Abstract
The effect of phosphate buffer on the kinetic behavior of jack bean urease covalently immobilized on chitosan membrane was studied in the pH range 5.76-8.19, and compared with that of the free enzyme in an attempt to elucidate the effects of heterogeneity of the system on its kinetics. The chemical inhibition by the buffer, occurring between pH 5.76 and 7.50, was found to consist of two antagonistic effects: a decrease in the intrinsic enzyme activity and a reduction in the degree of environment-related inhibition. The apparent kinetic constants of the immobilized urease: v x max., K x M and K i,buffer exhibited both pH- and buffer concentration-dependence, anomalous as compared to the free enzyme: the optimum pH and the pK i,buffer values were displaced toward more acidic pH values, and the pK x M values were leveled off. The anomalies were gradually suppressed by increasing the buffer concentration. The anomalous behavior of chitosan membrane-immobilized urease was accounted for by a combined effect of: a) the increase in local pH on the membrane produced by both the enzymatic reaction and the electric charge of the support, and b) diffusional limitations imposed on substrate and product in the external solution.
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