High-mannose

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. 

The enzymes are available from a variety of sources and are often specific for certain types of glycosidic linkages and also for their anomeric natures. The sites of action of endoglycosidases F and H are shown in Figure 47-5. F acts on both high-mannose and complex oligosaccharides, whereas FI acts on the former. 

Complex, Hybrid, High-Mannose Are the Three Major Classes of N-Linked Oligosaccharides... 

The protein-bound oligosaccharide is then partially processed by glucosidases and mannosidases if no additional sugars are added, this results in a high-mannose chain. 

The use of f.a.b.-m.s. to monitor hydrolysis and acetolysis reactions is illustrated by studies on a yeast high-mannose oligosaccharide containing two phosphoric esters (19). The molecular-ion region of the negative f.a.b. 

The compositions of high-mannose oligosaccharides isolated from urine of animals suffering from genetic or chemically induced mannosidoses have been assigned from their f.a.b.-mass spectra. After chromatography, some samples were sufficiently pure to be examined underivatized. Others required conversion into their peracetylated derivatives. 

F.a.b.-m.s. has contributed to a detailed analysis of the high-mannose carbohydrate chains of prostaglandin endoperoxide synthase from sheep. Oligosaccharides released by hydrazinolysis were analyzed after N-re-acetylation and reduction. The f.a.b. data defined the compositions of the oligosaccharides, and showed that hydrazinolysis had effected a partial fission of the chitobiose core. 

Townsend, R. R., Atkinson, R H., and Trimble, R. B., Separation of high-mannose isomers from yeast and mammalian sources using high pH anion-exchange chromatography, Carb. Res., 215, 211. 1991. 

Guttman, A. and Pritchett, T., Capillary gel electrophoresis separation of high-mannose type oligosaccharides derivatized by l-aminopyrene-3,6,8-trisul-fonic acid, Electrophoresis, 16, 1906, 1995. 

C-N.m.r., Anomeric Chemical-shift" Data67 for High-Mannose Oligopeptides Derived... 

Fig. 15.1. The A/-glycan biosynthetic pathway, illustrating how the three classes of A/-glycans (high mannose, hybrid and complex glycans) are derived from a common biosynthetic precursor. The trimannosyl-chitobiose core, which is common to all A/-glycans, is outlined on the precursor A/-glycan. Glc, o Man, GIcNAc.

A fourth family of A-glycans are referred to as hybrid glycans. These glycans share structural features of the high mannose and complex-type families. They usually retain two mannoses on the 6-arm of the trimannosyl core whilst complex-type antennae are elaborated on the 3-arm. Hybrid structures are frequentiy bisected and may also be core fucosylated (Fig. 15.1). It should be noted that whilst core fucosylation is common in complex-type, hybrid and truncated glycans, it is rarely found in high mannose glycans. 

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