β-glucosidase activity in the presence of copper and goethite

We investigated the effect of goethite and copper on almond P-glucosidase activity. The activity of beta-glucosidase was found to be inhibited at dissolved copper concentrations exceeding 0.2 mM. Copper was most influential in the pH range 5-5.5, at which the enzyme activity was reduced by 50% or more at total copper concentrations of 0.2 mh I compared with copper-free assays. At pH 4, the presence of 0.2 mM copper reduced the activity by 15% at most. Copper caused a shift of the pH optimum towards lower pH. Goethite did not influence beta-glucosidase activity significantly, although up to 95% of the enzyme was adsorbed on its surface. The adsorption seemed to be caused principally by non-electrostatic forces which were too weak to affect the structure of the enzyme. Goethite reduced the inhibitory effect of copper because of the strong affinity of copper for goethite, as observed in batch adsorption experiments. The sorption of the enzyme on goethite was not competitive with copper at concentrations less than 0.2 mM: at larger concentrations, however, the presence of the enzyme reduced copper adsorption. The influence of copper on enzyme activity as well as the influence of copper in combination with goethite could be described with a model combining Michaelis-Menten enzyme kinetics with a simple conditionally first-order reaction law for the binding of copper by the enzyme.