Biosynthesis of Other Polysaccharides

Synthesis of other polysaccharides involves many of the same mechanisms as for glycogen, particularly the use of nucleotide-linked sugars as activated biosynthetic intermediates and glycosyltransferase enzymes. Biosynthesis of several polysaccharides is described briefly below:

> Cellulose - a glucose homopolymer with (1->4) linkages between the units. UDP-glucose is used as an intermediate in some plant species. ADP-glucose and CDP-glucose are used by other plants. ADP-glucose is also the intermediate used in plant start synthesis and in the synthesis of glycogen in bacterial cells.

> Chitin - a structural polysaccharide in the exoskeletons of arthropods and mollusks composed of (1->4) linked N-acetylglucosamine. The activated intermediate is UDP-N-acetylglucosamine.

> Hyaluronic Acid - a heteropolymer of alternating units of glucuronic acid and N-acetylglucosamine. A two enzyme system alternately adds units from UDP-glucuronate and UDP-N-acetylglucosamine to the growing chain.

> Dextran - an (1->6) polymer of glucose with (1->2), (1->3), and (1->4) branch points. Dextran is the only polysaccharide listed here that does not use activated nucleotide sugars (or derivatives) in making the polymer. The polymerization, catalyzed by the enzyme dextran sucrase, is a transglycosylation of sucrose:

> > n Sucrose -> glucose n (dextran) + n fructose

> Dextran is made by several bacteria growing in the human mouth and contributes to dental plaque.

See also: Sucrose, Fructose