Glycosyltransferases carbohydrate biosynthesis

Keywords Natural products Carbohydrate Glycosyltransferase Chemoenzymatic synthesis Biosynthesis... 

Glycosyltransferases assemble polysaccharides from their monomeric units by adding activated nucleotide sugars to the growing carbohydrate chain [19]. The chemistry of glycosyltransferases is well known [20-22] and is illustrated in the biosynthesis of sialyl Lewis [23], an essential contributor to the cell adhesion process. In fact, without both the siahc acid and fucose components, the sialyl Lewis remnant is incapable of binding to selectins and effecting cell adhesion necessary for leukocyte extravasation [24-29]. 

The first enzymatic step involves the covalent attachment of the first carbohydrate unit to the side chain of a specific amino acid residue. Further sugar residues are added in sequential order by specific glycosyltransferases for each of the sugar residues. It appears that specific multienzyme systems are required for the biosynthesis of each type of polymer (153). Frequently the carbohydrate chain terminates with an N-acetylneuraminic acid residue. 

Phosphotransferases. There are several types of enzymes involved in the biosynthesis of the carbohydrate chains of glycoproteins. These include the phosphotransferases, the nucleotidyltransferases, and the glycosyltransferases of which there are two types. 

Some aspects of the mechanism of 0-specific polysaccharide biosynthesis in Salmonella newport and S. Kentucky have been reported. Cell envelope and soluble glycosyltransferase preparations from both organisms catalysed the formation of polyprenyl pyrophosphate oligosaccharides with carbohydrate units corresponding to the structures of the main-chain polysaccharides (Scheme 1). The D-glucosylation reaction occurs before the polymerization of the repeating units to the polysaccharide. 

---