1 Course 4. Biomolecules and their interactions Module 19: Structures and conformations of polysaccharide cellulose, amylase, chitin, carbohydrate conjugates OBJECTIVE The main aim of this module is to introduce the students the importance of polysaccharides their structures and to provide insights into how the physical properties of polysaccharides are relevant for their biological functions 1. INTRODUCTION Two or more monosaccharides linked to each other by glycosidic bond generate polysaccharides, also referred to as glycans. Homopolysaccharides consist of one type of monosaccharide while heteropolysaccharides contain more than one type of monosaccharide. Polysaccharides, as opposed to proteins and nucleic acids, form branched and linear polymers. The reason for this is that glycosidic linkage can form between any of the hydroxyl groups of the monosaccharide. Exoglycosidases and endoglycosdiases are enzymes that hydrolyze monosaccharide units from a polysaccharide. 1.1 Disaccharides A disaccharide consists of two sugars joined by an O-glycosidic bond. The hemiacetal OH of one monosaccharide and an OH of the second monosaccharide, dehydrate to establish the bond called a glycosidic bond. A glycosidic bond is formed between anomeric carbon and the alkoxy oxygen. The disaccharide lactose (Fig. 1) occurs naturally in milk. Upon hydrolysis it yields D-galactose and D- glucose. Its abbreviated name is Gal(β1 4)Glc. The disaccharide maltose (figure 1) contains two D-glucose residues joined by a glycosidic linkage between C-1 (the anomeric carbon) of one glucose residue and C-4 of the other. Figure 1
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Course 4. Biomolecules and their interactions
Module 19: Structures and conformations of polysaccharide cellulose, amylase, chitin,
carbohydrate conjugates
OBJECTIVE
The main aim of this module is to introduce the students
the importance of polysaccharides
their structures and
to provide insights into how the physical properties of polysaccharides are relevant
for their biological functions
1. INTRODUCTION
Two or more monosaccharides linked to each other by glycosidic bond generate polysaccharides, also
referred to as glycans. Homopolysaccharides consist of one type of monosaccharide while
heteropolysaccharides contain more than one type of monosaccharide. Polysaccharides, as opposed to
proteins and nucleic acids, form branched and linear polymers. The reason for this is that glycosidic
linkage can form between any of the hydroxyl groups of the monosaccharide. Exoglycosidases and
endoglycosdiases are enzymes that hydrolyze monosaccharide units from a polysaccharide.
1.1 Disaccharides
A disaccharide consists of two sugars joined by an O-glycosidic bond. The hemiacetal OH of one
monosaccharide and an OH of the second monosaccharide, dehydrate to establish the bond called
a glycosidic bond. A glycosidic bond is formed between anomeric carbon and the alkoxy oxygen.
The disaccharide lactose (Fig. 1) occurs naturally in milk.
Upon hydrolysis it yields D-galactose and D- glucose.
Its abbreviated name is Gal(β1 4)Glc.
The disaccharide maltose (figure 1) contains two D-glucose residues joined by a glycosidic
linkage between C-1 (the anomeric carbon) of one glucose residue and C-4 of the other.
Figure 1
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1.1.1 Formation of maltose
Maltose is formed when two molecules of glucose are linked to each other by an O-
glycosidic bond. This glycosidic bond is formed when a hydroxyl group of one
glucose molecule (the one on the right in Fig. 2) reacts or condenses with the
intramolecular hemiacetal of the glucose molecule (on the left). A water molecule is
eliminated during this condensation process, resulting in the formation of a glycosidic
bond. On the contrary, if the glycosidic bond is attacked by a water molecule, it is
referred to as hydrolysis. Glycsodic bonds can be attacked by acids but not by bases,
hence disaccharides can be hydrolyzed to release their monosaccharide components by
boiling with dilute acid. N-glycosidic bonds link the anomeric carbon of one sugar to a
nitrogen atom in glycoproteins and nucleotides.
Sugars that can be oxidized by cupric ion (as discussed in module 18) are referred to
as reducing sugars. This reaction can only occur with the linear forms of sugars, which
though exist in equilibrium with the cyclic forms of sugars in aqueous solution. When
the anomeric carbon is involved in the formation of a glycosidic bond, that sugar
cannot exist in the linear form and is referred to as a non-reducing sugar. In the case of
maltose, as the C-1 of the glucose molecule on the left is not involved in forming the
glycosidic bond, it can undergo reactions typical of reducing sugars. Sucrose on the
other hand is defined as a non-reducing sugar.
Figure 2
2. Polysaccharides
Polysaccharides also known as glycans consists of monosaccharides linked together by glycosidic
bonds.
They generally do not have defining molecular weights like proteins. Unlike proteins,
polysaccharides do not require a template for their synthesis.
Polysaccharides can be classified as homopolysaccharides or heteropolysaccharides.
Hydrolysis
Condensation
H2O
H2O
OH
HO
H
OH
CH2OH
H
H
H
HO
OH
H
OH
CH2OH
HOH
H
H
HO
O
Hemiacetal Acetal
OH
HO
H
OH
CH2OH
HH
OH
H
HO
OH
HO
H
OH
CH2OH
HOH
H
HO
α-D-Glucose β-D-Glucose
Hemiacetal
Alcohol
Maltose
α-D-glucopyranosyl-(1 4)-D-glucopyranose
H
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Homopolysaccharide contains only a single type of monomeric unit whereas
hetropolysaccharides consists of two or more different kinds of monomeric units.
2.1 Homopolysaccharides/Storage polysaccharides
Starch and glycogen are considered as storage homopolysaccharides, while cellulose and chitin
serve structural roles in plant cell walls and the exoskeletons of animals.
2.1.1 Starch
Starch is considered the most vital storage polysaccharide in plants while glycogen
serves the same purpose in animal cells. These polysaccharides are hydrated and
stored as large granule inside the cells.
Starch is the storage form of glucose in plants, where it is predominantly stored
in cholorplasts.
It is a branched chain of D-glucose.
It contains a mixture of amylose and amylopectin (Fig. 3).
Amylose is a linear unbranched polymer of α-D-glucose units in a repeating
sequence of α1 4 glycosidic linkages.
Amylose is an isomer of cellulose but these differ from each other in their
structural properties.
Cellulose is also a glucose polymer where glucose residues are linked via β
glycosidic linkages causing it to attain a fully extended conformation that can be
tightly packed.
Amlyose on the other hand, due to the α glycosidic linkages attains a helical
coiled structure that can aggregate irregularly (Fig. 3B).
Amylopectin is a branched polymer of α1 4 glycosidic linkages and with α1
6 branching points that occur at intervals of approximately 25-30 α-D-glucose
residues.
Amylopectin is considered as one of the largest molecules that exist in nature due
to their extensive branching.
It is a reducing sugar and is present abundantly in potatoes and in seed.
If starch were to be stored as monomers, it would increase the intracellular
osmotic pressure. Hence, storing glucose as starch keeps the osmotic pressure inside
the cell under check, preventing the cells from lysis.
Figure 3A
(α1 6) branch point
Reducing endNonreducing end
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Fig. 3B depicts the arrangement of amylose and amylopectin in starch
granules. A double helical structure is formed when amylopectin (red) coils
with itself or with amylose strands Amylose (blue). Starch is mobilized for
energy production when glucose residues are enzymatically cleaved from the
nonreducing ends.
Figure 3B
2.1.2 Glycogen
It is the major storage form of carbohydrate in animals, stored as granules largely
in the liver and in muscle.
The granules consist of several clusters of smaller granules. Each granule is
comprised of one branched glycogen molecule.
It is a highly branched form similar to amylopectin but is more extensively
branched than amylopectin as the α1 6 branching occurs every 8 to 14 D-glucose
residues (Fig.4).
Glycogen has many non-reducing ends, on which glycogen phophorylase can act
to mobilize the breakdown of glycogen to generate free glucose molecules.
Glycogen debranching enzyme acts on the branches.
Both these enzymes are associated, in a tightly bound form with the glycogen
granules making the mobilization of glycogen more efficient.
Glycogen granules are more tightly packed and have more branches than starch.