O-Glycan Ser Thr -Linked Oligosaccharides

Oligosaccharide chains linked by 0-glycosidic bonds to seryl or threonyl residues show more structural variability than do the asparagine-linked oligosaccharides the serine (threonine)-linked oligosaccharides range in size from 1 to 20 or more monosaccharide residues. The residues are added one at a time, directly on the protein, rather than preassembled on a lipid carrier. Assembly is not random, however, and the glycosyltransferases involved have acceptor specificities that render certain structures preferable. 

Polypeptide a-GalNAc transferase initiates O-glycan synthesis by attachment of GalNAc to Ser/Thr resides of proteins. There does not appear to be a specific amino acid sequence signal for GalNAc transfer however, Pro is often 

Synthesis of O-glycan cores (a) core 1 3-Gal-T (b) core 3 /33-GlcNAc-T (c) core 2 86-GlcNAc-T (d) core 4 j66-GlcNAc-T  

CHAPTER 16 Carbohydrate Metabolism III Glycoproteins, Glycolipids, GPI Anchors, Proteoglycans 

Processing reactions of 0-glycan core 1 (a) core 2 /16-GlcNAc-T (b) elongation fi3-GlcNAc-T (c) 1,66-GlcNAc-T (d) 6-SA-T I (e) a3-SA-T (f) a6-SA-T II (g) of2-Fuc-T. [Reproduced with permission from I. Brockhausen, Clinical aspects of glycoprotein biosynthesis. Crit. Rev. Clin. Lab. Sci. 30 80 (1993).] 

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