SN1 hydrolyses of glycosyl pyridinium salts, and quantification of the main source of catalytic power of E. coli(lacZ)-β-galactosidase

Abstract

The hydrolysis of the 3-chloro-1-(β-D-galactopyranosyl) pyridinium ion is independent of pH between pH 2 and 8; this hydrolysis and those of another four pyridine-substituted cations exhibit positive entropies of activation. Rates at 25 °C are between 10^–10 and 10^–12.5 times the k_cat values for the β-galactosidase-catalysed hydrolyses of the same compounds; for the 3-chloropyridinium salt ΔH^‡ is lowered by 21 kcal mol^–1 and ΔS^‡ by 25 cal mol^–1 K^–1. The α-deuterium kinetic isotope effect for the spontaneous hydrolysis of the β-D-galactopyranosylpyridinium ion is the same as that for its enzymic hydrolysis, indicating that both processes involve a galactosyl cation. The whole of the catalytic effect of the enzyme towards these substrates must therefore arise from non-covalent interactions with parts of the substrate other than the bond being cleaved. Arguments are presented that such interactions have comparable importance in the hydrolyses of oxygen glycosides.