Metabolism of Sugars and Sugar Nucleotides

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... 

Apart from the oxidation at C-6 to form uronic acids from hexoses, the reactions of sugar nucleotides show a marked tendency towards the formation of deoxy and deoxy-amino sugars, especially in prokaryotes. Moreover, many of the enzymes of sugar nucleotide metabolism have requirements for NAD or, less often, for NADP. Among such enzymes are the 4-epimerases, which catalyse conservative reactions, yet need catalytic amounts of NAD. 

In some respects it is surprising that ring-opening and rearrangement is not a more common feature of sugar nucleotide metabolism and it seems likely that the system for producing the branched chain of apiose is an evolutionary derivative of the normal decarboxylase reaction. 

The enzymes of sugar nucleotide metabolism (see Chapter 2) may also be varied in an analogous way by the combination of subunits and their mechanistic similarities hint that this may be so. 

Evolutionary and Mechanistic Implications of Sugar Nucleotide Metabolism... 

Figure 20-7. Summary of the interrelationships in metabolism of amino sugars. (At asterisk Analogous to UDPGIc.) Other purine or pyrimidine nucleotides may be similarly linked to sugars or amino sugars. Examples are thymidine diphosphate (TDP)-glucosamine and TDP-N-acetylglucosamine.

The important role of glyeosyl esters of nucleoside pyrophosphates (often referred to loosely as sugar nucleotides or nucleoside diphosphate sugars ) in carbohydrate metabolism is well known. Two comprehensive Chapters of this Series,1,2 as well as several surveys in other publications3-5 have already appeared. These reviews deal mainly with the metabolism of these compounds, and... 

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