O-glycosidases

A very interesting and useful apphcation of endo-A was reported recently by Wang and coworkers (40, 41). Based on the knowledge that some em/o-glycosidase effectively can transfer oxazoline derivatives of GlcNAc-ohgosaccharides, Wang and coworkers used endo-A on the oxazoline derivative of... 

Whether retaining or inverting, O-glycosidases have a number of common features ... 

Although one can write a canonical mechanism for retaining O-glycosidases, there are departures from this mechanism in acid-catalytic machinery and the nature of the nucleophile (for some families, not the carboxylate shown). In Table 5.4 are set out the nature of the catalytic groups, the protonation trajectory and the conformations of substrate and glycosyl enzyme, where at least two of these are known. 

Retaining and inverting O-glycosidases make use of a general acid/general base pair comprised of a carboxylic acid and carboxylate of aspartic acid or glutamic acid side chains (see also Eig. 21). These side chains have similar values in solution but the micro-environments of enzyme active sites allow each pK to be tuned so that each side chain is in the correct protonation state. This makes it possible for a carboxylate and a carboxylic acid to exist in the active site simultaneously. 

Glycosyl transferases and O-glycosidases modify glycoproteins and are important for polysaccharide catabolism. Inhibitors of these enzymes are potential drugs in the treatment of cancer, viral infection and diabetes. There are two mechanistic classes of glycosidases, retaining and inverting." ... 

As with glycoside hydrolases, the polysaccharide lyases (PL) have long been classified into sequence-related protein families (129) in the CAZy database (28). Presently, more than 20 PL families are listed in CAZy, and, because of significant recent attention, at least one three-dimensional structural representative is known for approximately one half of these (reviewed in Refe. (126,127), and (130) see also http //www.cazy.org/fam/acc PL.html). Not surprisingly, these endo-acting enzymes possess cleft-shaped active sites (126), akin to the polysaccharide erac/o-glycosidases vide supra). Characteristically, polysaccharide lyases bind their polyanionic substrates through basic amino acid residues and/or Ca++ ion complexation. Indeed, the activity of nearly all known pectate lyases strictly requires Ca++, which in some cases may directly affect the catalysis (126). 

The chondroitin sulphate in rat liver has been shown to be degraded by the combined actions of endo- and ej o-glycosidases, whereas that in human fibroblasts - in which hyaluronidase activity is not detected - is degraded by cxo-glycanases. 

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