are initiation, elongation and termination.
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3.2.1. 12.3.2.1. Initiation
The initiation is signified with AUG that is the initiation codon of mRNA. In eukaryotes this codon is
complementary to the anticodon part of tRNA bonding methionine.
For the initiation of the synthesis the initiation complex must be formed.
For the formation of initiation complex two subunits of the ribosome, mRNA and tRNA are required. The tRNA
contains the initial amino acid in activated form that is the methionine-tRNA. GTP provides the energy required
for the synthesis. Initiation factors and Mg ions (IF1, IF2, IF3,) play an important role. The steps of initiation
complex formation are shown in the following figure (Fig. 47).
12.5. ábra - Figure 47: The steps of initiation complex formation
The IF3 factor binds to the smaller ribosomal subunit, thus it prevents the larger ribosomal subunit's association.
This is necessary to ensure the connection of the smaller ribosomal subunit to mRNA. In the second step, the
initiation tRNA, IF2 and GTP arrive as a complex and reach the P site by means of IF1. In the third step the
larger ribosomal subunit ulinks to the already formed complex. The process also requires magnesium ions.
During the connection GTP decomposes to GDP and phosphoric acid and the dissociation of initiation factors
also takes place.
3.2.2. 12.3.2.2. Elongation
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In the elongation stage of protein synthesis amino acids connect to the initiator amino acid one after the other in
a certain sequence specified by the mRNA. The polypeptide chain is growing. The information supplied by the
mRNA transcribes in 5 '→ 3' direction forming polypeptide chain.
The rRNA moves along mRNA and it ensures the binding of tRNA. The involvement of new tRNA in the
process takes place by means of elongation factors (EF-T and EF-G).
The new amino acid aminoacyl-tRNA - EF-T - GTP arrives to the ribosome as a complex and binds to the A
binding site. The energy derived from GTP hydrolysis stabilizes the ribosome-aminoacyl-tRNA - mRNA
complex.
The amino acids on the two tRNA molecules are attached via a peptide bond. The amino group of the newly
arriving aminoacyl tRNA removes the amino acid from methionyl-tRNA by nucleophilic attack. The peptidyl
transferase enzyme forms peptide bond with the carboxyl group of cleaved methionine and the amino group of
newly arriving, but still RNA-bound amino acid. The formed dipeptide is located in A site and it is bound to the
second t RNA. The amino acid-free (blank) tRNA remains in P site.
In order to continue the chain elongation tRNA containing the dipeptide has to shift to the P-site. The amino
acid free tRNA has to leave the P-site. For the translocation of tRNA carrying dipeptide the factor EF-G and
GTP are required. With the displacement of the tRRNA, according to the codon instruction another activated
tRNA arrives into the vacant site. The subsequent steps of the process are the same as the previous ones.
In the final stage of protein synthesis (termination) the chain elongation is completed, the finished polypeptide
chain is released from its apparatus. The completion of the synthesis is signified to the synthesizing system by
mRNA via three nonsense codon (UAA, UGA or UAG). At this point the polypeptide chain is located at P site
bound to tRNA. The tRNA corresponding to base triplets would arrive to the A-site, but there is not such tRNA
molecule which anticodon part is complementary to these base triplet.
The role of terminator codons is to prevent the further elongation of the polypeptide chain. The recognition of
these codons takes place by means of three release factors (RF1, RF2, RF3).
3.2.3. 12.3.2.3. Termination
In the first step of completion a releasing factor ulinks to the termination codon. The specificity of peptidyl
transferase altered, thus it will be capable for hydrolyzing the ulinkage between the tRNA at the last site and the
amino acid (polypeptide). The polypeptide chain and the tRNA are released and removed from the ribosome. In
the next step, the mRNA and ribosomal disconnected. The ribosome dissociates and its subunits are ready to
participate in the synthesis of a further polypeptide.
During synthesis the formation of the spatial structure of the polypeptide already begins. The formation of
spatial structure does not need specific information, since it is determined by the amino acid sequence.
After termination the protein conformation is fully formed and meanwhile it undergoes through a maturation
process. Here the starting amino acid can be cleaved, and the oxidation, methylation, phosphorylation of sidechains can occur. The amino group of N-terminal end is often acetylated.
Protein synthesis takes place only when the cells have adequate energy, because the construction of this
macromolecule is a highly energy-inquiring process. This process consumes the most energy among all
processes taking place in cells.
ATP (AMP + ATP → PPA) is required for the activation of amino acids, and for their binding to tRNA. GTP is
used for the attachment of tRNA to the A-site. GTP is also required for the shift rRNA on mRNA. All in all the
cleavage of four high-energy phosphate binding can result one peptide bond. 1/6 part of the invested energy
only releases by the hydrolysis of this binding.
4. 12.4. The fate of dietary proteins in heterotrophic
organisms
4.1. 12.4.1. The quality of proteins
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The living organisms split dietary proteins to amino acids. The resulting amino acids as end products of
hydrolysis are utilized predominantly for the construction of its own body proteins. If necessary, they can be
precursors of sugars (gluconeogenesis). They can be utilized by the organism for lipid or chemical energy
production too.
In addition, they can be precursors of amino acids, hormones, porphyrins, purines, pyrimidines, alkaloids,
coenzymes as well.
Excess amino acids not used by the body are degraded. Their nitrogen content empties in various forms. Their
carbon skeleton decomposes into carbon dioxide and water. The various organisms have different amino acid
requirement. Organisms can synthesize most of the required amino acids, but they can not produce one part of
them, and they have to take it up by food. These types of amino acids are called essential amino acids.
Proteins have different values according to the satisfaction of amino acid requirement.
Proteins were classified on the basis of several aspects.
The biological value gives the percentage of the absorbed N that is incorporated into the organisms
It can be calculated as follows…
BV = ( ( Ni - Ne(f) - Ne(u) ) / (Ni - Ne(f)) ) * 100
Where:
• Ni = nitrogen intake in proteins on the test diet
• Ne(f) = (nitrogen excreted in faeces whilst on the test diet) - (nitrogen excreted in faeces not from ingested
nitrogen)
• Ne(u) = (nitrogen excreted in urine whilst on the test diet) - (nitrogen excreted in urine not from ingested
nitrogen)
The higher the biological value of the protein the more similar amino acid composition it has to human needs.
Digestibility also affects its value.
Among raw materials that are often applied in nutrition the whole egg protein biological value is 100% that of
milk is 93%. Among meat fish has 86%, beef has 85%, pork has 84% while chicken has 82% biological value.
The biological value of the vegetable proteins is smaller than that of meats. The highest value has soy protein
with 76% and potato with 74%. The BV of legumes is 66%, rice is 65%, wheat is 56%, while corn protein is
51%. Proteins of animal origin are called complete proteins while vegetable proteins are called incomplete
proteins according to the biological value.
The chemical score of a protein is based on the relationship of its amino acid composition and its nutritional
value. The examined protein is compared to the composition of a reference protein (FAO / WHO
recommendations, eggs, meat, milk, etc..) To calculate a food’s chemical score, the amount of each essential
amino acid provided by a gram of the food’s protein is divided by an “ideal” amount for that amino acid per
gram of food protein.
This value does not provide information on the absorption of the protein and the fate of intermediates in
metabolism. It is suitable for the determination of the limiting amino acids, thus this value can be used for the
protein supplements.
The net protein utilization (NPU) gives the amount of retained nitrogen is related to consumed nitrogen
percentage. This is the resultant of the certain protein’s digestibility, the absorption and utilization of the amino
acids.
The protein efficiency ratio (PER) shows the amount of weight gain which 1g protein causes. This value is
highly dependent on the quality of the protein.
The apparent digestibility can be calculated from the N content of the food and faeces, it is given in
digestibility %.
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Digestibility % = ((food N – faeces N)/food N)*100
The digestibility of milk, egg and meat are ~100%, while plant proteins are utilized in 80%.
The lower digestibility of plant proteins can be explained by the fact that there are their proteinase inhibitors in
them that inhibit the function of protein decomposing enzymes.
4.2. 12.4.2. The protein balance of the organism
The proteins as the parts of every cell (plant or animal) are being destroyed and synthesized continually. The
velocities of the intermediate metabolic processes are different in the different tissues. The lifetimes of the
proteins are different:
The blood plasma proteins live for ~ 10 days, the muscle proteins exist for ~ 100 days.
The N balance of the organism, which is approximately equivalent to the protein balance is equal to the
difference between the amount of intake N and emptied N.
Negative N - balance is in that body, which is in long-term shortage of protein. The organism does not receive
the proper amount and quality of protein. This is often coupled with a lack of energy. In this case, the organism
synthesizes sugar by protein breakdown complementing its energy in this way. After severe infections negative
N balance can also occur, while there is an increased N-loss.
Building new tissues or muscles reflect in a positive N balance. During pregnancy and breast-feeding mothers
have a positive protein (N) balance.
4.3. 12.4.3. The digestion of proteins
The breakdown of proteins to amino acids is carried out by proteolytic enzymes (proteases). These enzymes
break the peptide bonds between amino acids. They are hydrolases that hydrolyse bonds by water incorporation.
The process is extracellular. Among proteases peptidases (exopeptidase) cleave amino acids from the end of the
chain proteins, whereas proteinases are endopeptidases that cleave peptide bonds inside the chain.
The proteolytic enzymes are present in inactive form at the scene of the protein degradation that is the
gastrointestinal tract by vertebrates. For their activation a smaller or larger- peptide chain must be cleaved by
them. The activation is carried out by other enzymes, or the process is auto catalyzed.
Pepsin zymogen is a protein decomposition enzyme, the inactive form of which is pepsinogen. It works under
acid conditions (pH 1-2). It can be activated by a cleavage of peptide chain from its N-terminal end, which has a
molecular weight about 7000. This section contains the majority of basic amino acids of pepsinogen. Pepsin
cleaves bonds mainly between amino acids with aromatic and other nonpolar side-chains.
The partially digested proteins leave the stomach and get to the small intestine. Here the pH is neutral. Further
digestion of the proteins is carried out by chymotrypsinogen, trypsinogen, procarboxypeptidase A-B and
proelastase which are produced in the pancreas in inactive form. The activation of trypsinogen is started by
enteropeptidase, then it becomes autocatalytic process. In the process a hexapeptide is cleaved from the Nterminal end of trypsinogen. Trypsin cleaves peptide bond mainly next to carboxyl group in peptides.
Chymotrypsin activates trypsin.
In the small intestine the mixture of exo-and endopeptidases hydrolyze the partially decomposed proteins to
amino acids.
The amino acids are absorbed through the intestinal epithelium, which is an energy-requiring, active process.
Amino acids from blood stream get into the tissues, where they are involved in metabolism. The metabolic pool
of the cells is supplemented by amino acids formed from the degradation of tissue proteins. Intracellular protein
breakdown is carried out by cathepsins.
Proteases vary widely in their optimum pH, they can be acid, neutral and alkaline proteases.
4.3.1. 12.4.3.1. Proteases occur in each cell. Their role is wide ranged.
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• Proteases influence the composition of the cell protein substance.
• Proteases demolish the unnecessary proteins in order to build the necessary new protein molecules
• They promote the exit of secretory proteins from the cell by splitting the signal peptide.
• They activate inactive zymogens.
• They inactivate biologically active proteins that have already filled their functions.
• They inactivate defective proteins.
• They break down proteins to provide energy to the body during starvation.
4.3.2. 12.4.3.2. The common features of amino acid degradation pathways
The catabolic reactions of amino acids are catalyzed by multienzymes.
The substituents of α-carbon atom (carboxil-, amino groups) can be removed by similar catabolic pathway. The
fate of reaction products are various during the metabolic processes.
One way of amino acids degradation is the cleavage of carboxyl group.
During decarboxylation carbon dioxide split off from the carboxyl group of the amino acid and bioactive
amines are formed. The process is catalyzed by amino acid decarboxylases. They are characterized by narrow
substrate specificity. Generally, they can only decarboxylate one amino acid. Pyridoxal phosphate is required for
their operation. Glutamic acid decarboxylase breaks off carbon dioxide from L-glutamic acid. The resulting
biogenic amines is the γ-aminobutyric acid.
After decarboxilation the residual biogenic amines are still after physiologically active compounds that are
precursors of hormones, coenzymes or become building blocks of other materials. Some amino acids and
biogenic amines resulted from them are shown in the Table 7.
12.6. ábra - Table 7: Biogenic amines
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A small amount of amino acids only decomposes by decarboxylation to form biogenic amines. Another way of
amino acid breakdown is the removal of amino groups. During deamination amino group is split off and oxo
acids are formed.
The removal of amino group is carried out in the liver and kidneys in mammals. The process takes place by
transamination or oxidative deamination.
In transamination reaction one part of the amino group of amino acids attaches to a new α -keto acid and forms
another amino acid.
-amino acid + pyruvate ↔ α-keto acid + alanine
The process is catalyzed by transaminases. The process has a dual role. The amino-nitrogen of the amino acid is
held back and built into another amino acid while the carbon skeleton of the amino acids is transformed in a way
that it will be able to enter to the citric acid cycle.
During oxidative deamination ammonia is split off from amino acids (Fig. 48).
The process is catalyzed by NAD+, NADP+-specific amino acid dehydrogenase, amino acid oxidase or amine
oxidases
12.7. ábra - Figure 48: Oxidative deamination (amino acids)
During the process catalyzed by glutamate dehydrogenase enzyme glutamic acid transforms to α-keto glutaric
acid. The dehydrogenase enzyme is the only enzyme that operates under physiological conditions with
appropriate intensity. Glutamic acid has a central role in the storage the amino nitrogen by transamination.
Glutamic acid dehydrogenase enzyme has a role in the formation of nitrogen balance.
The amino acid oxidases catalyze the oxidative deamination of amino acids with using molecular oxygen as
acceptor and producing ammonia.
amino acid + O2 + H2O → α-ketoacid + NH3 + H2O2
Amino acid oxidases are flavoproteins. These enzymes are present in animal cells (for example: in endoplasmic
reticulum of the liver), appear in mushrooms, bacteria, but their occurence is not proved in higher ranked plants.
Plant amine-oxidases are less known, in the animal tissues mono- and diamine-oxidases are present.
amine + O2 → imine + H2O2
imine +H2O → aldehyde + NH3
In vertebrates the split amino-groups are emptied in the form of urea, uric acid or ammonium ion.
4.3.3. 12.4.3.3. The catabolism of carbon skeleton of amino acids in the
tricarboxylic acid cycle
The carbon skeleton of amino acids are broken down via citric acid cycle into CO2 and H2O (ATP is generated).
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Each of the 20 amino acids of proteins has a separate catabolic pathway. All 20 pathways converge into 5
intermediates, all of which can enter the citric acid cycle (Fig. 49).
• The acetyl-CoA pathway:
Alanine, glycine, serine, cysteine, isoleucine and thereonine are converted to pyruvate at first, then after
decarboxylation it transforms to acetyl-CoA.
Leucin, lysine, tryptopan, phenylalanine and tyrosine are converted into acetoacetyl-CoA at first, then it
transforms to acetyl-CoA.
Pyruvate converted from amino acids can also take part in the gluconeogenesis.
• The α-ketoglutaric pathway:
Glutamic acid, glutamine, proline, histidine and arginine are converted into glutamate that is then deaminated
by a transaminase and forms α-ketoglutarate that enters the cycle.
• Succinyl-CoA pathway:
Four amino acids are broken down through propionyl CoA and methyl-malonyl-CoA intermediers. It
transforms to succinyl-CoA that enters the cycle.
One part of the methionine, valine, threonine and leucine transforms through this pathway.
The sulphur content of methionine (when methionine is broken down) is built up into the cisteine.
• Fumarate pathway:
From a part of phenylalanine, tyrosine and fumaric acid aspartic acid forms which enters the cycle.
• Oxaloacetate pathway:
Aspartic acid and asparagine form oxaloacetate that enters the cycle.
The oxaloacetate can be the precursor of the glucose synthesis too.
12.8. ábra - Figure 49: Entering of carbon skeleton of amino acids the citric acid cycl
4.4. 12.4.4. Protein turnover
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Proteins are subject to continuous biosynthesis and degradation. The process is called protein turnover. Many of
the amino acids released during protein turnover are reutilized in the synthesis of new proteins.
In animals, protein intake can exceed the need for protein synthesis; the excess nitrogen is largely degraded.
Depending on the lifestyle and the development of an organization, the nitrogen coming from the amino acids
and other nitrogenous compounds can be excreted in different forms. Mammals excrete nitrogen in the form of
urea, reptiles and birds excrete as uric acid and fish excrete nitrogen as ammonia or as trimethylamine-oxide.
4.5. 12.4.5. Nitrogen excretion
4.5.1. 12.4.5.1. Nitrogen excretion in mammals, synthesis of urea (carbamide)
Those organisms can excrete most of their nitrogen as urea, whose liver contain arginase enzyme.
Urea is generated from two nitrogen atoms originated from two amino acids and from carbon dioxide in the
liver of mammals. The urea formation requires energy.
The first amino-group is incorporated from free ammonia. The ammonia is generated from glutamic acid in the
mitochondria. This process is catalyzed by glutamic acid dehydrogenase.
The first step in the formation of urea is the synthesis of carbamoyl phosphate from carbon dioxide and
ammonia by using two molecules of ATP. This process takes place in the mitochondrial matrix and is catalyzed
by the enzyme carbamoyl phosphate synthetase. The process is practically irreversible.
2 ATP + NH3 + CO2 + H2O → karbamoyl-phosphate + 2 ADP + Pi
This energy-rich group enters the cycle. In the second step, carbamoyl phosphate reacts with ornithine to form
citrulline. Ornithine can enter the mitochondrial matrix with the help of a specific carrier and there it is
connected to the carbamoyl group while inorganic phosphate breaks off. This reaction is catalyzed by carbamoyl
transferase.
Citrulline gets to the cytosol from the mitochondria. The cycle goes on there. Ornithine and citrulline are nonproteinogenic amino acids. They are present only in small amounts in mammals.
The second nitrogen of urea comes from aspartate, which reacts with citrulline to form argininosuccinate by
using ATP. This reaction is catalysed by argininosuccinate synthetase.
citrullin + aspartate + ATP → argininosuccinate + AMP + PPi
Aspartic acid ulinked to the carbonyl carbon atom of citrulline. The necessary energy for the reaction is
provided by ATP. This reaction is coupled with the hydrolysis of ATP to AMP.
In the next step, cleavage of argininosuccinate produces arginine and fumarate. This reaction is catalyzed by
argininosuccinate lyase. The fumaric acid returns to the mitochondria with the help of a carrier. Finally,
hydrolysis of arginine yields urea and ornithine. This reaction is catalysed by arginase.
arginine + H2O → ornithine + urea
Ornithine can get back into the mitochondria with the help of a transport system. The cycle is thus completed
(Fig. 50). Ornithine with taking up another carbamoyl phosphate is transformed to citrulline. Urea is transported
in the bloodstream to the kidneys then is excreted in the urine.
Synthesis of 1 mol urea:
2 NH3 + CO2 + 3 ATP + 3 H2O → urea + 2 ADP + AMP + 4 Pi
The excretion of urea is very energy-intensive process, but the ammonia, which is a cytotoxin, has to be
removed.
There is a relationship between the urea cycle and the citric acid cycle. The fumarate produced from
argininosuccinate in the urea cycle can enter the citric acid cycle.
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The aspartic acid originated from oxaloacetic acid as the starting compound of the citric acid cycle ulinks to the
citrulline in the urea cycle to form argininosuccinate.
12.9. ábra - Figure 50: Synthesis of carbamide
Mammals can excrete nitrogen as ammonium ion, too.
In the liver, ammonia is ulinked to the glutamic acid with the help of glutamate synthase to form glutamine. This
reaction requires energy.
glutamic acid + NH3 + ATP → glutamin + ADP + Pi
In the kidney, glutamine is cleaved hydrolitically by glutaminase to form ammonia, which is excreted in the
urine.
glutamine + H2O → glutamic acid + NH3
4.5.2. 12.4.5.2. Nitrogen excretion of birds and reptiles. Synthesis of uric acid
Birds and reptiles lack the arginase enzyme, so they are not able to synthesize urea, they excrete nitrogen as uric
acid.
The nitrogen arising in the process of deamination attaches to glutamate and gets into the liver with the blood
stream. The glutamine will be the one of the precursors of the purin-based uric acid in the liver. The synthesis of
purine skeleton is the sum of complex processes. Inosinic acid formed during reactions is a nucleotide
containing purine skeleton, which is considered as a direct precursor of uric acid.
The synthesis of uric acid is shown in the following equation:
2 glutamic acid + 2 formic acid + CO2 + aspartic acid + glycine + 6 ATP→
→ inosine + 2 glutamic acid + fumaric acid + 5 ADP + AMP + 5Pi + PPi
The organization uses 6 mol ATP to the synthesis of 1 mol uric acid, in which there are 4 mol N. For the
excretion of 1 mol ammonia (N) as carbamide or as uric acid 1,5 mol ATP is required.
4.6. 12.4.6. Disturbances of amino acid metabolism
The amino acid metabolism is regulated similarly to the other metabolic processes. The inherited metabolic
diseases might be caused by the deficiency or pathological overfunctioning of an enzyme. Enormous number of
(~ 80) hereditary anomalies are known by the researchers. One of the best-known amino acid metabolic disorder
is the phenylketonuria (PKU). Phenylketonuria is an autosomal recessive trait characterized by a mutation in the
gene of the phenylalanine hydroxylase. Those people whose phenylalanine-4-monooxygenase enzyme is
missing or works incomplete, are not able to break down the phenylalanine. Due to a lack of this enzyme
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phenylalanine can not transform into tyrosine, thus catecholamines are not formed in further changes. The
catecholamines are necessary for the development of nervous system. The phenylalanine is enriched in tissues,
than goes through the transformation processes, which anomalous products (phenyl pyruvate, phenyl-lactate,
phenyl acetate) are excreted in high dose in the urine.
The operating condition of the phenyl-hydrolase enzyme is the tetrahydrobiopterin, which is formed in the
process catalyzed by dihidrobiopterin reductase. In the absence of this enzyme the phenylketonuria can also
develop.
The weight of brain of patients with phenylketonuria is small, their lifetime is short (max.: 20 years). Treatment
of phenylketonuria includes the elimination of phenylalanine from the diet.
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13. fejezet - 13. OTHER
BIOCHEMICAL PATHWAYS
1. 13.1. The biochemical bases of the function of
skeletal muscle
The energy required for the functioning of muscles is obtained from biochemical processes. During muscle
contraction the glycogen concentration of muscle decreases and the lactate and phosphate contents of venous
blood flowing from muscle rises. For contraction, the muscle obtains energy not directly from the
decomposition of glycogen, but from ATP.
From the energetics viewpoint, important part processes taking place in the muscle are the following:
• By the hydrolysis of ATP, ADP and Pi are formed. This supplies energy for the contraction.
• The ADP obtains energy from the creatine phosphate for the resynthesis.
KrP + ADP → Kr + ATP
From the breakdown of 1 mol of creatine phosphate at most half mole of ATP can be resynthesized.
• For the resynthesis of creatine phosphate and ATP the energy is supplied by the breakdown -in anaerobic
conditions- of glycogen.
• For the resynthesis of glycogen the oxidation of pyruvic acid -aerobic- supplies the energy.
The majority of lactic acid originated from the breakdown of glycogen transform back into glycogen in the Cori
cycle. In the muscle beside the oxidation of carbohydrate, fat-burning is also going on.
The muscle contraction is a series of reactions, which take place between actomyosin, ATP and certain ions.
During the contraction of the muscle calcium ions are released. During relaxation of muscle chelate-like
compounds are released, which bind calcium ions to form complexes.
2. 13. 2. Factors influencing the quantity and quality
of the urine
The healthy kidneys are able to dilute or concentrate the urine in a wide range. The role of kidneys is to keep the
proper balance of salts, acids and water and to ensure the osmotic potential in the body. The quality and quantity
of urine is determined by three factors, the filtration, reabsorption and secretion.
The composition of ultrafiltrate formed in glomeruli is practically identical to the composition of blood plasma,
but does not contain a higher molecular weight protein. Its daily volume is 180 litres. Two kinds of processes
take place in the tubules, reabsorption and secretion. Reabsorption can be made by active and passive transport.
Glucose, phosphate ion, amino acids, bicarbonates, uric acid, sodium-, potassium- and chloride ions are
reabsorbed by active transport. Urea is reabsorbed by passive transport (from higher concentration to lower
one). Secretion is a result of active cell work.
In case of thirst, the decreasing uptake of high amount of liquids increases the water retraction of kidneys. As a
result, the quantity of the blood moving on the blood circulation hardly changes. The concentration of urine is
also influenced by hormonal factors. The hormonal effect upon the adiuretin (ADH) is responsible. The
adiuretic hormone (ADH), produced by hypothalamus and stored in the posterior lobe of hypophysis, is also
responsible for hormonal effect. With increasing water intake, the blood plasma is diluted, the osmotic
concentration decreases, thus the production of ADH also decreases. The urine becomes more dilute. In the
thirsty body the plasma thickens, the production of ADH increases. In the distal tubules ADH increases the
reabsorption of water, thus excreted urine becomes concentrated. The epithelium of tubules of kidneys is
impermeable to water molecules. ADH makes the epithelium more permeable to water.
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Kidneys play important role in the regulation of blood volume, extracellular volume and acid-base balance. In
the regulation of blood volume, the ADH is involved as previously described. When protein concentration of the
plasma decreases, the osmotic pressure of blood also decreases, this is perceived by osmoreceptors and thus
enhanced ADH secretion is induced.
The reabsorption of sodium ion is directed by hormone aldosterone. If the sodium concentration decreases in the
extracellular space, the osmotic potential decreases. The water flows into the intracellular space. The increase of
sodium ion concentration in cells enhances the water flow from intracellular space into extracellular space.
The amount of aldosterone is set by the renin-angiotensin system. Aldosterone in the kidneys increases the
reabsorption of sodium ions and enhances the excretion of potassium ions.
Kidneys play role in the regulation of pH and composition of body fluids. The blood pH varies between narrow
limits: from 7.35 to 7.45. In maintaining of this value, buffer systems in fluid spaces, the respiratory centre and
the kidneys are also involved. Such buffer systems are protein buffers and bicarbonate buffers. The hemoglobin
in the context of carbon dioxide transport is able to neutralize a significant amount of acid, thus is able to
influence the pH. The phosphate buffer is significant in tubules of kidneys and in the intracellular space.
Respiratory centre located in the medulla is able to detect the blood pH, thus in the case of acidification
respiration increases, while with increasing of pH respiration decreases it.
Kidneys excrete acidic or alkaline urine depending on the pH value of blood. Urine pH can vary from 4.5 to 8.5.
The plant food alkalizes the urine. In proximal tubule cells carbonic acid is formed from carbon dioxide by the
enzyme carbonic anhydrase, which immediately dissociate to bicarbonate and hydrogen ion. The hydrogen ion
gets into the lumen of the tubule, where forms carbonic acid with bicarbonate, and after that carbonic acid
dissociates to carbon dioxide and water.
Water excretes in the urine, carbon dioxide diffuses back to the tubule cells. In the distal tubule cells, ammonia
is produced from glutamine with cleaving of amino group by glutaminase. The ammonia diffuses to the lumen
and there it is united with hydrogen ion to form ammonium and is excreted in the urine.
The biochemical mechanism of excretion of hydrogen-ion and potassium-ion is common. The excretion of K+
and Na+ through kidneys is related to the concentration the hormone aldosterone. The excretion of Ca 2+ and
HPO42- depends on the hormone production of parathyroid glands.
3. 13. 3. The gastric juice and its separation
The pH of gastric juice is 0.9-1.5. Gastric juice contains three types of proteolytic enzyme, the pepsin (the pH
optimum is between 1.5 - 2), the rennin or chymosin (the pH optimum is 5.35) and cathepsin (the pH optimum
is between 3.0-5.0). It contains trace amounts of fat-degrading enzyme, lipase and also contains mucin.
The pepsin is released by the chief cells of stomach as an inactive form zymogen, pepsinogen. Hydrochloric
acid, which is released from parietal cells in the stomach lining activates pepsinogen to pepsin. Chymosin
converts caseinogen (milk protein) into insoluble casein in the presence of calcium ions.
3.1. 13. 3. 1. The mechanism of the hydrochloric acid production
of the stomach
The mechanism of hydrochloric acid production by parietal cells is still not fully understood. The parietal cells
are able to concentrate the hydrogen ions of the blood. When acid secretion is stimulated the outflow of
hydrogen-carbonate into venous blood leaving from a stomach lining results in an elevation. The activity of
carbonic anhydrase of the parietal cells increases.
The essence of Davenport theory of gastric acid production is that parietal cells decompose water contained
therein to hydrogen and hydroxide ions.
The carbonic anhydrase enzyme catalyses the reaction between carbon dioxide and hydroxide ions originated
from water to form bicarbonate ions.
The hydrogen ions are secreted actively by using ATP from the cytoplasm of parietal cells and mixed in the
canaliculi.
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The energy obtained may be derived from the oxidative degradation of glucose. The bicarbonates get into the
blood, but chemically equivalent amount of chloride ions get from the blood plasma to the cell. The migration of
chloride ion (so far) is passive, requiring no power process. The central role of carbon anhydrase is supported by
the fact that substances, which retard the activity of the enzyme also inhibit the formation of hydrochloric acid
(Fig. 51).
13.1. ábra - Figure 51: Steps of gastric acid secretio
In terms of the secretion of gastric juice, two states can be distinguished, the basal phase and the digestive
phase. In the basal phase variable amount of hydrochloric acid- and pepsin are being secreted. This is called
basal secretion. In the digestive phase, the gastric juice production adapts to the quality of the food. This is
ensured by a neuro-humoral regulation.
An example is that if you eat meat, large amount of gastric juice is produced, wherein the amount of
hydrochloric acid is high, while the amount of pepsin is medium. If we eat bread, moderate amounts of gastric
juice is produced, which contains little hydrochloric acid and a lot of pepsin.
4. 13. 4. The control of metabolic processes
4.1. 13. 4. 1. The control of lipid metabolism
Lipid metabolism is in strong connection with glucose catabolism, it depends on the carbohydrate supply of the
body. The liver plays a major regulating role in the process of metabolism. Fats are the only nutrient forms that
can avoid the controlling function of the liver, because they can get into the lymphatic system and can get into
the adipose tissue directly.
The mobilisation of the stored fat from the adipose tissues is under hormonal regulation. Adrenaline, glucagon
and ACTH ulink to the membrane of the fat tissue. This initiates the activation of the adenylate cyclase system.
The intracellular lipase is activated by adenylate cyclase-cAMP system. This enzyme catalyzes the hydrolysis of
triglycerides stored in fatty tissue. The reaction products get into the site of application via circulation. Fatty
acids are transported by serum albumin in the circulation.
A large amount of acetyl groups originated from the β-oxidation of fatty acids decomposes to carbon dioxide
and water in the citric acid cycle and in the terminal oxidation. This process is in control of respiration that
works in the citric acid cycle and in terminal oxidation.
If there is large amount of ATP and small amount of ADP in the system the resulting energy is not utilized in
anabolic processes by the body. Then the high concentration of ATP inhibits citrate synthetase, isocitrate
dehydrogenase and α-ketoglutaric acid dehydrogenase enzymes allosterically. The activity of these enzymes
decreases, the citric acid cycle slows down.
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If a large amount of food gets into the organism the cell glucose concentration increases and its degradation
becomes faster. The quantity of acetyl-CoA generated through pyruvic acid increases. The resulting acetyl-CoA
concentration can become so large that it exceeds the absorption capacity of citric acid. This has an inhibitory
effect on the pyruvate dehydrogenase enzyme, thus a part of acetyl-CoA gets into the cytoplasm from the
mitochondria as citric acid. The fatty acid synthesis requires large amount of acetyl-CoA and NADPH + H+.
NADPH + H+ are only formed in a few enzyme catalyzed processes. These are malate, isocitrate dehydrogenase,
glucose-6-phosphate dehydrogenase enzyme-catalyzed reactions. The lack of NADPH + H+ can influence the
fatty acid synthesis.
The fatty acid synthesis can be influenced by the formation of malonyl-CoA. In the process, the carboxylation of
acetyl-CoA is catalyzed by acetyl-CoA carboxylase enzyme. This enzyme with acyl-ACP synthetase that is the
central enzyme in the synthesis is allosterically regulated.
These enzymes are activated by citric acid and isocitric acid in large amount, while they are inhibited by acylCoA derivatives.
4.2. 13. 4. 2. The function of adenylate cyclase - cAMP system
Hormones influence the pathways of biochemical processes catalyzed by enzymes. They exert their effects in
two ways.
1. They regulate gene activity. They interact with repressor molecules that inhibit the function of DNA in the
nucleus, thus the synthesis of enzyme proteins is started.
2. They activate adenylate cyclase system. In this case, they connect to the cell membrane and induce the
synthesis of secondary messenger compounds that activate the enzymes.
In both cases, they act by enzymes; they change the enzyme concentration or activity.
1. During gene activation, steroid hormones initiate enzyme protein synthesis in the nucleus of target organ
cells. The non-polar steroid hormones are able to pass through the target cell membrane. In the cytosol they
are bound to a steroid-specific receptor.
The formed hormone-receptor complex is able to enter the nucleus through the nucleus membrane. In the
nucleus the DNA chains that presented as chromosomes are surrounded by proteins. These repressor proteins
prevent the binding of DNA dependent RNA polymerase and thus they also prevent the mRNA synthesis as
DNA dependent RNA polymerase initiates the mRNA synthesis.
To construct a particular protein (e.g. an enzyme that catalyzes a biochemical process) the hormone
transforms to a hormone receptor complex in the cytosol of the cell, enters the nucleus and there cleaves
repressor protein from the DNA molecule containing the code for the protein synthesis (at DNA stage that
starts mRNA transcription). The RNA polymerase is then able to bind to the chain. The mRNA synthesis can
start. The hormone can only start up the transcription of DNA to mRNA, the RNA polymerase activation in
such a form that binds to the receptor. The outcome of hormonal effect is the messenger RNA transcription
that is necessary for protein synthesis. Rewriting of information carried by the mRNA into protein takes
place in the ribosomes.
2. The hormone attaches irreversibly to the hormone-receptor proteins localized in the membrane of the target
cell. As a result, the configuration of the receptor changes, thus it is able to bind the protein type carrier
molecules on the inside surface of cell membrane (G protein). As a result of connection, G-protein undergoes
conformational changes. Due to the altered conformation, it can connect to enzymes that catalyze synthesis
of secondary messengers.
The adenosine 3’, 5’ cyclic-monophosphate (cAMP) is a secondary messenger that is formed by adrenaline
hormone. The cAMP is synthesised from ATP by the enzyme adenylate-cyclase with a loss of pyrophosphate.
The cAMP is also called secondary messenger.
The main role of cAMP is the activation of protein kinases. It also influences membrane transport processes.
Protein kinases activate other proteins by chemically adding phosphate groups to them.
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By cleaving macroergic phosphate groups, protein kinases attach phosphate groups to proteins that become
active.
The cAMP provides connection between hormones that transfer stimulus as chemical information and enzymes
that catalyze the metabolic processes directly.
After the termination of the hormonal effects, the unnecessary cAMP molecules hydrolyse to AMP by the
enzyme, phosphodiesterase.
Phosphodiesterase enzyme is inhibited by methylated xanthines, caffeine, theobromine thus the effect of the
hormones is elongated in the presence of them.
The cAMP influences the permeability of cell membranes and the transport processes.
4.2.1. 13.4. 2. 1. The presentation of adenylate-cyclase system operation
through the mobilization of glycogen
Adrenaline is able to mobilize glucose from the stored glycogen quickly.
The effect of this hormone prevails through a mediator, by increasing the activity of the phosphorylase. The
liver cells respond to the signal of adrenaline and glucagon, while the muscle cells respond to adrenaline
hormone. Adenilate-cyclase connected to the membrane of the cell is activated. One part of ATP transforms to
cAMP in the cell as an effect of protein kinase. Activation occurs allosterically.
Protein kinase activates phosphorylase kinase by using ATP. Phosphorylase kinase is sensitive to the
concentration of Ca2+ ions as well. This is another controlling factor in the muscle. Muscle contraction is started
up by Ca2+ ions released as an effect of nerve impulses.
In addition to contraction, they also affect the operation of the phosphorylase kinase.
The active phosphorylase kinase converts the inactive phosphorylase-B to active phosphorylase-A. Two
phosphorylase B molecules (dimer) form a tetramer in the muscle and become active in this way. In the liver,
both of them are in dimeric form (Fig. 52).
13.2. ábra - Figure 52: cAMP system
The reduction of activity is carried out by phosphorylase phosphatase; it transforms the active form A into
inactive form B. The synthesis of glycogen is controlled by a system that is similar to the previous one, but it
has an opposite effect.
4.2.2. 13. 4. 2. 2. Hormone control of carbohydrate metabolism
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Complex carbohydrates are broken down in the process of digestion to monosaccharides that transform to
glucose. In the small intestine glucose is absorbed and get into the bloodstream. Glucose in the blood is called
blood sugar.
Glucose molecules get into the tissues via bloodstream, where they become the main energy supply carrier of
the cells. Some tissues, such as brain tissue do not have oxidizable carbohydrate reserves, thus they are reliant
on blood glucose very much.
Carbohydrates from food sources and the body need to determine what kind of processes the glucose steps into.
The conversion speed of carbohydrates in the living body is high although mammals have constant blood sugar
level. In humans it ranges from 4.0 to 5.5 mmol / l. If the body wants to utilize blood sugar as energy, it
decomposes it in the cells and depending on the circumstances, it transforms to lactic acid or pyruvic acid.
Excess glucose is stored as glycogen in the liver and muscles, which become available later. The endocrine
system keeps blood glucose level at a constant level. The glucose originated from foods, digested and absorbed
from the gut can get into the blood and can cause increased blood sugar level. This compound might also come
from the breakdown of glycogen stored in the liver. Sugar generated from the breakdown of amino acids, lactic
acid and oxaloacetic acid by gluconeogenesis also may increase the blood sugar level.
The glucose concentration of blood can be reduced by the sugar utilization and oxidation of muscle and other
tissues. The blood sugar level is also reduced when glycogen is synthesized in the liver and muscle. The
formation and the storage of fat, produced from glucose - which takes place in the liver and in adipose tissues can also reduce the blood sugar level.
The regulation of the processes mentioned above is performed by several hormones collectively (Fig. 53).
Insulin has central role in the regulation, which reduces the blood sugar level. Insulin promotes the uptake of
glucose by most cells and also helps the breakdown of it. Insulin enhances the formation and storage of
glycogen in the muscles. This hormone increases the synthesis of fats in the liver, inhibits the breakdown of
glycogen in the liver to glucose and also inhibits the release of glycogen into the bloodstream.
Glucagon is secreted by the pancreas effects on the process of raising the blood sugar level. This hormone
enhances the breakdown of glycogen in the liver and helps the release of glucose into the blood stream.
Glucagon enhances the resynthesis of glucose from lactic acid in the liver.
Adrenalin stimulates the increase of blood sugar level, because it enhances the breakdown of glycogen into
glucose in the liver. Adrenalin promotes the breakdown of muscle glycogen into lactic acid. The resulting lactic
acid may be a precursor of resynthesis of glucose.
Cortisol raises the blood glucose level, because it increases the glucose resynthesis from amino acids in the
liver. Cortisol enhances the amount of required amino acids, because this hormone inhibits protein synthesis,
increases the breakdown of proteins in the muscles.
Somatotropin (growth hormone) decreases the blood glucose levels, because enhances the synthesis of
proteins. It inhibits the conversion of amino acids to glucose.
Gluconeogenesis is controlled by adenohypophysis by its hormonal activity (ACTH, STH) in the body.
Thyroxine, LTH (luteotropin) and androgens can also affect the carbohydrate balance.
13.3. ábra - Figure 53: The outline of the neourohormonal control of the carbohydrate
metabolism and the blood-sugar level
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Hypoglycemia (decreased blood glucose level) is an abnormally diminished content of glucose in the blood. The
cause of hypoglycemia might be the damage of a central nervous system. Hypoglycemia can cause
unconsciousness, which might be irreversible. The hypoglycemia induces sympato-adrenal symptomes.
Hyperglycemia is a condition in which an excessive amount of glucose circulates in the blood. Because of the
excitement of the sympato-adrenal system, the glucose and lactic acid content of blood inrease. This occurs due
to mobilization of compounds from depots and due to enhance of gluconeogenesis. The sucking capacity of
kidneys decreases, glucose is secreted in the urine. Thus, a biologically important compound is lost.
The glucose transport between organs and its hormonal regulation are shown in 54 Figure.
13.4. ábra - Figure 54: The glucose transport between organs and its hormonal
regulation
5. 13. 5. The role of liver in the intermediate
metabolism
The liver plays a central role in the intermediate metabolism like gluconeogenesis and glycogen metabolism.
Liver is the scene of anabolism, catabolism and storage of glycogen. The liver cells are only able to convert
galactose to glucose. Like other cells, liver cells are also able to isomerize fructose into glucose. The lactic acid,
glycerol, oxaloacetic acid and other intermediates also are converted into glucose in the liver. The
gluconeogenesis or conversion of amino acids to glucose takes place in the liver.
Liver plays a role in the lipid metabolism:
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• Ketone bodies are produced here from large amounts of acetyl-CoAs originated from degradation of fatty
acids.
• The synthesis of plasma-phosphatides takes place in the liver. Plasma-phosphatides are important during
transports of lipids from the liver to the plasma.
• Cholesterol is also formed in the liver.
• Bile acids formation takes place in the liver. Bile acids react with taurine and glycine here.
• The storage of carotene takes place here. Carotene is converted to vitamin A in the liver.
The liver plays crucial roles in nitrogen metabolism, too. It has central role in the metabolism of amino acids
and in the maintenance of dynamic balance of proteins. The synthesis of urea and trans-amination, deamination
processes of amino acids also take place in the liver. The trans-amination and deamination processes create new
amino acids. Without these reactions, the gluconeogenesis does not occur and ketone bodies are not formed.
Without the work of the liver amino acids accumulate and are excreted in the urine. The formation of choline
and creatine are also take place here. Some specific plasma proteins, albumins, globulins, prothrombin,
fibrinogen and the majority of coagulation factors are also synthesized in the liver. Because of this liver is
important organ of blood clotting and defense mechanism of the body. The majority of uric acid also forms in
the liver.
Liver has important role in the porphyrin metabolism:
• The breakdown of hemoglobin to bilirubin is localized in the liver. (This process takes place in the spleen and
in the bone-marrow, too).
• The excretion of bilirubin esterified with glucuronic acid occurs in the bile into the intestine. The
esterification reaction takes place in the liver.
• Liver is also a detoxification organ.
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14. fejezet - 14. BIOCHEMICAL
PATHWAYS IN THE FOOD INDUSTRY
During the storage and treatment of food raw materials produced in large quantities by food processing
industries for human and animal nutrition, biochemical processes may take place which might be favourable and
unfavourable.
The biochemical processes can be or must be influenced, enhanced or reduced by changing of the
circumstances.
One of the basic processes is the fermentation, which in the narrow sense refers to anaerobic catabolism of
carbohydrates. The end product of this process is lactic acid or alcohol and carbon dioxide.
The fermentation in the food technology is a common pathway, which is necessary in order to obtain the desired
quality of product. The processes that take place spontaneously can adversely affect quality, therefore it is
essential to avoid them.
1. 14. 1. The application of the fermentation in the
food industry
The fermentation is carried out by enzymes produced by microbes, or fermentation is occurred by exposed
tissue enzymes. During the fermentation flavours may also develop. This is also true for yeast, lactic acid
fermentation. The hetero-fermentative lactic acid bacteria also produce more flavour components. The resulting
acetic acid and acetaldehyde are major flavour components of dairy products. Yeast produces mainly ethanol as
the end product of the fermentation, but they are also able to convert amino acids by trans-amination and
decarboxylation reactions. In these processes alcohols, aldehydes, acids and esters are formed.
The cocoa beans, coffee before roasting, the tobacco and the tea are fermented before further transformation.
During tea fermentation tissue enzymes work. The fermentation of yellow tea is shorter than black tea’s one. In
the case of green tea, there are not fermentation processes.
The lactic acid fermentation is known as microbiological preservation procedure for centuries.
Acidification of cabbage, cucumber happens with lactic acid fermentation. In this process, lactic acid bacteria
produce lactic acid from the sugar content of raw material used to preserve. At the 0,7 to 1% lactic acid content
the activity of lactic acid bacteria and other microbes ceases, so the product can be stored for a long time.
At the production of alcoholic beverages (beer, wine, brandy) fermentation processes can take place. At the
alcoholic fermentation various yeast strains are used, which convert the fermentable sugars of liquid into
alcohol.
The reaction sequence of the process is: hexose → pyruvic acid → acetaldehyde → ethanol.
The formation of pyruvic acid to acetaldehyde is conducted by yeast during anaerob metabolism. This process is
catalyzed by pyruvic acid decarboxylase. Then acetaldehyde is reduced to ethanol by the NAD-specific alcohol
dehydrogenase enzyme.
The alcohol content of wines and spirits is originated from the sugar content of fruit juice of ripe grapes, fruits.
Beer is brewed from barley. Barley contains starch that has to be broken down to simply sugars before the yeast
can make alcohol. Therefore malting step is needed. Malting is the process in which inactivated enzymes
become active again and degrade starch into fermentable sugars.
2. 14. 2. The biochemical processes of cereals
germination
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Germination replaces the resting stage that is fixed duration and is due to external or internal circumstances. In
seed germination, characteristic and fast reactions take place, plant hormones work intensively.
The enzyme activity increases, so the breakdown and usage of reserved nutrients are started. The changes
concern the seed coat, the nourishing tissues and the germ as well. There are active energy and material circles
between them.
In the first stage of germination, the seed swells greatly and germ reactivation occurs. In order to start the
process appropriate temperature, humidity and oxygen are required. At this stage macromolecules, cell
organelles and phytohormones are reactivated. Due to the swelling and water absorption, the dormant enzymes
are also activated. The biochemical processes start. The oxidation processes of free sugars, amino acids occur.
The protein synthesis starts as well.
In the second stage of the germination, which clearly can not be separated from the previous stage, the
mobilization of reserved nutrients occurs. With the help of activated enzymes mobilization and hydrolysis of
starch, storage proteins, lipids, phosphates take place. For faster processes a large amount of hydrolase enzyme
is needed. At this stage, the system is able to synthesize this enzyme. The stored energy in the form of lipids is
converted into monosaccharides.
3. 14. 3. Respiration during storage
During the storage of foods, the cells respire, so quality may change. The breathing process mainly refers to the
breakdown of carbohydrates to carbon dioxide and water. In aspect of food industry, the carbohydrates
decomposed during respiration counts losses. There is an important requirement during food processing and storage to minimize the breathing losses. Many factors affect the intensity of respiration, such as external
conditions (temperature, humidity, atmosphere composition), the varieties and agrotechnical factors. The
oxygen concentration influences the carbohydrate degradation speed as well beside the direction of the process.
3.1. 14. 3. 1. Respiration of grain during storage
Oligo- and monosaccharides with low molecular weights required for respiration are found in grain in small
amount, or originated from degradation of reserved polysaccharides. In the dormant state of grain the βamylases are the most active amylolytic enzymes, they produce maltose. There can also be found other sources
(glucose, fructose, maltose, galactose, sucrose, raffinose) in the grain for respiratory processes, and their supply
may also come from degradation of several reserved compounds.
The respiration rate of grains is mainly affected by the moisture content, temperature, oxygenation and the lives
of the seeds.
The increase of the moisture content of germ enhances the respiration, the function of glutamic acid
decarboxylase increases. The quantity of microbes also increases.
With the increasing of the temperature, the velocity of the respiration enhances. Its effect is influenced by the
oxygen supply and the concentration of CO2. The anaerobic fermentation may occur under a particular oxygen
concentration level.
The production, ripening, harvesting and drying processes of grains might be called “the life of the grain”,
collectively.
Overall, we can say that the fuller ripening, the storage on the dry place reduces the velocity of the breathing.
3.2. 14.3.2. The respiration of fruits and vegetables
Fruits and vegetables like other foodstuffs of plant origin respire during storage. There might be considerable
differences between the species (even 50%) in the respiration intensity. The breathing intensity changes during
the growth of the fruit, too. Increased respiration intensity decreases initially then approaching the ripening
stage the respiration rises again.
In the case of vegetables such phenomena can not be detected. The respiratory intensity of vegetables grows
steadily and in the end of storage is stronger than in the beginning of the picking and storage. Different tissues
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have different respiration. The respiratory intensity of dermal tissues is much greater than vascular and ground
tissues. This can be explained by improved oxygen supply and greater enzyme activity (oxido-reductase). The
increase of temperature both in case of fruits and vegetables can also improve the breathing.
The temperature of storage can influence the biochemical processes in varying degrees, as a result we get the
right quality stored product. The optimum storage temperature depends on the species, variety and the maturity.
The potatoes become sweet stored at low temperature, its pleasure value decreases, and the non-enzymatic
browning increases. During storage the amount of mono- and oligosaccharides, especially sucrose greatly
increases. The formation of mono- and oligosaccharides and change of respiration rate are different depending
on the temperature.
The increase of sugar content depends on the balance of the next biochemical processes:
• breakdown of starch into mono- and oligosaccharides,
• resynthesis of starch from mono- and oligosaccharides,
• formation of sucrose from monosaccharides,
• respiratory process.
The ability of the resynthesis decreases with the time of the storage and decreasing of the temperature mostly.
Thus increases the concentration of mono- and oligosaccharides.
The oxygen and carbon dioxide tension of containers significantly affects the intensity of respiration. The
decrease in oxygen and increase in carbon dioxide reduces the respiration. The decrease of oxygen
concentration below the critical level favours the anaerobic breakdown of sugar in the case of fruits and
vegetables, which can lead to deterioration in quality.
3.3. 14. 3. 3. The ripening of fruits
The ripening is the process by which a variety of fruit reaches the proper size and color, develops the taste,
flavor and aroma. During ripening the fruit becomes suitable for consumption or industrial processing.
During ripening the ground color of fruit peel becomes yellow from green, the cover color becomes red or blue
from yellow.
The hardness of flesh can also change during this process. The ripe fruit is generally flexible, soft. Most of the
unripe fruits are sour, bitter, while ripe fruits are sweet or slightly sour. The ripe fruit has a noticeable
characteristic flavor as well.
Among the fruits, there are some, which are able to continue ripening after being picked. These fruits depending
on their ripening stage will ripe within longer or shorter period. Its condition is that a certain degree of maturity
of fruit must have attained before its removal from the plant.
Such fruits are apple, pear, quince, apricot, peaches and strawberries. During the ripening, process the
metabolism of the fruit changes. Many enzyme activity increases. The starch is converted into sugars, acids are
converted to pyruvic acid then to carbon dioxide and water. The fruit becomes sweeter and slightly sour. The
cellulose and protopectin content of fruit decrease and proportionally the amount of dissolved pectin increases.
The protein content increases as well.
The chlorophyll begins to decompose, but formation of carotenoids, anthocyanins and flavourings start. These
processes are followed by the change of permeability of cells and cell organelles. In case of most fruits during
the ripening the respiration and CO2 production increases, that can only slow down at the aging stage.
The fruits can be classified based on their intensity of respiration at the ripening stage. As citrus fruit ripens
CO2 production reduces. The bananas and tomatoes are characterized by a gradual increase in CO2 production
and the mature state occurs after the maximum CO2 production.
In the case of third group (peach and strawberries) the CO 2 production increases, but the mature state of fruit
occurs at the maximum CO2 production.
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Ripening agent, ethylene produced in the fruit speeds up the ripening process.
In foods during their storage and processing, often brown pigments, flavours are developed. Brown pigments
may be developed through either enzymatic (enzymatic browning) or non-enzymatic processes (non-enzymatic
browning (NEB)).
The non-enzymatic browning (NEB) means that the interaction between free amino group and reducing sugar,
under certain circumstances, causes the browning. In that process aroma compounds and brown pigments,
melanoidins are produced. The reaction is promoted if the mole proportion of the sugar and amino-compounds
is 3:1, the temperature is high and the pH is slightly acidic, neutral or alkaline (pH 3-11). In order to run the
reaction at least 10% water content is required.
Enzymatic browning
In this process, melanins are formed by the certain enzymes of body tissue. The melanins may be also a normal
life functioning products: pigments of hair, fur, feathers. Often, due to injuries like peeling or pressing of fruits,
vegetables are formed these pigments.
In the damaged cells, due to the damage of endoplasmic network substrates come into contact with enzymes
being not in contact before.
The enzymes are called polyphenol oxidases (PPO). These enzymes contain Cu 2+ ions, which are activated by
reduction of the copper. Enzymes can bind molecular oxygen and can oxidize monophenols to O-diphenols. In
the meantime, the copper is reconverted to Cu2+ and becomes inactive again. With the oxidation of O-diphenol
to quinone the enzyme is activated again. The enzyme has a phenol hydroxylase and polyphenol oxidase
activity.
The substrates may be tyrosine, catechins, flavones, cinnamic acid, leukoantocianidins, etc. The enzymatic
browning can also be beneficial at the fermentation process of tea and cocoa. In adverse cases, the process may
be deferred and inhibited. In such cases the inactivation or inhibition of enzymes to be solved. Inactivation most
easily can be done by heating. The process can be delayed by the exclusion of the air or by application of
reducing agent.
The reducing of browning tendency can be achieved by such variety of plant cultivation, in which the enzyme
activity is low or the substrate concentration is low.
4. 14. 4. The biochemistry of meat ripening
The main function of muscle tissue in the living animal is to ensure the mechanical work. To do this, a large
amount of energy is required, so it is important to ensure adequate energy supply in the muscle.
Direct source of energy is the ATP, which is derived from the breakdown of carbohydrates. In the case of large
and sustained muscle work in the energy supply the role of lipids are also appreciated. The muscles contain
large amounts of creatine phosphate, which is involved in the ATP supply with the help of creatine kinase.
After the animal has been slaughtered, the conditions of the biochemical processes are changed in the muscular
tissue. The connection of muscular tissue with the liver breaks up, and there will be no nutrient supply any
longer. The muscle does not receive fresh blood, so that the oxygen-supply breaks off. The process has taken
place in the changed circumstances is called meat ripening.
The final product, which undergoes chemical changes between changed circumstances, is called meat.
In practical terms, the post-mortem transformations can be divided into three phases. The first stage is the stage
before rigor mortis. The meat is soft yet, the quantity of the ATP and the creatine phosphate decrease, the
anaerobic processes start.
The second stage is the stage of the rigor mortis. The glycogen in the muscle is converted into lactic acid
causing a fall in pH, muscles become stiff or rigid one known as muscle rigor, proteins are denatured.
The third stage is the stage of the post rigor mortis. The meat becomes tender again. In secondary processes,
the flavour and organoleptic characteristics improve (Fig. 55).
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14.1. ábra - Figure 55: The biochemistry of meat ripening
After slaughtering of animal the oxygen supply of muscle tissues stops. Due to the lack of oxygen the function
of mitochondrial electron transport chain is not possible. ATP is formed only in the anaerobic glycolysis, but its
quantity is only a fraction of that produced under aerobic conditions. When the ATP decrease reaches a
threshold that is no longer sufficient to inhibit actin-myosin connection, rigor mortis occurs.
In the absence of ATP the condition of muscle relaxation can not come to be. Lactic acid is reduced from
pyruvic acid -under these circumstances- originated from glycolysis. Lactic acid is not able to get out of the
meat, so it is accumulated. The acidic pH is shifted to neutral, the pH reaches 5.3 to 5.5 value. These
biochemical processes lead to changes in the consistency and the water absorbing capacity of meat
The rigor mortis disappears after a while because some proteolytic enzymes, cathepsins operate under such
conditions.
The membranes are damaged in the state of rigor mortis, the enzymes can get out and can expose their effects.
The changes in consistency may be related to the ions redistribution within the protein system.
The meat ripening processes are influenced by those effects (pre-slaughter effects) which may influence the
glycogen content of the muscle tissue. Such effect is the stress as well.
Maturation can be accelerated – it is often used in practice – with proteolytic enzyme preparations. In meat
industry, it is used for beef because it has very hard consistency.
Earlier there were often used proteolytic enzymes with plant origin such as papain and phycin, but today
microbial enzyme preparations are rather used.
After slaughtering the negative effects of cooling can be reduced by electrical stimulation. In this case, the
shortening of muscle strength occurs (cold shortening phenomenon). If within one hour after slaughtering the
meat is treated from 1.5 to 2 s time period at a voltage of 500-600V, the shortening can be reduced.
5. 14. 5. Changes of colour through the meat
processing
The colour of meat indicates the freshness of meat for the customers. Today, the inclusion of additives give
meat prolonged intense red colour. These added compounds (KNO 2-KNO3) prevent natural biochemical
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processes. The colour (intensity and brightness) of meat is based on the quantity of principal muscle pigment—
myoglobin. Myoglobin has a protein portion, globin (96%) and a heme portion (4%). Original colour is crimson.
The colour change of myoglobin is due to heme can bind different groups (ions). Each heme residue contains
one central coordinately bound iron atom that is in the Fe2+ (ferrous) oxidation state. The Fe2+ is being chelated
with six ligands. Heme is a square planar molecule containing four pyrrole groups, whose nitrogen form
coordinate covalent bonds with four of the iron's six available positions.
One position is used to form a coordinate covalent bond with the side chain of a single histidine amino acid of
the protein. The sixth bond is an unstable connection with water, which can be exchanged with other ion groups
or ions. In living organisms, where the oxygen partial pressure is high, the sixth bond is a connection with
oxygen. The resulting complex is called oxy-myoglobin that has bright red colour. If instead of oxygen, carbon
monoxide is bound, it is called carboxy-myoglobin, and when nitrogen monoxide is bound, it is called nitrosomyoglobin. Both forms arise during smoking, pickling. Their colours similar to the oxy-myoglobin are red.
In products exposed to heat protein is denaturated, thus myochromogens are formed. If the partial pressure of
oxygen is reduced, the oxidation number of iron is changed in the hem.
The Fe(II) is oxidized to Fe(III), hem is transformed to hemin and myoglobin is transformed to metmyoglobin.
In this case, the sixth bond is not attached to oxygen, but is attached to hydroxide ion. The metmyoglobin has
grayish red colour. The product denatured by heat is the metmyochromogen. The formation of the
metmyoglobin is helped by heat, UV light and by decrease of pH.
Myoglobin products in which the oxidation number of iron is +2 are red. The distribution of myoglobin in meat
is not equable. Its concentration and thus the intensity of the red colour depends on the species, sex, age and life
style of the animal. The more myoglobin contains meat, the darker red the colour (Fig.56).
14.2. ábra - Figure 56: Changes of colour through the meat processing
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15. fejezet - 15. RECOMMENDED
REFERENCES
Wiliam H. Brown, Judith A. McClarin, Introduction to Organic and Biochemistry. 3rd edition Copyright 1981.
by Willard Grant Press, 20 Providence Street, Boston, Massachusetts 02116. ISBN: 0-87150-738-2.
Christopher K. Mathews; K. E. van Holde, 1990. Biochemistry, The Benjamin/ Cummings Publishing
Company, 390 Bridge Parkway, ISBN: 0-8053-5015-2.
Karla L. Roehrig, Carbohydrate Biochemistry and metabolism. Copyright 1984 by THE AVI PUBLISHING
COMPANY, INC. Westport, Connecticut. ISBN: 0-87055-447-6.
James Darnell, Harvey Lodish, David Baltimore, Molecular Call Biology. Copyright 1986 by Scientific
American Books, Inc. ISBN: 0-7167-6001-0.
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16. fejezet - Questions
1. How can the living organism be classified according to their type of metabolism?
2. How can carbohydrates be classified?
3. What does the reductive sugars expression mean?
4. What kind of biological functions do the polysaccharides have?
5. What are the similarities and differences between amilose and amilopectine?
6. What kind of bonds stabilise the spatial structure of proteins?
7. What are the biological functions of proteins?
8. What are the biological functions of phospholipids and neutral fats?
9. What are the similarities and differences between the primary and secondary functions of DNA and RNA?
10.
What are the biological functions of nucleoside-triphosphates?
11.
How can the vitamins expound their effects?
12.
Which type of gland produces hormones and how can the hormones be classified by their chemical
structure?
13.
What does the fact mean that the hormonal regulation has hierarchical order (through an example)?
14.
What are the similarities and the differences between the hormones and the tissue hormones,
phytohormones?
15.
What kind of parts do the enzymes consist of, what is the function mechanism of the enzymes, and
what factors influence the function of the enzymes?
16.
What is the essence of photosynthesis, what are the stages of photosynthesis?
17.
What does the expression of photo-phosphorylation mean?
18.
What are the biochemical processes of the catabolism of glucose and what are the places of these
reactions in the cell?
19.
Where is it produced and how much energy is recovered from the entire oxidative catabolism of one
mole of glucose?
20.
What is the importance of pentose phosphate cycle?
21.
What kind of fermentation processes do you know?
22.
What is the similarity and the difference between the fermentation processes taking place in the silo
and the first stomach?
23.
What is the essence of gluconeogenesis, where and why does this process take place?
24.
Describe thesteps of glycogen mobilization (Cory cycle)!
25.
What substances and what kind of enzyme complex are necessary to the biosynthesis of fatty acids?
26.
Describe the steps of fatty acid synthesis!
27.
How does the catabolism of fat begin and what enzymes catalyze this process?
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Questions
28.
Describe the steps of catabolism of palmitic acid .
29.
How many ATPs are formed in the entire oxidative catabolism of one mole of palmitic acid?
30.
What is the process of ketogenesis?
31.
Where is it formed and what is the role of cholesterol in the human body?
32.
What is the role of glyoxylate cycle?
33.
What is the difference between glyoxylate cycle and citric acid cycle?
34.
How can plants take up nitrogen, and how can they transform it into organic compounds?
35.
Describesome pathways of essential aminosynthesis!
36.
What is the transcription process of the protein synthesis?
37.
What is the translation process of the protein synthesis?
38.
How can you determine the quality of proteins?
39.
What kind of protein-digesting enzymes do you know?
40.
Where do these enzymes work and what circumstances influence their functions?
41.
What are the biogenic amines and what is their significance?
42.
Review the catabolism of carbon skeleton of amino acids in the tricarboxylic acid cycle!
43.
Review the urea cycle!
44.
How can birds and reptiles excrete the nitrogen?
45.
What kind of chemical processes do muscles obtain energy for their function from?
46.
What kind of factors influence the quantity and quality of the urine?
47.
Describe the hypotheticalmechanism of the formation of gastric juice.
48.
What are the functions of adenylate cyclase-cAMP system?
49.
What kind of hormones regulate the carbohydrate metabolism?
50.
What are the roles of liver in the metabolism?
51.
What is fermentation in food processing used for?
52.
Review the biochemical processes going during ripening of fruits!
53.
What does non-enzymatic browning mean?
54.
What is enzymatic browning and how does it affect food products?
55.
Review the biochemistry of meat ripening!
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