Sugar nucleotide precursors

The main function of the ester 34 in bacterial cells seems to be its participation in the biosynthesis of the glycopeptide cell-wall polymer. If this process is blocked, there results the accumulation of a high concentration of sugar nucleotide precursors in the cell. A number of these compounds have been isolated the simplest one is the ester of uridine 5 -pyrophosphate with N-acetylmuramic acid [2-acetamido-3-0-(D-l-carboxyethyl)-2-deoxy-D-glucose] (37), first obtained from Staphylococcus aureus cells that had been treated with penicillin7,151 or Gentian Violet.144 An intermediate in the biosynthesis of 37 was isolated and shown to be the 3 -enolpyruvate ether152,153 (38). 

Most of the precursor pools turnover within a few minutes [12], and the size of various sugar nucleotide pools have been determined by methods with different reliability. But even with the best methods, it is hard to interpret the measured numbers and translate them into a clear picture because the relative distribution of the precursors in the cytosol and Golgi is not known. The average cellular concentration of sugar nucleotide precursors may not be very meaningful, since cytosol is an operational definition (100,000 g supernatant) that may not detect compartments of cytoplasmic organization or substrate channeling. 

Many other oligosaccharides and polysaccharides are synthesized by similar routes in which sugars are activated for subsequent reactions by attachment to nucleotides. In the glycosylation of proteins, for example (see Fig. 27-34), the precursors of the carbohydrate moieties include sugar nucleotides and lipid-linked oligosaccharides. 

From this brief survey, it is seen that there were few features of carbohydrate metabolism in plants that escaped Hassid s touch, and much that we now know about the role of sugar nucleotides in the interconversion of carbohydrates in plants is a direct result of his persistent effort. From the incorporation of labelled precursors into monosaccharides, to the conversion of the monosaccharides into their glycosyl phosphates, to the action of the pyrophosphorylases in the synthesis of glycosyl esters of nucleoside pyrophosphates, to the interconversion of the resulting sugar nucleotides, to the polymerization of the activated monosaccharides to yield disaccharides and the homopolysaccharides, and, finally, to the modification of the polysaccharides by methylation—in summary, to almost every aspect... 

This brief summary does not exhaust the list of the known transformations of glycosyl nucleotides. It is clear, however, that, from a-D-gluco-pyranosyl phosphate, it is possible to obtain a wide variety of sugar nucleotides. These may, in turn, serve as precursors of the innumerable glycosides, including oligo- and poly-saccharides, found in Nature. 

Table 30.2. Some Sugar Nucleotides That Are Precursors for Transferase Reactions...

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