Active-site-directed irreversible inhibition of E. coliβ-galactosidase by the ‘hot’ carbonium ion precursor, β-D-galactopyranosylmethyl-p-nitrophenyltriazene

Abstract

The title compound deactivates Na⁺- and Mg²⁺- saturated E. coli β-galactosidase at 25 °C and pH 7·0 with a maximum rate of (4·02 ± 0·17)× 10^–3s^–1, its binding being governed by a K of 73 ± 11 µM; the enzyme is protected against this irreversible inhibitor by the competitive inhibitor methyl 1-thio-β-_D-galacto-pyranoside to an extent predicatble from its kinetically-determined K_i, whilst removal of Mg²⁺ increases the maximum deactivation rate [to (5·7 ± 0·7)× 10^–2s^–1] and weakens the binding [K= 207 ± 47 µM], inhibition being quantitatively associated with attachement of the β-D-galactopyranosylmethyl group to the protein.