Page 49 - Color Atlas of Biochemistry

40 Biomolecules
Polysaccharides: overview
Polysaccharides are ubiquitous in nature.
They can be classified into three separate
groups, based on their different functions.
Structural polysaccharides provide mechani-
cal 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 substitutes (plasma expanders) and food-
be stored in large quantities within the cell.
A. Polysaccharides: structure
Polysaccharides that are formed from only
one type of monosaccharide are called homo-
glycans, 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 homogly-
can. Amylopectin, the branched component of
vegetable starch (see p. 42), has a very similar polymer, is used as a starch substitute in dia-
structure. Both molecules mainly consist of
α1 4-linked glucose residues. In glycogen,
on average every 8th to 10th residue car-
ries —via an α1 6 bond—another 1,4-linked
chain of glucose residues. This gives rise to
branched, tree-like structures, which in ani-
mal glycogen are covalently bound to a
protein, glycogenin (see p.156).
The linear heteroglycan murein,a struc-
tural polysaccharide that stabilizes the cell
walls of bacteria, has a more complex struc-
ture. Only a short segment of this thread-like
molecule is shown here. In murein, two differ-
ent components, both β1 4-linked, alter-
nate: 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-dimen-
sional network (not shown). Synthesis of the
network-forming peptides in murein is inhib-
ited by penicillin (see p. 254).
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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B. Important polysaccharides
The table gives an overview of the composi-
tion and make-up both of the glycans men-
tioned above and of several more.
In addition to murein, bacterial polysac-
charides 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 chromato-
graphic separation of macromolecules after
chemical treatment (see p. 78). Dextrans are
also used as components of blood plasma
stuffs.
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 to modify the consistency of these prod-
ucts.
The starches,the most importantvegetable
reserve carbohydrate and polysaccharides
from plant cell walls, are discussed in greater
detail on the following page. Inulin,a fructose
betics’ dietary products (see p.160). In addi-
tion, it serves as a test substance for measur-
ing renal clearance (see p. 322).
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 and mus-
culature in particular (A, see pp.156, 336). The
formationand breakdownofglycogen are
subject to complex regulation by hormones
and other factors (see p.120).