N-acetylhexosamine

A feasible way of introducing acid-stable linkages into carbohydrates is N-deacetylation. This can be achieved with hydrazine.59,70,71 The use of sodium hydroxide-sodium benzenethioxide in aqueous dimethyl sulfoxide for this purpose has also been described72 The difference in the acid hydrolysis of N-acetylhexosamine-con-taining carbohydrates before and after N-deacetylation was used in the study of complex glycoprotein saccharides from human erythrocyte membranes.73-75 Methylation analysis of the glycopeptides prepared... 

Further developments are shown in Figure 4. On the basis that glucosamine reacted with pyruvic acid in the presence of alkali to yield pyrrole-2-carboxylic acid, in 1% yield, Gottschalk (21) proposed that sialic acid was formed by an aldol condensation reaction between N-ace-tylglucosamine and pyruvic acid. Kuhn and Brossmer (15) and Zilliken and Glick (22) showed that the reverse reaction also took place under alkaline conditions. Cornforth, Firth, and Gottschalk (23) synthesized crystalline N-acetylneuraminic acid (NANA) from N-acetylglucosamine and oxaloacetic acid (pH 11, 20°C). Under conditions less subject to misinterpretation, Heimer and Meyer (24) found that Vibrio cholerae enzymes cleaved NANA into an N-acetylhexosamine and pyruvic acid. 

In this connection Kochetkov and co-workers (73) have reported that hog stomach A + H substance contains several N-acetylhexosamine residues bound by (1 - 3) linkages to the N-acetyl-D-galactosamine residue which is linked to serine or threonine of the peptide backbone, but no oligosaccharides have yet been isolated. 

Valley U, Nimtz M, Conradt HS, Wagner R (1999), Incorporation of ammonium into intracellular UDP-activated N-acetylhexosamines and into carbohydrate structures in glycoproteins, Biotechnol. Bioeng. 64 401-417. 

Figure 9 Selective ceramide deacylation of N-acetylhexosamine that contains GSLs. Cangliotriaosyl ceramide is shown as example. In organic solvent, 1 N base hydrolysis results in the cleavage of both amides, but acetylation in aqueous buffer only affects the amino sugar.

The N-acetylhexosamines can usually be quantitatively released (as the hexosamines) from glycoproteins with 4 N HCl at 100°C for 6 hr (Spiro 1972). However, under these conditions the other carbohydrate components will undergo varying amounts of destruction. 

Also, levels of uridine triphosphate (UTP) and uridine diphosphate glucose (UDP-glucose) fall dramatically within the first hour after the administration of galactosamine. There is also a concomitant rise in UDP-hexosamines and UDP-N-acetylhexosamines ffmure 7.40b). 

FIGURE 7.40b The effect of galactosamine on biochemical parameters. The graph shows the effect on ATP (A), UTP (t), UDP-glucose (0), UDP-N-acetylhexosamines (D) and UDP-hexosamines (M). Adapted from Decker and Kepler (1974) Rev. Physiol. Biochem. Pharmac., 77, 78. 

Horwitz, A. L., and Dorfinan, A., The enzymatic defect in Morquio s disease The specificity of N-acetylhexosamine sulfatases. Biochem. Biophys. Res. Commun. 80, 819-825 (1978). 

Matalon, P., Arbogart, B., and Dorfman, A. Morquio s syndrome Deficiency of a chondroitin sulfate N-acetylhexosamine sulfate sulfatase. Biochem. Biophys. Res. Commun. 61, 6-13 (1974a). 

Fig. 6. Transformed ESIMS spectrum of the heterogeneous GPI species attached to the C-terminal peptide Glu22TSer231. A, (Peptide 221-231 ).Ea.P.Hex3.(HexNAc) (P.Ea). HexN.Ino.P, [Ea = ethanolamine Hex, hexose HexN, hexosamine HexNAc, N-acetylhexosamine I, inositol P, phosphate Sia, sialic acid.]...

Eukaryotic PNPTs are localized in the membrane of the endoplasmic reticulum (ER) where they catalyze the first step in N-linked glycoprotein biosynthesis resulting in a Dol-PP-GlcNAc intermediate. In contrast, bacterial PNPTs such as WecA, MraY, WbpL, and WbcO utilize different N-acetylhexosamine substrates and they also differ in their susceptibility to selective inhibitors. Several regions of conserved amino acid sequence can be found in bacterial and eukaryotic members of the PNPT family. It is plausible that all the members of this family utilize a common enzymatic mechanism for the formation of the phosphodiester bond. However, bacterial and eukaryotic PNPTs differ in their substrate specificity for various N-acetylhexosamine substrates and also they can discriminate the type of polyisoprenol phosphate. Und-P contains 11 isoprene units all of which are fully unsaturated, while Dol-P can be made of 15-19 isoprene units that have a saturated ct-isoprene. The ct-isoprene is the phosphorylated end of the molecule, which participates in the phosphodiester bond formation with the N-acetylhexosamine-l-P. Therefore, the ability of eukaryotic and bacterial enzymes to exquisitely discriminate their lipid substrate is likely a reflection of evolutionary divergence. 

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