Download UNIT 3 Structure and function of saccharides and carbohydrates

PRT3402- Agricultural Biochemistry
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UNIT 3
Structure and function of saccharides and carbohydrates
Introduction to Unit
Saccharides and carbohydrates are the most abundant class of biological molecules.
Plants are the main producers of carbohydrates via photosynthesis. They have many
special role in living cells whivh include energy provision when oxidized to drive
metabolic processes. They act as energy storage molecules as as markers for cell
recognition. In this unit you will learn about the basic components of carbohydrates
which are saccharides. Starting with the structure of monosaccharides the build-up
into oligosaccharides, polysaccharides and other polymers sucha starch, chitin and
cellulose will be elucidated.
Learning Outcomes
At the end of this unit the students will be able to:
1. Recognise the basic chemistry and structure of simple sugars or
saccharides and the existence of stereoisomers
2. Describe the formation of more complex structures of carbohydrates
derived from monosaccharides.
3. Describe the various structure of and function of biologically important
carbohydrate polymers.
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TOPIC 1: STRUCTURE OF SACCHARIDES, STEREOISOMERS,
OLIGOSACCHARIDES AND THEIR DERIVATIVES
Main Points
1.1
Carbohydrates can be classified as monosaccharides, oligosaccharides and
poly saccharides. The term saccharides are derived from the Latin
‘sakcharon’ meaning sugar. Another name for monosaccharides is ‘simple
sugars’. It contain between 3-7 C atoms and consists of a single polyhydroxy
aldehyde (aldoses) or ketone unit (ketoses). Depending on the number of
carbon atoms, different names are derived:
No. of Carbon
Name of
monosaccharides
3
triose
4
tetrose
5
pentose
6
hexose
7
heptose
8
octose
Monosaccarides made of aldehydes and ketones are called aldoses and
ketoses respectively. Meanwhile monosaccharides that form five member
rings and six member rings are called furanoses and pyranoses respectively.
1.2
Triose is the simplest 3 C monosaccharide and can exist as aldehyde (aldose)
or ketones (ketose). Glyceraldehyde and dihydroxyacetone has the same
atomic composition but differ only in the position of hydrogens and double
bonds.
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1.3
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Other common monosaccharides include: Glucose (6C), Mannose (6C),
Fructose (6C),
Ribose (5C), Galactose (6C) and Erythrose (4C). Most
abundance form of monosaccharide is D-glucose (6-Carbon).
1.4
Stereoisomers are compounds with the same molecular formula but different
spatial arrangements of their atoms. The 3D arrangement of atoms around
the carbon atom is such that if four groups are attached to it, they can be
arranged in two different ways. Such C is described as asymmetric or chiral.
The two molecules with different 3D arrangement are mirror images of each
other. Mirror images of each other are called enantiomers. Two sugars that
differ in configuration at only one chiral center are called epimers for e.g Dglucose and D-mannose are epimers.
1.5
Molecule with n chiral centers can have:
2n stereoisomers
Glyceraldehyde = 1 chiral center
21 = 2 streoisomers
By convention, one is a D isomer, the other L isomer.
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CHO
CHO
H
C
OH
HO
C
CH2OH
CH2OH
D-glyceraldehyde
L-glyceraldehyde
CHO
H
C
CHO
OH
HO
C
CH2OH
D-glyceraldehyde
1.6
H
H
CH2OH
L-glyceraldehyde
For 6 carbon aldoses it will have 4 chiral centres. Thus: 24 = 16 streoisomers
which is 8 D isomers and 8 L isomers.
O
H
O
C
H – C – OH
HO – C – H
H – C – OH
H – C – OH
CH2OH
C
HO – C – H
H – C – OH
HO – C – H
HO – C – H
CH2OH
D-glucose
1.7
H
L-glucose
The following structures depict the relationship of D-aldoses and their
stereoisomers.
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1.8
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When sugars cyclize, they form furanose or pyranose structures. In aqueous
solution, aldotetroses (4C) and other monosaccharides with 5 or more C
atoms occur mostly as cyclic structures. For example C1 aldehyde and C5
1.9
-OH react to form a 6C ring pyranose structure.
Cyclization of glucose
produces a new asymmetric center at C1, e.g α-glucose (-OH below the ring)
and β-glucose (-OH above the ring). A pair of stereoisomers that differ in
configurations around C-1 are called anomers. Hence C-1 carbon is called
anomeric carbon whilst α-glucose and β-glucose are anomers.
Glucose (cyclic form)
1.10
Many of the monosaccharides plays very important functions in biological
metabolism as indicated below:
Number of Carbon
Triose (3 C)
Type of Monosaccharide
Glyceraldehyde
Dihydroxyacetone
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Role/Function
The 3-phosphate
intermediate in
glycolysis
The 1-phosphate
intermediate in
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Pentoses (5 C):
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D-Arabinose
L-Arabinose
D-Ribose
2-D-Deoxyribose
Hexoses (6 C)
D-Glucose
D-Galactose
D-Mannose
D-Fructose
Heptulose (7 C)
D-Sedoheptulose
glycolysis
Plant glycosides
(defence), cellwalls
Contituents of cell
walls, plant
glycoprotein
Constituents of
Deoxyribonucleic acid
(DNA)
Energy source
Milk (part of lactose)
Polysaccharides
structures
Intermediate in
glycolysis
Intermediate in Calvin
cycle in photosynthesis
1.11. Monosaccharides can be added with other chemical groups which form other
derivates of importance. Some of these derivatives include sugar phosphates,
sugar acids, sugar alcohols, amino sugars and deoxy sugars.
1.12. Sugar phosphates are made by esterifying a phosphate group to one of the
hydroxyls. The phosphate group gives molecules a negative charge at neutral
pH. Some examples include glyceraldehyde phosphate, glucose-1-phosphate,
glucose-6-phosphate and fructose-6-phosphate. Many of these sugar esters
are important metabolic intermediates in the generation of ATP (adenosine
triphosphate). These are shown below.
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1.13. Sugar acids are monosaccharides with carboxyl groups. These include
gluconic acid, glucuronic acid and ascorbic acid (Vitamin C). Glucuronic acid
can be found in fermented drink. Some structures of these sugar acids are
shown below.
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1.14. As the name suggests sugar alcohols contain alcohol groups when the
carbonyl part of the sugar is reduced. This group of sugars are known as
alditols and include erythritol, D-mannitol and D-glucitol (or sorbitol). Mannitol
occurs naturally in pineapples, olives, asparagus, sweet potatoes and carrots
whereas sorbitol is found naturally in fruits and vegetables, xylitol is found
naturally in fruit, vegetables, and cereals and Erythritol is found in fruit and
fermented food and also tasted as sweet as table sugar. For this reason
mannitol, sorbitol, xylitol are used to sweeten sugarless chewing gum and
mints.The structures of these alditols are shown below.
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1.15. Amino sugars are made by replacing hydroxyl of a sugar with an amine group.
Some examples areβ –D-glucosamine and D-galactosamine. β–D-Nacetylglucosamine, muramic acid and N-acetylmuramic acid (sialic acid) are
some derivatives of the amino sugars. Glucosamine is part of the structure of
the polysaccharides chitosan and chitin, which compose the exoskeletons of
crustaceans. Glucosamine is also used for the treatment of osteoarthritis.
Galactosamine is a hepatotoxic or liver-damaging agent. These amino sugars
and their derivatives are shown below.
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1.16. In deoxy sugars, one of the OH groups is replaced with hydrogen. Deoxy
sugars are constituents of DNA – deoxyribonucleic acid (See Unit 5).
1.17. Oligo in Greek means “few”, hence oligosaccharides consists of two to ten
simple sugars. Oligosaccharides and polysaccharides are formed when
monosaccharides are joined through glycosidic bonds. The simplest
oligosaccharides, disaccharides include sucrose and lactose. Other
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disaccharides are trehalose, maltose, gentiobiose and cellobiose.
Disaccharides are quite common whilst trisaccharides are frequently found.
Oligosaccharides with four or six sugars are usually bound to other molecules.
Maltose and trehalose consists of 2 glucose units while sucrose consists of
glucose and fructose.
1.18. Maltose is a disaccharide of glucose units joined by -1,4 linkage while
trehalose is a disaccharide made by joining two glucose units together via
(1->1) bonds. Sucrose is a disaccharide, composed of a glucose joined
through carbon 1 in -linkage to carbon 2 of fructose. Sucrose is a common
table sugar.
1.19. Maltose is form from the glycosidic linkage between the C1 hydroxyl of one
glucose and the C4 hydroxyl of another glucose.
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1.20. Other common disaccharides include: Lactose -milk sugar composed of
galactose & glucose with β(14) linkages and Cellobiose which is the
product of cellulose breakdown. The β(14) glycosidic linkage is a zig-zag
between 2 glucoses where one is actually flipped over the other.
1.21. Features that distinguish disaccharides from each other include:
a). The two specific sugar monomers may be of the same kind or they may be
different).
b). The carbons involved in the linkage (the most common linkages are 1→1,
1→2 (as in sucrose), 1 → 4 (as in lactose, maltose and cellobiose).
c) The order of monomeric units, if they are of different types.
d). The anomeric configuration of the hydroxyl group on carbon 1 of each
residue (can either be α or β) – may have major effect on the shape of the
molecule which is a feature recognized by enzyme.
1.22.
Some properties of oligosaccharides are describe in the table below:
Sugars
Sucrose
Characteristics
 Energy provider in many
organism
 Found in many fruits, roots,
seeds and honey
Lactose


Animal energy source
Found in milk
Trehalose


Energy source for insects
Found in insect blood, yeast
and Fungi
Maltose

Derived from starch and
glycogen polymers
Found in plant (starch), and
animal (glycogen-energy
storage- animal starch)


Cellobiose

Derived from cellulose
polymers
Cellulose is structural
component of cell wall
1.23. Oligosaccharides are also found as part of glycoproteins and play a role in cell
recognition/identity. Oligosaccharides form the blood group antigens. In some
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cells, these antigens are attached as O-linked glycans to membrane proteins.
Alternatively, the oligosaccharide may be linked to a lipid molecule to form a
glycolipid. These oligosaccharides determine the blood group types in
humans.
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TOPIC 2: STRUCTURE AND FUNCTIONS POLYSACCHARIDES
Main Points
2.1
Polysaccharides are polymers of monosaccharides and their derivatives and
they are also known as glycans. Homopolysaccharides or homoglycan
contains one kind of monosaccharide molecules while heteropolysaccharides
or heteroglycancan have more than one kind of monosaccharide molecules.
2.2
The table below described some common polysaccharides, the monomeric
units and the linkages that form the polymers.
2.3
Polysaccharides
Monomeric units
Linkages
Glycogen
Cellulose
Chitin
Amylopectin
Amylose
D Glucose
D Glucose
N-Acetyl-D-glucosamine
D Glucose
D Glucose
 1>6 branches
 1>4
 1>4
1>6 branches
 1>4
Linkages between the individual units require special enzymes to break them
down. For example, the α1->4 linkages between glucose units in glycogen,
amylose, and amylopectin, are readily broken down by all animals, but only
ruminant animals (cows, horses, etc.) contain symbiotic bacteria with an
enzyme (cellulase) that can break down the 1->4 linkages between individual
glucose units in cellulose.
2.4
Polysaccharides differ from one another due to:
- Component of monosaccharides
- Length of their chains
- Amount of chain branching also distinguished polysaccharides from
protein, nucleic acids which occur only as linear polymers.
2.5
Functions of polysaccharides include:
1)
Storage materials
e.g. Starch, glycogen and dextran
-metabolized to produce ATP
2)
Structural components
Chitin - skeletons of arthropods
Cellulose – green plants
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3)
2.6
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Protective substances
Starch is the polysaccharide storage materials in plants and exists in two
different forms: α-amylose and amylopectin. Starch in nature consist of 10-30
% α-amylose and 70-90 % amylopectin. Cornstarch is made-up of 25% αamylose and 75% amylopectin. α-amylose is made up of D-glucose units with
α(1→4) linkages. It is a linear polymer consisting of 300 to 3000 or more of
glucose units. It is an energy storage material, insoluble in water and used as
gelling agents. The structure of starch amylase is shown below:
2.7
Amylopectin is a polymer of highly branched chain of glucose units. Branches
occur every 12-30 residues with an average branch length of 24-30 residues.
The linear way of polymerization is through α(1→4) glycosidic bonds while the
branching is through α(1→6) bonds as shown below. It is soluble in water and
its uses include pastes, adhesives, and lubricants.
2.8
Glycogen is another storage polymer made from polysaccharides but it is
mainly in animals unlike starch in plants. It is found mainly in the liver. The
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structure is similar to amylopectin but highly branched. The polymer has
branches for every 8-12 glucose units.
2.9
Dextran is the storage polysaccharides found in yeasts and bacteria. It is
made-up of straight chains of D-glucose units link by α(1→6) linkages, and it
branches by α-1,3 linkages. In the medical field it is use to reduce blood
viscosity.
2.10
Polysaccharide that functions as structural components include cellulose
which is the main constituents of many plants and chitin which is the
protective skeletons of arthropods. Cellulose is a linear polymer linked
through glucose β(1→4) linkages. Cellulose is the most abundant natural
polymer in nature, found in the cell wall of plant cells. It gives structural
integrity and strength to the plants. The β (1→4) linkages of cellulose
generate a planar structure and the parallel cellulose chains are linked
together by a network of hydrogen bonds. This results in cellulose having a
great mechanical strength but limited extensibility.
Structure of cellulose
2.11
Chitin is a homopolymer of N-acetyl-β-D- glucosamine, a derivative of glucose
with β,(1→4) linkages. It is the structural material in exoskeleton of many
arthropods such as crabs, shrimps and molluscs. Chitin can be used for many
purposes such as binder in dyes, fabrics, and adhesives. It can be used as
fertilizer to improve crop yields and as surgical threads in surgical operations.
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2.12
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Saccharides can also be linked to non-sugar components. Proteoglycans are
complexes of polysaccharides consisting of glycosaminoglycans and specific
proteins. Peptidoglycans are heteroglycan chains linked to peptides. The
repeating unit of peptidoglycans is a disaccharide composed of N-acetyl-Dglucosamine and N-acetylmuramic acid joined by a β-1,4 glycosidic bond.
Short chains of amino acids (peptides) are linked as branch to the
heteroglycans. Many of the peptidoglycans are present in bacterial cell wall.
The structure and linkages is presented below.
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