Role of the substituent at C-5 of the pyranose ring in catalysis by E. coli(lacZ)β-galactosidase

Abstract

Removal of the C-5 hydroxymethyl group of the galactopyranosyl substrates of E. coliβ-galactosidase has only a slight effect—usually a decrease—on the rate of the conformation-change step with aryl glycosides, on the rate of cleavage of the C–N bond of the glycosylpyridinium ion, or on the rate of hydrolysis of the glycosyl-enzyme. Values of K_m are all substantially increased. Substrate destabilisation of the particular type commonly envisaged for lysozyme catalysis therefore plays no part in catalysis by this particular glycosidase, and the C-5 hydroxymethyl group plays a role only in binding.