Glycoproteins N-linked

FIGURE 9.26 The carbohydrate tnoiedes of glycoproteins may be linked to the protein via (a) serine or threonine residues (in the O-linked saccharides) or (b) asparagine residues (in the N-linked saccharides), (c) N-Linked glycoproteins are of three types high mannose, complex, and hybrid, the latter of which combines structures found in the high mannose and complex saccharides. 

Galactose Hexose Gal UDP-Gal Often found subterminal to NeuAc in N-linked glycoproteins. Also found in core trisaccharide of proteoglycans. 

Glucose Hexose Glc UDP-GIc Present during the biosynthesis of N-linked glycoproteins but not usually present in mature glycoproteins. Present in some clotting factors. 

Mannose Hexose Man GDP-Man Common sugar in N-linked glycoproteins. 

The Biosynthesis of N-Linked Glycoproteins Involves Dolichol-P-P-Oligosaccharide... 

Thus, the initial steps involved in the biosynthesis of the N-linked glycoproteins differ markedly from those involved in the biosynthesis of the O-hnked glycoproteins. The former involves Dol-P-P-ohgosaccharide the latter, as described earlier, does not. 

Figure 47-4. Structures of the major types of asparagine-linked oligosaccharides. The boxed area encloses the pentasaccharide core common to all N-linked glycoproteins. (Reproduced, with permission, from Kornfeld R, Kornfeld S Assembly of asparagine-linked oligosaccharides. Annu Rev Biochem 1985 54 631.)...

Figure 47-5. Schematic diagram of the pentasaccharide core common to all N-linked glycoproteins and to which various outer chains of oligosaccharides may be attached. The sites of action of endoglycosidases F and H are also indicated.

Studies have actually revealed two type I interferon receptor polypeptides. Sequence data from cloning studies place both in the class II cytokine receptor family. Both are transmembrane N-linked glycoproteins. Studies using isolated forms of each show that one polypeptide (called the a/p receptor) is capable of binding all type I interferons. The other one (the ap receptor) is specific for IFN-a-B (a specific member of the IFN-a family). Both receptors are present on most cell types. 

An intramolecular acetal has also been introduced by the treatment of a mixture of a 1-thio-mannoside, having a methoxybenzyl protecting group at C-2 and an alcohol with DDQ [71] (Scheme 4.4c). Activation of the thioglycoside with methyl triflate gave a P-mannoside as the only anomer. This approach was employed for the synthesis of the core pentasaccharide of N-linked glycoproteins. 

Both in viruses and tumors, specific N-linked glycoproteins are involved in the recognition phenomena responsible for the pathology. The biosynthesis of these... 

Mannose is primarily found in mammalian N-linked glycoproteins as part of core structures containing the tri-Man core [a-D-Man-(1 3)-a-D-Man-(1 6)-Man]. However, in the glycoproteins of yeast and molds, mannose, primarily a-D-Man-(1 2)-a-D-Man and a-D-Man-(1 6)-a-D-Man, has been found a-linked to Ser or Thr. 244 Expression of glucoamylase G1 in Aspergillus niger 245 and human IGF-I (somatomedin C) in Saccharomyces cerevisiae 246 resulted in proteins glycosylated with mannose at Ser and Thr. 

Synthesis of N-linked glycoproteins, o = N-acetylglucosamine = mannose = glucose i... 

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