Enzymatic properties of alpha-D-galactosidase from Trichoderma reesei

The kinetics of hydrolysis of a number of natural and synthetic substrates [melibiose, raffinose, stachyose, methyl alpha-D-galactopyranoside, and p-nitrophenyl alpha-D-galactopyranoside (PNPG)], catalyzed by alpha-D-galactosidase from the fungus Trichoderma reesei, has been studied, A number of N-acyl-alpha-D-galactopyranosylamines, which are competitive inhibitors of alpha-D-galactosidase, have been synthesized, and the K-I values for these compounds have been obtained. The inhibiting properties of the competitive inhibitors of D-galactose, 1,5-anhydro-D-galactitol, and 2-deoxygalactose have been compared, and reasons for differences in K-I values between these compounds have been discussed. It has been shown that alpha-D-galactosidase exhibits transglycosylating activity; the main product of transglycosylation in the reaction with PNPG is p-nitrophenyl 6-O-alpha-D-galactopyranosyl-alpha-D-galactopyranoside. The hydrolysis inhibition in the presence of a substrate has been shown to correlate with the substrate transglycosylation. Data of steady-state kinetics together with data of presteady-state kinetics obtained by the stop-flow method suggest that an intermediate galactosyl-enzyme complex is formed in the reaction and is of particular importance in the processes under study, A minimal kinetic scheme describing the experimental data obtained is proposed. (C) 1996 Elsevier Science Ltd.