ABSTRACT
Secretory and cytoplasmic glycosylation makes use of nine different monosaccharides (Fig. 2.1), e.g., for the synthesis of glycoproteins (N-and O-glycosylation; Sections 2.4, and 2.5), or the group of the linear GAGs, also known as proteoglycans (Section 2.6). The first nucleotide sugar is attached to an Asn or a Thr/Ser residue, respectively, by the catalytic action of glycosyltransferases (Chapters 6-10), and bound via its C(1)OH to form an a or b linkage. This sugar may now serve as an acceptor for the next sugar molecule with a glycosidic linkage being established between the C1(OH) or C2(OH) of the donor and one of the accessible positions (2,3,4,6) of the acceptor substrate, and so forth. Glycosylation of ceramide results in the formation of GSLs (see Chapter 11). Many proteins are anchored by glycosylphosphatidylinositol (Fig. 11.17) to the cell surface; its biosynthesis requires phosphatidylinositol as acceptor substrate.
