Polysaccharide chains biosynthesis

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

The sugar nucleotides (an uninformative name that has been used for glycosyl nucleotides, or more strictly, glycosyl esters of nucleoside di- or mono-phosphates) were discussed in this Series12 in 1973. Since then, accumulation of new data about these derivatives has continued, and now, about 35 representatives of this class are known to participate in the biosynthesis of polysaccharide chains of bacterial polymers (for a survey, see Ref. 13). These include glycosyl esters of uridine 5 -diphosphate (UDP), thymidine 5 -diphosphate (dTDP), guanosine 5 -diphosphate (GDP), cytidine 5 -diphosphate (CDP), cytidine 5 -monophosphate (CMP), and adenosine 5 -diphosphate (ADP). 

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. 

Synthetic moraprenyl a-D-glucopyranosyl diphosphate serves as an acceptor for D-mannosyl transfer. Among the synthetic moraprenyl diphosphates of disaccharides329 (compounds 21-24) tested as D-mannosyl acceptors, the a-(l- 3) derivative 23 was found to be the most efficient.330 An analogous mechanism of polysaccharide-chain assembly was shown331,332 to operate in the biosynthesis of the polysaccharide 20. 

The situation that exists in the case of the biosynthesis of neutral polysaccharide chains of streptococcal cell-wall is still very uncertain. In S. sanguis, assembly of the trisaccharide derivative (45, X = an unidentified, hydro-phobic group) from UDP-GlcNAc, dTDP-Rha, and UDP-Glc on an... 

The material presented in previous sub-sections clearly shows that both of the possible mechanisms of polysaccharide chain-assembly may operate in the biosynthesis of bacterial polysaccharides. There is no clearcut, mechanistic difference in the biosynthesis of O-specific chains of lipopoly-saccharides, exocellular polysaccharides, and carbohydrate chains of Grampositive, cell-wall polymers for every class of polymer, the existence of both mechanisms of chain assembly was demonstrated. 

---