Comparative biochemistry of nucleotide-linked sugars.

Nucleotide-linked sugars have 2 general biochemical functions: they are i) intermediates in the formation of monosaccharides found in complex carbohydrates and ii) glycosyl donors of these monosaccharides. Few sugars arise by reactions not involving nucleotide-linked intermediates. Of these few, glucose, mannose, and N-acetylglucosamine are important in that they are transformed after attachment to nucleotides into most other monosaccharides. Several different nucleotides are involved in these transformations. What factor governs the choice of a particular nucleotide carrier for a given reaction is not apparent, but the use of different nucleotides separates pathways of synthesis and offers a means for their independent control by creating reactions unique to the synthesis of certain products and therefore suitable for regulation. Carrying sugars on different nucleotides may also be advantageous by increasing the accuracy of synthesis of complex carbohydrates. For example, a transferase responsible for the transfer of fucose from GDP-fucose is less likely to transfer galactose from UDP-galactose by mistake than galactose from GDP-galactose. The role of nucleotides in the synthesis of complex carbohydrates thus appears related to the specificity of enzymes that catalyze the modification and transfer of nucleotide-linked sugars.