Anion inhibition of CO2 hydration catalyzed by human carbonic anhydrase II. Mechanistic implications

Five monovalent anions, I-, N3-, SCN-, NCO- and Au(CN)2-, were investigated as inhibitors of CO2 hydration catalyzed by human carbonic anhydrase II (carbonate hydro-lyase, EC 4.2.1.1). Predominantly uncompetitive inhibition patterns were observed at pH near 9 in all cases. While Dixon plots of Km/V vs. inhibitor concentration were linear, all the investigated anions except NCO- gave nonlinear Dixon plots of 1/V vs. inhibitor concentration. The anion SCN- was also tested at pH 7.4 and 6.4. Essentially noncompetitive patterns of inhibition of CO2 hydration were obtained at both these pH values. These results are analyzed in terms of two rivaling mechanism models, a kinetic scheme originally proposed by Steiner et al. (Steiner, H., Jonsson, B.-H. and Lindskog, S. (1975) Eur. J. Biochem. 59, 253-259) and a rapid-equilibrium kinetic scheme proposed by Pocker and Deits (Pocker, Y. and Deits, T.L. (1982) J. Am. Chem. Soc. 104, 2424-2434). It is concluded that the observed steady-state inhibition patterns are compatible with both models, but that discriminatory data, strongly favouring the model of Steiner et al., are available in the literature.