Polyprenyl glycosyl diphosphates

The ability of nucleoside glycosyl diphosphates to serve as glycosyl phosphate donors in the biosynthesis of polyprenyl glycosyl diphosphates (see Section 11,3) is well documented. It is not yet known whether they may function as immediate precursors of glycosyl phosphate residues not infrequently present as components of bacterial polymers, or whether some other intermediates are involved. 

In addition to Glc-pp-Bpr,77,79,80 identified polyprenyl glycosyl diphosphates include derivatives of a-D-galactopyranose,81 85 2-acetamido-2-deoxy-a-D-glucopyranose,73,74,86"89 and 2-acetamido-2-deoxy-a-D-galac-topyranose.90 Also, several analogous N-acetylmuramylpentapeptide... 

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. 

It seemed of interest to check whether the type of biosynthetic mechanism involved may be related to the structure of the polymeric chain formed. In the block mechanism of chain assembly, formation of polyprenyl glycosyl diphosphates through transfer of glycosyl phosphate is a necessary step of the process. Thus far, only UDP-activated monosaccharides have been found to participate in this reaction (see Section 11,3). Consequently, the presence of a monosaccharide residue of this group in the main chain of a polymer may be regarded as a necessary condition for assembly of the chain through the block mechanism. 

In abbreviations for polyprenyl glycosyl phosphates, p stands for phosphate, and pp" for diphosphate groups. The abbreviation Bpr means a bacterial undecaprenyl group. For the moraprenyl group (see Ref. 57), the abbreviation Mpr is used, and the symbol Pre stands for any unidentified, lipid acceptor having the properties of polyprenol. 

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