N-Linked glycosylation

This chapter discusses N-linked glycosylation. N-linked glycans, and the biosynthetic pathways that generate them, act as the cornerstone of our understanding of mammalian glycobiology. In spite of their diversity, all N-linked glycans on glycoproteins are generated through a common pathway, which starts in the endoplasmic reticulum with the transfer of a preformed oligosaccharide to an asparagine residue in the sequence Asn-X-Ser/Thr within a nascent polypeptide chain. The three major classes of N-linked oligosaccharides (high mannose, complex, and hybrid) result from the differential action of glycosidases and glycosyltransferases in the Golgi apparatus. The restricted specificities of these glycosidases and glycosyltransferases require that they act in a defined order. The structures found on a particular glycoprotein depend primarily on the glycosyltransferases being expressed in the cell type in which the glycoprotein is synthesized. Many glycoproteins are heterogeneous because a variety of different oligosaccharides can be attached to a particular asparagine residue.