Catalytic consequences of experimental evolution. Part 1. Catalysis by the wild-type second β-galactosidase (ebg°) of Escherichia coli: a comparison with the lacZ enzyme

Abstract

β-D-Galactopyranose is the initial product of the hydrolysis of β-D-galactopyranosyl fluoride by ebg° enzyme. Transfer to methanol of a β-D-galactopyranosyl residue from either m-nitrophenol or 3-bromopyridine is seven times more favourable than to water. Breakage of the aglycone–glycone bond limits the rate of, and is the first irreversible step in, the hydrolysis of both aryl galactopyranosides and galactosylpyridinium ions. In the transition state for this process the anomeric carbon atom is ca. 1/3 of the way towards sp² hybridisation, the fissile bond is largely broken, and a proton is transferred a little way towards the aglycone oxygen of the leaving group.