N-Glycan Asn-Linked Glycoproteins

The pathway for de novo synthesis of dolichol phosphate is the same as for cholesterol until production of famesyl pyrophosphate (Chapter 19). Two salvage pathways provide for the reutilization of dolichol and dolichol pyrophosphate once transfer of oligosaccharide from the dolichol to glycoprotein has occurred. A significant portion of the dolichol in liver is present as the free alcohol or as a fatty acid ester, and dolichol absorbed from the diet may enter this pool. Phosphorylation of dolichol is catalyzed by a dolichol kinase via cytidine triphosphate (CTP). Removal of one phosphate 

The lipid-oligosaccharide is built by sequential attachment of mannose, N-acetylglucosamine, and glucose units 

CHAPTER 16 Carbohydrate Metabolism 111 Glycoproteins, Glycolipids, GPI Anchors, Proteoglycans, and Peptidoglycans 3UDP-  

The secondary or tertiary structure can prevent glyco-sylation of sites within those regions, and disruption of tertiary structure allows glycosylation to occur at new sites in certain glycoproteins. Availability of active transferase and of lipid-linked oligosaccharide also affects the rate of glycosylation. 

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