Summary

Summary Chapter 7.2

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Course
Biochemistry I (BIOL2080U)

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University Of Ontario Institute Of Technology (UOIT )

Summary study book Lehninger Principles of Biochemistry of Nelson David L., Albert L. Lehninger, David L. Nelson, Michael M. Cox, University Michael M Cox (7.2) - ISBN: 9780716743392 (Chapter 7.2)

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7.2 Polysaccharides

Most carbohydrates found in nature occur as polysaccharides, polymers of medium to high
molecular weight (Mr>20,000). Polysaccharides, also called glycans, differ from each other
in the identity of their recurring monosaccharide units, in the length of their chains, in the
types of bonds linking the units, and in the degree of branching. Homopolysaccharides
contain only a single monomeric species; heteropolysaccharides contain two or more
different kinds (Fig. 7-12).

Some homopolysaccharides serve as storage forms of monosaccharides that are used as
fuels; starch and glycogen are homopolysaccharides of this type. Other
homopolysaccharides (cellulose and chitin, for example) serve as structural elements in
plant cell walls and animal exoskeletons.

Heteropolysaccharides provide extracellular support for organisms of all kingdoms. For
example, the rigid layer of the bacterial cell envelope (the peptidoglycan) is composed in
part of a heteropolysaccharide built from two alternating monosaccharide units (see Fig. 6-
28). In animal tissues, the extracellular space is occupied by several types of
heteropolysaccharides, which form a matrix that holds individual cells together and provides
protection, shape, and support to cells, tissues, and organs.

FIGURE 7-12 Homopolysaccharides and heteropolysaccharides. Polysaccharides may
be composed of one, two, or several different monosaccharides, in straight or branched
chains of varying length.

Unlike proteins, polysaccharides generally do not have defining molecular weights. This
difference is a consequence of the mechanisms of assembly of the two types of polymer. As
we shall see in Chapter 27, proteins are synthesized on a template (messenger RNA) of
defined sequence and length, by enzymes that follow the template exactly. For
polysaccharide synthesis there is no template; rather, the program for polysaccharide
synthesis is intrinsic to the enzymes that catalyze the polymerization of the monomeric
units, and there is no specific stopping point in the synthetic process; the products thus vary
in length.

Some Homopolysaccharides Are Storage Forms of Fuel

The most important storage polysaccharides are starch in plant cells and glycogen in animal
cells. Both polysaccharides occur intracellularly as large clusters or granules. Starch and
glycogen molecules are heavily hydrated, because they have many exposed hydroxyl
groups available to hydrogen- bond with water. Most plant cells have the ability to form
starch (see Fig. 20- 5), and starch storage is especially abundant in tubers (underground
stems), such as potatoes, and in seeds.

Starch contains two types of glucose polymer, amylose and amylopectin (Fig. 7-13).
Amylose consists of long, unbranched chains of D-glucose residues connected by (α1→4)
linkages (as in maltose). Such chains vary in molecular weight from a few thousand to more
than a million. Amylopectin also has a high molecular weight (up to 200 million) but unlike
amylose is highly branched. The glycosidic linkages joining successive glucose residues in
amylopectin chains are (α1→4); the branch points (occurring every 24 to 30 residues) are
(α1→6) linkages.

, FIGURE 7-13 Glycogen and starch. (a) A short segment of amylose, a linear polymer of
D-glucose residues in (α1→4) linkage. A single chain can contain several thousand glucose
residues. Amylopectin has stretches of similarly linked residues between branch points.
Glycogen has the same basic structure, but has more branching than amylopectin. (b) An
(α1→6) branch point of glycogen or amylopectin. (c) A cluster of amylose and amylopectin
like that believed to occur in starch granules. Strands of amylopectin (black) form double-
helical structures with each other or with amylose strands (blue). Amylopectin has frequent
(α1→6) branch points (red). Glucose residues at the nonreducing ends of the outer branches
are removed enzymatically during the mobilization of starch for energy production.
Glycogen has a similar structure but is more highly branched and more compact.

Glycogen is the main storage polysaccharide of animal cells. Like amylopectin, glycogen is
a polymer of (α1→4)-linked glucose subunits, with (α1→6)-linked branches, but glycogen is
more extensively branched (on average, a branch every 8 to 12 residues) and more
compact than starch.

Glycogen is especially abundant in the liver, where it may constitute as much as 7% of the
wet weight; it is also present in skeletal muscle. In hepatocytes glycogen is found in large
granules (see Fig. 15-26), which are clusters of smaller granules composed of single, highly
branched glycogen molecules with an average molecular weight of several million. The large
glycogen granules also contain, in tightly bound form, the enzymes responsible for the
synthesis and degradation of glycogen (see Fig. 15-42).

Because each branch in glycogen ends with a nonreducing sugar unit, a glycogen molecule
with n branches has n + 1 nonreducing ends, but only one reducing end. When glycogen is
used as an energy source, glucose units are removed one at a time from the nonreducing
ends. Degradative enzymes that act only at nonreducing ends can work simultaneously on
the many branches, speeding the conversion of the polymer to monosaccharides.

Why not store glucose in its monomeric form? It has been calculated that hepatocytes store
glycogen equivalent to a glucose concentration of 0.4 M. The actual concentration of
glycogen, which is insoluble and contributes little to the osmolarity of the cytosol, is about
0.01 μM. If the cytosol contained 0.4 M glucose, the osmolarity would be threateningly
elevated, leading to osmotic entry of water that might rupture the cell (see Fig. 2-13).
Furthermore, with an intracellular glucose concentration of 0.4 M and an external
concentration of about 5 mM (the concentration in the blood of a mammal), the free-energy
change for glucose uptake into cells against this very high concentration gradient would be
prohibitively large.

Dextrans are bacterial and yeast polysaccharides made up of (α1→6)- linked poly-D-
glucose; all have (α1→3) branches, and some also have (α1→2) or (α1→4) branches. Dental
plaque, formed by bacteria growing on the surface of teeth, is rich in dextrans, which are
adhesive and allow the bacteria to stick to teeth and to each other. Dextrans also provide a
source of glucose for bacterial metabolism. Synthetic dextrans are components of several
commercial products (for example, Sephadex) used in the

fractionation of proteins by size-exclusion chromatography (see Fig. 3-17b). The dextrans in
these products are chemically cross-linked to form insoluble materials of various sizes.

Some Homopolysaccharides Serve Structural Roles

Cellulose, a tough, fibrous, water-insoluble substance, is found in the cell walls of plants,
particularly in stalks, stems, trunks, and all the woody portions of the plant body.

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