Biosynthesis of Oligosaccharyl Dolichol

Abstract

Oligosaccharyl dolichol is a biosynthetic intermediate in the process of attaching an oligosaccharide to particular asparagine residues in proteins via an N-glycosidic bond. These glycoproteins, often referred to as N-linked glycoproteins or asparagine-linked glycoproteins, generally have 1-20'%) carbohydrate by weight which represents 1 -10 oligosaccharides attached to the protein backbone. N-linked glycoproteins are membrane-associated or soluble products of a biosynthetic system found in the secretory pathway of eukaryotic cells. These glycoproteins are diverse in structure and function and often have other co-/post-translational modifications such as lipids or sugars linked via 0-glycosidic bonds. For example, the insulin receptor, low-density lipoprotein receptor, ovalbumin, thyroglobulin, transferrin, immunoglobulins, lysosomal hydrolases, HMG-CoA reductase, and many Golgi glycosy1 transferases are N-linked glycoproteins.The function of the oligosaccharide moiety on these N-linked glycoproteins varies from one N-linked glycoprotein to another. These functions include having a role in the folding of the protein, being important for the biological half-life, being an important component of ligand-receptor interactions, affecting three-dimensional structure. and affecting solubility (see [ 11 and later Chapters of this volume).The cotranslational addition of oligosaccharide from oligosaccharyl dolichol to protein is necessary for life, as will be discussed below in this Chapter. Oligosaccharyl transferase, a multi-subunit, ER enzyme, catalyzes the transfer of oligosaccharide from oligosaccharyl dolichol to asparagine residues in an Asn-X-Ser(Thr) consensus sequence of protein (see next chapter and [2, 31). Thus, the core of each oligosaccharide of N-linked glycoproteins is not synthesized by a step-wise addition of monosaccharides onto a protein substrate, as is the case for oligosaccharides on many types of glycoproteins. Instead, the oligosaccharide core of N-linked glycoproteins is first assembled step-wise on a lipid carrier, transferred en bloc to protein, and then modified by numerous glycosidases, glycosyltransferases, and sulfoCarbohydrates in Chemistry and Biology 38 3 Biosynthesis of' Oligosaccharyl Dolichol Figure 1. Structure of oligosaccharyl dolichol. The structure of this amphipathic phosphopolyisoprenyl glycolipid is given in this Figure. As mentioned in the text, the major isomer of dolichol in hamsters is N = 17 (4) while the major isomer of dolichol in S. pombe is N = 14,15 (5). M = mannose; GlcNAc = N-acetylglucosamine; G = glucose. transferases (most of which are subjects of other chapters in this volume) in order to reach its final. mature structure. Oligosaccharyl DolicholOligosaccharyl dolichol is a large, amphipathic phosphopolyisoprenyl glycolipid; its structure is shown in Figure 1. This glycolipid has solubility properties similar to large, complex glycosphingolipids, in that it is not soluble in the standard lipid solvent, chlorofomxmethanol 2 : 1 but is soluble in a more polar sol...