Glycoproteins - Hughes R. 1985

Biosynthesis
Glycosyltransferases

The Enzymes directly involved in The Biosynthesis of glycoprotein N- and O-glycans are collectively known as glycosyltransferases. The general reaction catalyzed by a glycosyltransferase is illustrated in Fig. 3.1: a monosaccharide residue is transferred from an activated high-energy donor (X) to a suitable acceptor (Y). The acceptor can be an asparagine, Serine, Threonine, or hydroxylysine residue within a polypeptide chain, or the carbohydrate sequence of an incompletely synthesized glycan. The monosaccharide transfer reaction can be repeated; for instance, during Glycogen synthesis, a glucose residue is transferred from a high-energy donor to the non-reducing end of the growing polysaccharide chain (Y). Glycolipids are formed in a similar manner by transferring the first monosaccharide to the primary hydroxyl group of ceramide, followed by the sequential addition of subsequent Monosaccharides.

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Fig. 3.1. Reaction catalyzed by a glycosyltransferase.

Individual glycosyltransferases typically exhibit high Specificity toward groups X and Y. In essence, these enzymes belong to the group of biologically active Proteins whose Research Methods and characteristics are still insufficiently developed. Nevertheless, certain aspects of the Catalytic Mechanism of several glycosyltransferases have already been elucidated, and many of these enzymes have been successfully isolated in homogeneous form.