Polysaccharides units, biosynthesis

Figure 48-9. Structure of heparin. The polymer section illustrates structural features typical of heparin however, the sequence of variously substituted repeating disaccharide units has been arbitrarily selected. In addition, non-O-sulfated or 3-0-sulfated glucosamine residues may also occur. (Modified, redrawn, and reproduced, with permission, from Lindahl U et al Structure and biosynthesis of heparin-like polysaccharides. Fed Proc 1977 36 19.)...

An understanding of the structures of the molecules of heparin and heparan sulfate has come, in part, from studies with degradative enzymes, but also with biosynthetic enzymes. Both polysaccharides are based on the disaccharide unit which, in the initial stage of biosynthesis, consists of... 

BIOSYNTHESIS OF BACTERIAL POLYSACCHARIDE CHAINS COMPOSED OF REPEATING UNITS... 

Polyprenyl glycosyl diphosphates operate mainly as membrane-linked glycosyl acceptors in the biosynthesis of carbohydrate chains of bacterial polymers. In reactions of polymerization of repeating units during polysaccharide synthesis, the polyprenyl diphosphate derivatives serve as donors of growing polysaccharide chain, but monosaccharide transfer from a polyprenyl glycosyl diphosphate has never been detected. 

A modification of the monosaccharide units of polysaccharides may obviously be effected at different stages of the biosynthesis of a polymer (a) prior to formation of the activated form of a monosaccharide, (b) at the level of glycosyl nucleotides, (c) at the stage of formation of oligosaccharide intermediates, and (d) after the synthesis of a polymeric chain. 

The most complete list of modified monosaccharide units of bacterial polysaccharides may be found in a review by Kenne and Lindberg.98 The present discussion is limited only to examples where some data on biosynthesis are available. 

In the biosynthesis of the capsular polysaccharide from Xanthomonas campestris, the modification was shown265 to occur at the level of a polyprenyl pentasaccharide diphosphate intermediate prior to polymerization of the repeating units, and enolpyruvate phosphate was a precursor of the pyruvic acid residues. A similar observation was made during a study of the biosynthesis of Rhizobium meliloti exopolysaccharide.266... 

The industrially important, extracellular polysaccharide (31) of Xan-thomonas campestris131,341,342 is another example of complete assembly of the repeating unit during the biosynthesis of a branched polysaccharide. 

Biosynthesis of repeating units of the main chain of these polysaccharides (see Section V,2,a) includes the consecutive transfer of a-D-galacto-pyranosyl phosphate from UDP-Gal, the L-rhamnopyranosyl group from dTDP-Rha, and the D-mannopyranosyl group from GDP-Man onto a poly-... 

Dextran is a unique polysaccharide because of its structure (only glucose units), purity, defined branching pattern depending on the microbial sources and defined molecular weight. Today, it is produced on a commercial scale resulting from optimised biotechnological processes for the biosynthesis of dextran using preferably Leuconostoc mesenteroides. 

In 1967, two possible general mechanisms for the biosynthesis of linear polysaccharides were proposed.86 The first was the primer mechanism already discussed, and the second was the sequential addition of monomer units to the reducing end by the insertion between a carrier and the growing polysaccharide chain. This latter mechanism had been shown for Salmonella sp. O-antigenic polysaccharide87 then in 1973, it was shown for the bacterial cell-wall polysaccharide murein88 in which the carrier was a polyisoprenoid pyrophosphate. 

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