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LECTURE 3 –STRUCTURE AND PROPERTIES OF CARBOHYDRATES • Structure, properties and function of carbohydrates and their derivatives • Classification of carbohydrates Course outcome: • Ability to differentiate basic structure, properties, functions and classification of important biomolecules. LECTURE 3 -CARBOHYDRATES Section Section Section Section TOPICS: 1. Role & Significance of Carbohydrates 2. Monosaccharide 3. Oligosaccharides 4. Polysaccharides Sect.1. ROLE OF CARBOHYDRATES As a major energy source for living organisms As a means of transporting energy (glucose is a principal energy source in animal and plants) ( exp: sucrose in plant tissues) As a structural material . As a precursor for other biomolecules ( cellulose in plants, chitin in insects, building blocks of nucleotides) (purine, pyrimide) Sect 1. SIGNIFICANCE OF CARBOHYDRATES Carbohydrates are the most abundant biomolecules in nature, having a direct link between solar energy and the chemical bond energy in living organisms. Source of rapid energy production Structural building blocks of cells Components of several metabolic pathways Recognition of cellular phenomena, such as cell recognition and binding (e.g., by other cells, hormones, and viruses) CARBOHYDRATES Carbohydrate : compounds contains H, C & O with the comp : (CH2O)n (Hydrate of carbon) Carbohydrates : Consist of sugar (saccharum) Sugars : compound that contains alcohol & carbonyl functional group Carbonyl func.group : >C=o Adehyde aldose Ketone ketose Examples: Classification Carbohydrate Mono saccharide Glucose, fructose Ribose (aldopentose) Deoxy ribose Oligo saccharide disaccharides Glycoproteins (bacterial cell walls Poly saccharide cellulose, chitin, starch, glycogen, glucoaminoglycans Glyconoconjugates glycoproteins and proteoglycans Sect2. MONOSACHARIDES Sect. 2. Monosacharides Sub sections : 2.1 Properties & classification 2.2 Stereoisomers 2.3 Cyclic structure 2.4 Important Reactions 2.5 Important monosach 2.6 glycoproteins and proteoglycans 2.7 Monosaccharide derivatives 2.1 Monosach Properties& classification Colorless, crystalline solids Soluble in water but insoluble in nonpolar solvents One of the carbon atoms is double-bonded to an oxygen atom to form a carbonyl group; each of the other carbon atoms has a hydroxyl group. – Carbohydrates with an aldehyde (-CHO) functional group are called aldoses e.g. glyceraldehyde (CH OH-CHOH-CHO) Those with a keto group (-C=O) are ketoses 2 e.g.dihydroxyacetone (CH2OH-C=O-CH2OH) – Classified according to the number of carbon atoms they contain Monosacharides : Exp. aldoses & ketoses Aldotriose Ketotriose Aldotetrose Ketotetrose Aldopentoses Ketopentose Aldohexose Ketohexose 2.2. MONOSACCHARIDES STEREOISOMERS Isomers: same chemical formulas, different structures Total no of possible isomers can be determined by Van Hoff’s rule: compound with n chiral C atoms has a max of 2n possible stereoisomers. Chiral: asymmetric carbons, i.e carbon atom with four different substituents Eg: n = 4, there are 16 stereoisomers (8-L stereoisomers, 8-D stereoisomers). In optical isomers- the ref C is the asymmetric C that is most remote from the C=O carbon. In D-aldose family sugars, the OH group is to the right on the chiral C atom farthest from the most oxidized C (aldehyde group) in the molecule. D- and L- enantiomers Stereoisomers that are not enantiomers (mirror-image) are called diastereoisomers. Eg: aldopentoses, D-ribose and L-ribose are enantiomers. The D-ribose and D-arabinose are diastereomers because they are isomers but not mirror image. Diastereomers that differ in the configuration at a single asymmetric C atom are called epimers. Eg: D-glucose and D-galactose are epimers because they differ only in the configuration of the OH group at C-4. D-mannose and D-galactose are not epimers- differ more than 1 C. 2.2. MONOSACCHARIDES STEREOISOMERS The simplest aldose, glyceraldehyde, contains one chiral center (the middle carbon atom) and has two different optical isomers, or enantiomers the projection in which the carbohydrate backbone is drawn vertically with the carbonyl shown on the top. 2.3 Cyclic structure of monosacharides • in aqueous solution, monosaccharides with five or more carbon atoms in the backbone occur predominantly as cyclic (ring) structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl group along the chain. • Sir Norman Haworth showed that the linear form of glucose (and other aldohexose) could undergo intramolecular reaction to form a cyclic hemiacetal. • the analogous intramolecular reaction of ketose sugar yields a cyclic hemiketal. The new chiral center in cyclic (c1) is called anomeric carbon In aldose sugars, the OH group of the newly formed hemiacetal occurs on C-1 (the anomeric carbon). The OH group may occur either below the ring (down position)- α-anomeric form. Or above the ring (up position)-β anomeric form. In Fischer projections, the α–anomeric OH occurs on the right and β–anomeric OH occurs on the left. Pyranoses& Furanoses Pyranoses: six-membered ring compounds ( resemble pyran ) Furanoses : fivemembered rings, (resemble furan) The structure systematic names glucose & fructose become HAWORTH STRUCTURES An English chemist W.N. Haworth gave a more accurate picture of carbohydrate structure. Haworth Structures To convert from traditional Fischer formula of a Dpentose or D-hexose to a Haworth formula, the following steps should be followed: Draw a 5 or 6-membered ring with the O placed as shown below: Starting with anomeric carbon to the right of the ring O, place OH group either above or below the plane of the ring. Group that pointing to the left in Fischer projection should go above (β-) the plane of the ring, and those pointing right should go below the ring (α-) In D-sugars, the last C position (eg: C-6 glucose) is always up FISHER AND HAWORTH FORMS OF SUGAR SUMMARY OF SUGAR STRUCTURES ISOMERS- compounds that have the same chemical formula e.g. fructose, glucose, mannose, and galactose are isomers of each other having formula C6H12O6. EPIMERS- refer to sugars whose configuration differ around one specific carbon atom e.g. glucose and galactose are C-4 epimers and glucose and mannose are C-2 epimers. ENANTIOMERS- a special type of isomerism found in pairs of structures that are mirror images of each other. The mirror images are termed as enantiomers and the two members are designated as D- and L- sugar. The vast majority of sugars in humans are Dsugars. CYCLIZATION OF SUGARS- most monosaccharides with 5 or more carbon atoms are predominately found in a ring form, where the aldehyde or ketone group has reacted with an alcoholic group on the same sugar group to form a hemiacetal or hemiketal ring. Pyranose ring- if the ring has 5 carbons and 1 oxygen. Furanose ring- if the ring is 5-membered (4 carbons and 1 oxygen 2.4.IMPORTANT REACTIONS IN MONOSACCHARIDES Monosaccharides undergo the following reactions : 1. 2. 3. 4. 5. 6. Mutarotation Oxidation Reduction Isomerization Esterification Glycoside formation IMPORTANT REACTIONS IN MONOSACCHARIDES Details 1. Mutarotation – alfa and beta forms of sugars are readily interconverted when dissolved in water. Mutarotation produces an equilibrium mixture of α and β- forms in both furanose and pyranose ring structures. 2 Oxidation and reduction in presence of oxidising agents, metal ions (Cu2+) and enzymes, monosacchs undergo several oxidation reactions e.g. Oxidation of aldehyde group (R-CHO) yields aldonic acid; of terminal CH2OH (alcohol) yields uronic acid; and of both the aldehyde and CH2OH gives aldaric acid. The carbonyl groups in both aldonic and uronic can react with an OH group in the same molecule to form a cyclic ester known as a lactone. sugars that can be oxidized by weak oxidizing agent ie. Benedict’s reagent, called reducing sugars. Because the reaction occurs only with sugars that can revert to open chain form, all monosaccharides are reducing sugars. 3. REDUCTION reduction of the aldehyde and ketone groups of monosacchs yield sugar alcohols (alditols) Sugar alcohols e.g.sorbitol, are used commercially in processing foods and pharmaceuticals. sorbitol- improves the shelf-life of candyit helps prevent moisture loss. IMPORTANT REACTIONS (Cont) 4. ISOMERIZATION Monosaccharides undergo several types of isomerization e.g. D-glucose in alkaline solution for several hours contain D-mannose and D-fructose. Both isomerization involves an intramolecular shift of a H atom and a relocation of double bond. The conversion of glucose to mannose is termed s epimerization. 5 ESTERIFICATION Free OH groups of carbohydrates react with acids to form esters. This reaction an change the physical and chemical propteries of sugar. 6. GLYCOSIDE FORMATIONHemiacetals and hemiketals reaction with alcohols to form the corressponding aceta or ketal. On the contrary when a cyclic hemiacetal or hemiketal form of monosaccharide reacts with alcohol, the new linkage is called glycosidic linkage and the compound glycoside. Alfa & beta GLYCOSIDIC BOND REDUCING SUGARS All monosacchs are reducing sugars. They can be oxidised by weak oxidising agent such as Benedict’s reagent Benedict's reagent is a solution of copper sulfate, sodium hydroxide, and tartaric acid. Aqueous glucose is mixed with Benedict's reagent and heated. The reaction reduces the blue copper (II) ion to form a brick red precipitate of copper (I) oxide. Because of this, glucose is classified as a reducing sugar. 2.5 IMPORTANT MONOSACCHARIDES GLUCOSE FRUCTOSE GALACTOSE D-Glucose: D-glucose (dextrose) is the primary fuel in living cells especially in brain cells that have few or no mitochondria. Cells such as eyeballs have limited oxygen supply and use large amount of glucose to generate energy Dietary sources include plant starch, and the disaccharides lactose, maltose, and sucrose Important monosaccharides. Cont FRUCTOSE – D-fructose (levulose) is often referred as fruit sugar and is found in some vegetables and honey – This molecule is an important member of ketose member of sugars – It is twice as sweet as sucrose (per gram basis) and is used as sweeting agent in processed food products – It is present in large amounts in male reproductive tract and is synthesised in the seminal vesicles. Important monosaccharides. Cont.... GALACTOSE – is necessary to synthesize a variety of biomolecules (lactose-in mammalary glands, glycolipids, certain phospholipids, proteoglycans, and glycoproteins) – Galactose and glucose are epimers at carbon 4 and interconversion is catalysed by enzyme epimerase. – Medical problems – galactosemia (genetic disorder) where enzyme to metabolize galactose is missing; accumulation of galactose in the body can cause liver damage, cataracts, and severe mental retardation 2.7.MONOSACCHARIDE DERVATIVES URONIC ACIDS – formed when terminal CH2OH group of a mono sugar is oxidised – Important acids in animals – D-glucuronic acid and its epimer L-iduronic acid – In liver cells glucuronic acid combines with steroids, certain drugs, and bilirubin to improve water solubility therby helping the removal of waste products from the body – These acids are abundant in the connective tissue carbohydrate components. Mono sugar derivatives AMINO SUGARS – – Sugars in which a hydroxyl group (common on carbon 2) is replaced by an amino group e.g. D-glucosamine and D-galactosamine – common constituents of complex carbohydrate molecule found attached to cellular proteins and lipids – Amino acids are often acetylated e.g. Nacetyl-glucosamine. Mono sugar derivatives DEOXYSUGARS – monosaccharides in which an - H has replaced an – OH group – Important sugars: L-fucose (formed from D-mannose by reduction reactions) and 2-deoxy-D-ribose – L-fucose – found among carbohydrate components of glycoproteins, such as those of the ABO blood group determinates on the surface of red blood cells – 2-deoxyribose is the pentose sugar component of DNA. GLYCOSIDIC BONDS • Monosaccharides can be linked by glycosidic bonds (joining of 2 hydroxyl groups of sugars by splitting out water molecule) to create larger structures. • Disaccharides contain 2 monosaccharides e.g. lactose (galactose+glucose); maltose (glucose+glucose); sucrose (glucose+fructose) • Oligosaccharides – 3 to 12 monosaccharides units e.g. glycoproteins • Polysaccharides – more than 12 monosaccharides units e.g. glycogen (homopolysaccharide) having hundreds of sugar units; glycosaminoglycans (heteropolysaccharides) containing a number of different monosaccharides species. Section 3 DISACCHARIDES AND OLIGOSACCHARIDES DISACCHARIDES AND OLIGOSACCHARIDES Configurations: alfa or beta ( 1,4, glycosidic bonds or linkages; other linkages 1,1; 1,2; 1,3; 1,6) Digestion of disaccharides and other carbohydrates aided by enzymes. Defficiency of any one enzyme causes unpleasant symptoms. The undigestible dissacharide sugar pass into large intestine and digested by bacteria (fermentation) in colon produces gas [bloating of cramps]. Most common defficiency, an ancestoral disorder, lactose intolerance caused by reduced synthesis of lactase Important sugars of Disaccharides LACTOSE (milk sugar) disaccharide found in milk; composed of one molecule of galactose (OH group in C-1) and glucose (OH group at C-4) linked through beta(1,4) glycosidic linkage (anomeric C of galactose is in β-configurations); because of the hemiacetal group of the glucose component, lactose is a reducing sugar Lactose intolerance Lactose (milk sugar) in infants is hydrolyzed by intestinal enzyme lactase to its component monosacch for absorption into the bloodstream (galactose epimerized to glucose). Most adult mammals have low levels of betagalactosidase. Hence, much of the lactose they ingest moves to the colon, where bacterial fermentation generates large quantities of CO2, H2 and irritating organic acids. These products cause painful digestive upset known as lactose intolerance and is common in the African and Asian decent. MALTOSE ( malt sugar) An intermediate product of starch hydrolysis; it is a disaccharide with an alfa(1,4) glycosidic linkage between two D-glucose molecules; in solution the free anomeric carbon undergoes mutarotation resulting in an equilibrium mixture of alfa and beta – maltoses; it does not occur freely in nature SUCROSE common table sugar: cane sugar or beet sugar produced in the leaves and stems of plants; it is a disaccharide containing both alfa-glucose and beta-fructose residues linked by alfa,beta(1,2)glycosidic bond. CELLOBIOSE degradation product of cellulose containing two molecules of glucose linked by a beta (1,4) glycosidic bond; it does not occur freely in nature OLIGOSACCHARIDE SUGARS Oligosaccharides are small polymers often found attached to polypeptides in glycoproteins and some glycolipids. They are attached to membrane and secretory proteins found in endoplasmic reticulum and Golgi complex of various cells Two classes: N-linked and O-linked Section 4 POLYSACCHARIDES 4.1. Intro to Polysaccharides 4.2. Classification of Polisacharides 4.2.1. Homosacharides 4.2.2. Heteropolysacharides 4.1. Intro to Polysaccharides Composed of large number of monosaccharide units connected by glycosidic linkages Classified on the basis of their main monosaccharide components and the sequences and linkages between them, as well as the anomeric configuration of linkages, the ring size (furanose or pyranose), the absolute configuration (D- or L-) and any other substituents present. (http://www.lsbu.ac.uk/water/hypol.html) Polysaccharides are more hydrophobic if they have a greater number of internal hydrogen bonds, and as their hydrophobicity increases there is less direct interaction with water Divided into homopolysaccharides (e.g.Starch, glycogen, cellulose, and chitin) & heteropolysaccharides (glycoaminoglycans or GAGs, murein). 4.2. Classification of Polisacharides 4.2.1.HOMOPOLYSACCHARIDES Found in abundance in nature Important examples: starch, glycogen, cellulose, and chitin Starch, glycogen, and cellulose all yield Dglucose when they are hydrolyzed Cellulose - primary component of plant cells Chitin – principal structural component of exoskeletons of arthropods and cell walls of many fungi; yield glucose derivative N-acetyl glucosamine when it is hydrolyzed. STARCH (Homopolysaccharide) A naturally abundant nutrient carbohydrate, (C6H10O5)n, found chiefly in the seeds, fruits, tubers, roots, and stem pith of plants, notably in corn, potatoes, wheat, and rice, and varying widely in appearance according to source but commonly prepared as a white amorphous tasteless powder. Any of various substances, such as natural starch, used to stiffen cloth, as in laundering. Two polysaccharides occur together in starch: amylose and amylopectin Amylose – unbranched chains of D-glucose residues linked with alfa(1,4,)glycosidic bonds Amylopectin – a branched polymer containing both alfa(1,4,) and alfa(1,6) glcosidic linkages; the alfa(1,6) branch points may occur every 20-25 glucose residues to prevent helix formation Starch digestion begins in the mouth; alfa-amylase in the saliva initiates hydrolysis of the gycosidic linkages Amylose amylopectin GLYCOGEN (Homopolysaccharide) Glycogen is the storage form of glucose in animals and humans which is analogous to the starch in plants. Glycogen is synthesized and stored mainly in the liver and the muscles. Structurally, glycogen is very similar to amylopectin with alpha acetal linkages, however, it has even more branching and more glucose units are present than in amylopectin. Various samples of glycogen have been measured at 1,700600,000 units of glucose. The structure of glycogen consists of long polymer chains of glucose units connected by an alpha acetal linkage. The branches are formed by linking C # 1 to a C # 6 through an acetal linkages. In glycogen, the branches occur at intervals of 8-10 glucose units, while in amylopectin the branches are separated by 12-20 glucose units. STRUCTURE OF GLYCOGEN CELLULOSE (Homopolysaccharide) Cellulose is found in plants as microfibrils (2-20 nm diameter and 100 - 40 000 nm long). These form the structurally strong framework in the cell walls. The microfibrils are held together by hydrogen bonding and may contain 12,000 glucose units each. Cellulose is mostly prepared from wood pulp Cellulose is a linear polymer of β-(1 4)-D-glucopyranose units in 4C1 conformation. The fully equatorial conformation of β-linked glucopyranose residues stabilizes the chair structure, minimizing its flexibility Cellulose has many uses as an anticake agent, emulsifier, stabilizer, dispersing agent, thickener, and gelling agent but these are generally subsidiary to its most important use of holding on to water. Water cannot penetrate crystalline cellulose but dry amorphous cellulose absorbs water becoming soft and flexible. Purified cellulose is used as the base material for a number of water-soluble derivatives e.g. Methyl cellulose, carbomethycellulose The ability to digest cellulose is found only in microbes that possess enzyme cellulase. Certain animal species (termites and cows) use such organisms in their digestives tracts to digest cellulose. The breakdown of cellulose makes glucose available to both the microbes and their host. Cellulose also make up the dietary fibre. Cellulose as polymer of β-D-glucose Cellulose in 3D CELLULOSE CHITIN (Homosaccharide) Chitin is a polymer that can be found in anything from the shells of beetles to webs of spiders. It is present all around us, in plant and animal creatures. It is sometimes considered to be a spinoff of cellulose, because the two are very molecularly similar. Cellulose contains a hydroxy group, and chitin contains acetamide. Chitin is unusual because it is a "natural polymer," or a combination of elements that exists naturally on earth. Usually, polymers are man-made. Crabs, beetles, worms and mushrooms contain large amount of chitin. Chitin is a very firm material, and it help protect an insect against harm and pressure Structure of the chitin molecule, showing two of the Nacetylglucosamine units that repeat to form long chains in beta-1,4 linkage. CHITOSAN A spinoff of chitin that has been discovered by the market is chitosan. This is a man-made molecule that is often used to dye shirts and jeans in the clothing industry. Chitosan can be used within the human body to regulate diet programs, and researchers are looking into ways in which it can sure diseases. Chitin, the polysaccharide polymer from which chitosan is derived, is a cellulose-like polymer consisting mainly of unbranched chains of N-acetyl-D-glucosamine. Deacetylated chitin, or chitosan, is comprised of chains of D-glucosamine. When ingested, chitosan can be considered a dietary fiber. CHEMICAL STRUCTURE OF CHITOSAN http://www.pdrhealth.com/drug_info/nmdrugprofiles/nutsupdrugs/chi_0067.shtml 4.2.2.HETEROPOLYSACCHARIDES Are high-molecular-weight carbohydrate polymers more than one kind of monosaccharide Important examples include glycosaminoglycans (GAGs) – the principle components of proteoglycans and murein, a major component of bacterial cell walls. Glycoaaminoglycans (GAGs) GAGs are linear polymers with disaccharides repeating units. Many of their sugar residues are amino derivatives. The repeating units contain hexuronic acid (a uronic acid contain 6-C atoms) except for keratan sulphate – contains galactose. Usually N-acetylglucosamine sulphate is also present except in hyaluronic acid which contain N-acetylglucosamine. Many disacharide units contain both carboxyl nd sulfate functions groups. GAGs are classified according to their sugar residues, the linkages between residues and the presence and location of sulphate groups. 5 classes has been distinguished: hyaluronic acid, chondroitin sulfate, dermatan sulfate, heparin and keratan sulfate THE SPECIFIC GAGs OF PHYSIOLOGICAL SIGNIFICANCE Hyaluronic acid Occurence : synovial fluid, ECM of loose connective tissue Hyaluronic acid is unique among the GAGs because it does not contain any sulfate and is not found covalently attached to proteins. It forms non-covalently linked complexes with proteoglycans in the ECM. Hyaluronic acid polymers are very large (100 10,000 kD) and can displace a large volume of water. Hyaluronic acid (D-glucuronate + GlcNAc) Dermatan sulfate (L-iduronate + GlcNAc sulfate) Occurence : skin, blood vessels, heart valves Chondroitin sulfate (D-glucuronate + GalNAc sulfate) Occurence : cartilage, bone, heart valves ; It is the most abundant GAG. Heparin and heparan sulfate (D-glucuronate sulfate + N-sulfo-D-glucosamine) Heparans have less sulfate groups than heparins Occurence : Heparin :component of intracellular granules of mast cells lining the arteries of the lungs, liver and skin Heparan sulfate : basement membranes, component of cell surfaces Keratan sulfate ( Gal + GlcNAc sulfate) Occurence : cornea, bone, cartilage ; Keratan sulfates are often aggregated with chondroitin sulfates. MUREIN (Peptidoglycan) Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of eubacteria. The sugar component consists of alternating residues of β-(1,4) linked N-acetylglucosamine and N-acetylmuramic acid residues. Attached to the N-acetylmuramic acid is a peptide chain of three to five amino acids. The peptide chain can be cross-linked to the peptide chain of another strand forming the 3D mesh-like layer. Some Archaea have a similar layer of pseudopeptidoglycan. SUMMARY Monosaccharides, the simplest carbohydrates, are classified as aldoses or ketoses. The cyclic hemiacetal and hemiketal forms of monosacchs have either alfa or beta configuration at their anomeric carbon. Monosacch derivatives include aldonic acids, uronic acids, deoxy sugars, amino sugars, alfa & beta glycosides. Disaccharides simplest polysaccharides occuring as hydrolysis products of larger molecules e.g. Lactose,sucrose Oligosaccharides play important roles in determining protein structure and in cell-surface recognition phenomena. Oligosacchs with 3 or more sugar residues are mostly found in plants. Summary contd. -1 POLOYSACCHARIDES consist of monosacchs linked by glycosidic bonds. Cellulose and chitin are structural polysacchs with beta(1-4) linkages that adopt rigid and extended structures. The storage polysacchs starch and glycogen consist of alfa-glycosidically linked glucose residues Glycosaminoglycans are unbranched polysacchs containing uronic acids and amino sugars that are often sulfated END NOTES The destiny of a nation depends on the manner in which it feeds itself. We eat to live, NOT, live to eat. Lower your carbohydrate consumption, but balance it with the right amount of protein and fat.