Page 26 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: FPP 2nd Revised Pages
Encyclopedia of Physical Science and Technology EN002C-64 May 19, 2001 20:39

230 Biopolymers

Cell walls of bacteria often contain and may be coated its lubricating and cushioning effects, and direct injection
by extremely complex polysaccharides. These polysac- of this polysaccharide can help patients suffering from
charides seem to be constructed of regular repeats— osteoarthritis.
frequently of four or more sugar units. A great variety of Heparan sulfate and heparin have backbones based
different polysaccharides have been found, some linear, on alternating glucuronic acid and N-acetyl glucosamine
some branched, some containing amino sugars, and oth- residues (Fig. 15). In both cases extensive modification
ers containing uronic acids; the structure depends on the of the monosaccharide units takes place after polymer
bacterial source. The outer membrane of Gram negative synthesis. The N-acetyl group on the glucosamine can
bacteria contains lipopolysaccharide (i.e., polysaccharide be removed and replaced with sulfate; D-glucuronic acid
chains covalently linked to lipid). Again the polysaccha- can be converted to L-iduronic acid and sulfate groups
ride is made from regular repeats. can be attached at carbons 3 and/or 6 of the N-sulfated
Many of these polysaccharides act as antigens and stim- glucosamine and at carbon 2 of the iduronic acid. Hep-
ulate the formation of antibodies (see Section II.A.2), if arin differs from heparan sulfate in having a higher con-
the bacteria invade mammals. It is sometimes possible to tent of N-sulfated glucosamine (>85%) and iduronic acid
confer immunity against one bacterial strain by immuniz- (>70%). Both heparin and heparan sulfate occur mainly
ing a mammal at risk with the bacterial polysaccharide attached to proteins as proteoglycans (see next section).
only, without the need for using intact bacteria. This pro- Heparin is used as a blood anticoagulant. A specific pen-
cedure is obviously less hazardous than if whole bacteria tasaccharide sequence in the polysaccharide binds to a
were employed. protein, antithrombin, and prevents a series of reactions
taking place that would lead to blood clotting.
6. Animal Polysaccharides
7. Peptidoglycan, Proteoglycan, and Glycoprotein
Glycogen is a glucan that acts as a short-term energy re-
serve in animals although it is also found in some bacteria, Several biopolymers contain both oligopeptide or protein
algae, and fungi. It is much like amylopectin in struc- and oligosaccharide or polysaccharide covalently bonded
ture but is more highly branched, with approximately 7% together in their molecules.
of the glucose residues carrying branches. The compact Peptidoglycans consist essentially of polysaccharide
structure of glycogen allows for efficient storage and fast chains cross-linked by oligopeptides, producing a cage-
breakdown by enzymes to glucose when living organisms like arrangement which can be the major structural com-
require energy. ponent of a bacterial cell wall. The polysaccharide chains
Glycogen metabolism is of medical interest, for individ- consist of disaccharide repeats based on glucosamine;
uals are known that lack one or another enzyme important one example is shown in Fig. 16a. The structure of the
for glycogen synthesis or breakdown. Where an enzyme cross-linking peptides depends on the bacterial source,
of glycogen breakdown is missing, the individual is said and one example is given in Fig. 16b. In these pep-
to have a glycogen storage disease. These genetic diseases tides, D-amino acids can be found, unlike the case of
are usually serious, and may be fatal, as in Pompe’s dis- proteins, where only L-amino acids occur. Penicillins
ease, characterized by lack of one particular glucosidase are antibacterial agents, because they act as inhibitors
in subcellular organelles called lysosomes. of peptidoglycan synthesis, by preventing cross-link for-
Another group of polysaccharides, the glycosaminogly- mation. Other polymers, the teichoic acids, are found in
cans, are mostly of animal origin. They all contain mod- bacterial cell walls covalently bonded to peptidoglycan
ified amino sugars and disaccharide repeats along their through a phosphorylated derivative of the disaccharide
linear chains (Fig. 15). Hyaluronic acid is found in skin, shown in Fig. 16a. These acids are linear polymers of
connective tissues, and joint fluid, while chondroitin and glycerol phosphate, CH 2 CHOH CH 2 OPO 2 O or
keratan sulfates occur in cartilage and bone. Chondroitin ribitol phosphate CH 2 (CHOH) 3 CH 2 O PO 2 O
and dermatan sulfates form part of the structure of skin. and may contain sugar rings as side chains or in the main
All of these, except hyaluronic acid, exist in tissues co- backbone chain.
valently bonded to protein, that is, as proteoglycans (see Proteoglycans contain protein and polysaccharide in the
Section II.B.7). The polysaccharide molecules may form same molecule. In structural tissues of the fiber and ma-
a network that impedes the flow of water and so may trix type, proteoglycans frequently occur in the matrix.
be important for the correct hydration of tissues and in Thus, in plant cell walls, cellulose fibers are embedded in
some cases for lubrication and shock absorption in joints. a matrix which contains, among other polymers, arabino-
Hyaluronic acid can be used in eye surgery because of galactans attached to protein. In animals collagen fibers