Polysaccharides: overview

Polysaccharides: overview

Polysaccharides are ubiquitous in nature.

They can be classified into three separate

groups, based on their different functions.

Structural polysaccharides provide mechanical

stability to cells, organs, and organisms.

Waterbinding polysaccharides are strongly

hydrated and prevent cells and tissues from

drying out. Finally, reserve polysaccharides

serve as carbohydrate stores that release

monosaccharides as required. Due to their

polymeric nature, reserve carbohydrates are

osmotically less active, and they can therefore

be stored in large quantities within the cell.

A. Polysaccharides: structure 

Polysaccharides that are formed from only

one type of monosaccharide are called homoglycans,

while those formed from different

sugar constituents are called heteroglycans.

Both forms can exist as either linear or

branched chains.

A section of a glycogen molecule is shown

here as an example of a branched homoglycan.

Amylopectin, the branched component of

vegetable starch, has a very similar

structure. Both molecules mainly consist of

α1
4-linked glucose residues. In glycogen,

on average every 8th to 10th residue carries

—via an α1
6 bond—another 1,4-linked

chain of glucose residues. This gives rise to

branched, tree-like structures, which in animal

glycogen are covalently bound to a

protein, glycogenin .

The linear heteroglycan murein, a structural

polysaccharide that stabilizes the cell

walls of bacteria, has a more complex structure.

Only a short segment of this thread-like

molecule is shown here. Inmurein, two different

components, both β1
4-linked, alternate:

N-acetylglucosamine (GlcNAc) and

N-acetylmuraminic acid (MurNAc), a lactic

acid ether of N-acetylglucosamine. Peptides

are bound to the carboxyl group of the lactyl

groups, and attach the individual strands of

murein to each other to form a three-dimensional

network . Synthesis of the

network-forming peptides in murein is inhibited

by penicillin .

B. Important polysaccharides 

The table gives an overview of the composition

and make-up both of the glycans mentioned

above and of several more.

In addition to murein, bacterial polysaccharides

include dextrans—glucose polymers

that are mostly α1
6-linked and α1
3-

branched. In water, dextrans form viscous

slimes or gels that are used for chromatographic

separation of macromolecules after

chemical treatment . Dextrans are

also used as components of blood plasma

substitutes (plasma expanders) and foodstuffs.

Carbohydrates from algae (e. g., agarose

and carrageenan) can also be used to produce

gels. Agarose has been used in microbiology

for more than 100 years to reinforce culture

media (“agar-agar”). Algal polysaccharides are

also added to cosmetics and ready-made

foods tomodify the consistency of these products.

The starches, themost important vegetable

reserve carbohydrate and polysaccharides

from plant cell walls, are discussed in greater

detail on the following page. Inulin, a fructose

polymer, is used as a starch substitute in diabetics’

dietary products. In addition,

it serves as a test substance for measuring

renal clearance .

Chitin, a homopolymer from β1
4-linked

N-acetylglucosamine, is the most important

structural substance in insect and crustacean

shells, and is thus the most common animal

polysaccharide. It also occurs in the cell wall

of fungi.

Glycogen, the reserve carbohydrate of

higher animals, is stored in the liver andmusculature

in particular . The

formation and breakdown of glycogen are

subject to complex regulation by hormones

and other factors .