The beta-glucosidase from Botryodiplodia theobromae. Mechanism of enzyme-catalysed reactions

The effects of pH and temperature on Michaelis constant (Km) and maximum velocity (Vmax.) and of NaCl on the activity of the high-molecular-weight beta-glucosidase (beta-D-glucoside glucohydrolase EC 3.2.1.21) from cultures of Botryodiplodia theobromae Pat. have been studied. 2. Donor binding and inhibition of activity by glucose were dependent on the ionization of a group (pK 6.0) that appeared to be an imidazole group. 3. Catalytic activity and the stimulation of activity by glycerol were dependent on the ionization of two groups, which appeared to be a carboxy group and an imidazole group. 4. The Arrhenius activation energy (Ea) calculated from results obtained at pH 4.0 and 5.0 was about 45--46kJ.mol-1. 5. The enthalpies (delta H0) calculated from results obtained at pH 4.0 and 5.0 were similar (about -4kJ.mol-1), whereas at pH 6.5 the value was about -33kJ.mol-1. 6. The entropies (delta S0) calculated from these results at 37 degrees C were -21, -22 and -118J.K-1.mol-1 at pH 4.0, 5.0 and 6.5 respectively. A low concentration of NaCl (16.6 mM) stimulated enzymic activity and decreased the Km for the donor, whereas high concentrations (up to 500 mM) inhibited enzymic activity, increased the Km and had no effect on Vmax. 8. Plots of initial velocity data obtained in the presence of dioxan as 1/v against the ratio of the molar concentration of dioxan to that of water were linear. 9. The results are discussed in terms of the enzyme mechanism.