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 -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 (-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.
O-糖基化是哺乳动物糖蛋白丝氨酸和苏氨酸残基常见的共价修饰。本章描述了通常被称为黏蛋白的糖蛋白的结构、生物合成及功能。在黏蛋白中，O-聚糖通过N-乙酰半乳糖胺（GalNAc）部分经O-糖苷键与丝氨酸或苏氨酸的-OH共价α连接，其结构被命名为黏蛋白O-聚糖或O-GalNAc聚糖。本章重点关注高度O-糖基化的黏蛋白和类黏蛋白糖蛋白，不过也简要讨论了仅携带一个或几个O-GalNAc聚糖的糖蛋白。还存在几种非黏蛋白O-聚糖类型，包括α连接的O-岩藻糖、β连接的O-木糖、α连接的O-甘露糖、β连接的O-GlcNAc（N-乙酰葡糖胺）、α或β连接的O-半乳糖以及α或β连接的O-葡萄糖聚糖（在第12章以及第16 - 18章中讨论）。然而，在本章中，除非另有说明，术语O-聚糖指的是黏蛋白O-聚糖。黏蛋白糖蛋白普遍存在于细胞表面和体液的黏液分泌物中。
