O-GalNAc Glycans

Excerpt

O-glycosylation is a common covalent modification of serine and threonine residues of mammalian glycoproteins. This chapter describes the structures, biosynthesis, and functions of glycoproteins that are often termed mucins. In mucins, O-glycans are covalently α-linked via an N-acetylgalactosamine (GalNAc) moiety to the -OH of serine or threonine by an O-glycosidic bond, and the structures are named mucin O-glycans or O-GalNAc glycans. The focus of this chapter is on mucins and mucin-like glycoproteins that are heavily O-glycosylated, although glycoproteins that carry only one or a few O-GalNAc glycans are also briefly discussed. There are also several types of nonmucin O-glycans, including α-linked O-fucose, β-linked O-xylose, α-linked O-mannose, β-linked O-GlcNAc (N-acetylglucosamine), α- or β-linked O-galactose, and α- or β-linked O-glucose glycans (discussed in Chapters 12 and 16–18). In this chapter, however, the term O-glycan refers to mucin O-glycans, unless otherwise specified. Mucin glycoproteins are ubiquitous in mucous secretions on cell surfaces and in body fluids.