BIOSYNTHESIS AND PROCESSING OF GLYCOPROTEINS

Abstract

This chapter focuses on biosynthesis and processing of glycoproteins. Considerable progress has been achieved providing a clearer understanding of the mode of biosynthesis and assembly of glycosylamine-linked carbohydrate side chains of glycoproteins. The carbohydrate moiety is linked to the polypeptide chain by way of a glycosylamine linkage utilizing the amide nitrogen of an Asn residue in the protein and the linkage region invariably consists of trisaccharide Man-β1, 4-GlcNAc-β1, 4-GlcNac-β-Asn. Beyond the invariant trisaccharide linkage region of the sidechain, two or more branch structures usually occur, each of which contains Man, GlcNAc, Gal, and one of the sialic acids. In biosynthesis of the carbohydrate sidechains of this type, two separate groups of enzymes participate in its biosynthesis, assembly, and attachment to the polypeptide chain. A group of glycosyltransferases, primarily localized in the Golgi apparatus appears to be solely responsible for the sequential addition of peripheral carbohydrate residues of the oligosaccharide side chain, including the NeuNAc-Gal-GlcNAc trisaccharide at the reducing termini of completed chains and fucose residues that may be located in terminal nonreducing positions elsewhere in the oligosaccharide.