Identification of Sites of Glycosylation

Abstract

Glycoproteins typically contain three types of glycans (1), the so-called N-linked glycans which are attached via an amide bond to asparagine in a Asn-Xxx-Ser(Thr) motif where Xxx can be any amino acid except proline; the O-linked glycans that are attached to serine or threonine, and glycosylphosphatidylinositol lipid anchors attached to the carboxy-terminus of some proteins. Glycoproteins containing N-linked glycans typically possess from 1-20 glycosylation sites that may or may not be occupied, usually with a range of carbohydrate structures. Each individual glycoprotein is known as a “glycoform”. All of these N-linked carbohydrate structures contain a trimannosyl-chitobiose [Manα 1→(Manα 1 →6)Manβ 1 →-4GlcNAcβ 1→ 4GlcNAc] pentasaccharide core with one or more glycan chains (antennae) attached to each of the nonreducing mannose residues (see Fig. 1). Glycans containing only mannose in the antennae are termed “high-mannose,” those with galactose and GlcNAc in both antennae are termed “complex” and glycans with both mannose and GlcNAc on different antennae are known as “hybrid” glycans. Fucose, sialic acid, other monosaccharides and sulphate are frequently also present. O-linked glycans are usually smaller, lack a common core structure and are usually found in groups on adjacent or closely spaced amino acids. Several structural types are recognised (see Fig. 1). Unlike the case of N-linked glycans, there is no consensus sequence of amino acids directing O-linked glycosylation.