Enzymes glycosidases, mechanism

With the interest in glycosidase inhibitors, compounds lending insight to the structures and mechanisms of these enzymes have received much attention. Recently, Lehmann, et al.,27 prepared a series of diastereotopic C-glycosides designed to be substrates for p-D-galactosidase. The premise was that in order to confirm this enzyme s mechanism of action, proposed from an extrapolation of lysozyme activity,28 31 the structural analysis of products resulting from the enzyme s effect on its substrates would be helpful. Therefore, the structures... 

Figure 21 Chemical probes for monitoring glycan-processing enzymes, (a) Mechanism-based probe for labeling active exo-glycosidases in cell lysates/ (b) FRET-based reporter for OGT activity in cells. ...

Another unusual glycosidase mechanism classified is encountered in family 4, which requires a unique NAD" and Mn " " assisted catalysis. Those enzymes do not follow the classical double-displacement mechanism but act through multiple catalytic steps involving oxidation-elimination-addition and reduction reactions. Other interesting examples can also be found in the case of trans-sialidases and a-glucan lyases. ... 

G. Legler and L. M. O. Osama, Mechanism of action of glycosidase splitting enzymes. IV. Purification and properties of a P-glucosidase from Aspergillus oryzae, Hoppe Seyler s Zeitschr. Physiol. Chem., 349 (1968) 1488-1492. 

The glycosidases act by two different mechanisms which is revealed by the stereochemistry at the anomeiic centre of the product (McCarter and Withers, 1994). In one type of glycosidases the anomeiic centre is directly attacked by a hydroxide to give a product with inverted stereochemistry at the anomeiic centre. In the other mechanism, the anomeric centre is attacked by the carboxylate group of a glutamic acid residue to form an intermediate in which the carbohydrate moiety is covalently bound to the enzyme similar to in epoxide hydrolases (Figure 2.16) and serine hydrolases. Attack on this intermediate by a nucleophile leads to the net result which is retention of the stereochemistry at the anomeric centre. 

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