THE BIOSYNTHESIS OF ANIMAL GLYCOPROTEINS

Abstract

This chapter discusses biosynthesis of animal glycoproteins. The term “glycoprotein” covers a multitude of different biopolymers with only a single common characteristic, the presence of both carbohydrate and protein joined in covalent linkage. Glycoproteins are defined as conjugated proteins containing as prosthetic group one or more heterosaccharides with a relatively low number of sugar residues, lacking a serially repeating unit and bound covalently to the polypeptide chain. The monosaccharide components of mammalian glycoproteins and proteoglycans are all synthesized by way of nucleotide sugars. In vivo, these compounds normally originate from glucose, but certain other monosaccharides may also serve as precursors of their respective nucleotide derivatives. All interconversions of glucose to the sugars found in glycoproteins occur at or prior to the nucleotide sugar stage. The l-iduronic acid residues of heparin are formed at the polymer level by epimerization of D-glucuronic acid residues. Because 5-epimerization of UDP-D-glucuronic acid to UDP-L-iduronic acid has been demonstrated, it is possible that different pathways exist for the formation of polymeric iduronic acid units.