The effect of methanol and dioxan on the rates of the β-galactosidase-catalysed hydrolyses of some β-<scp>d</scp>-galactopyranosides: rate-limiting degalactosylation. The pH-dependence of galactosylation and degalactosylation

Abstract

1. The effect of methanol on the beta-galactosidase-catalysed hydrolysis of some nitrophenyl beta-d-galactopyranosides has been studied under steady-state conditions. 2. The initial fractional rate of increase of k(cat.) as a function of methanol concentration with 2,4- and 3,5-dinitrophenyl beta-d-galactopyranosides, but not with the other substrates studied, indicated that degalactosylation of the enzyme was rate-limiting. 3. The decrease in k(cat.) at high methanol concentrations for these substrates is considered to arise from causes other than galactosylation becoming rate-limiting. 4. Both galactosylation and degalactosylation of the enzyme require protonation of a group of pK(a) approx. 9.