Glycosyl pyrophosphates

The conversion of thymidine 5 -(6-deoxy-Q -D-xi/lo-hexopyranosyl-4-ulose pyrophosphate) into thymidine 5 -(D-fucosyl and 6-deoxy-D-glycosyl pyrophosphates) was accomplished by reduction with sodium borohydride.329,330 The reaction products were separated by paper chromatography in a borate-containing solvent-system. 

Although no mechanistic studies of this hydrolysis have been reported, the data available suggest that the mechanism is similar to that proposed for the hydrolysis of a-D-glucopyranosyl phosphate in weakly acidic media,332 namely protonation of the glycosyl pyrophosphate derivative and slow heterolysis of the resulting monoanion (79) to produce a cyclic carbonium ion (80), a species considered to be an intermediate in the hydrolysis of glycosides (for a review, see Ref. 333). 

Both mechanisms seem to be consistent with incorporation of a label from tritiated water into 101 (at C-2),412 but not into the unreacted glycosyl pyrophosphate esters,413 and also with kinetic evidence414 for liberation of the pyrophosphate 6 prior to release of 101. No requirement for added NAD has been reported, and a search for pyridine nucleotide firmly bound to the enzyme may lead to conclusive evidence for the mechanism. 

One of the most impressive findings has been the discovery of lipid intermediates in the biosynthesis of polysaccharides (see Refs. 2 and 465.) At least two structurally different types of these compounds exist the intermediate may be an isoprenoid alcohol ester of the glycosyl pyrophosphate or the analogous derivative of the glycosyl phosphate. Derivatives of the first type are formed by reaction between the sugar nucleotide and the alcohol phosphate, for example, undecaprenyl phosphate (120), which participates in the biosynthesis of Salmonella lipopolysaccharide.466... 

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