10.3. The p«a values vary from 9.2 to 9.7 correlating reasonably well with the activity profiles. When bicarbonate is added to the samples the 113-Cd resonances shift upfield to new characteristic positions. The inhibitors CN", SH", and SCN” bind directly to the metal ion with their C, S, and N atoms, respectively. The results are best explained by assuming a rapid exchange between three species in which the open coordination site of the metal ion is occupied by'hydroxide, water, or bicarbonate.
Another approach has been to study kinetic properties of the active enzyme. A number of monovalent anions were investigated as inhibitors of carbon dioxide hydration catalyzed by human carbonic anhydrase II. Predominantly uncompetitive inhibition patterns were observed at pH near 9 in all cases. The inhibition of human carbonic anhydrase II by the organic compounds tetrazole, 1,2,4-triazole, 2-nitrophenol, and chloral hydrate was also investigated. These inhibitors, together with phenol, can be classified in three groups depending upon the kinetic patterns of inhibition of carbon dioixde hydration at pH near 9. The first group, represented by tetrazole and 2-nitrophenol, yields predominantly uncompetitive inhibition under these conditions in analogy with simple, inorganic anions. The second group, represented by 1,2,4-triazole and chloral hydrate gives rise to essentially noncompetitive inhibition patterns whereas phenol, representing the third group, is a competitive inhibitor of carbon dioxide hydration.
These results are analyzed in terms of two rivaling mechanism models, a kinetic scheme originally proposed by Steiner et al. (Eur. 3. Biochem. (1975) 59, 253-259) and a rapid-equilibrium kinetic scheme proposed by Pocker and Deits (3. Am. Chem. Soc. (1982) 104, 2424-2434). It is concluded that the observed steady-state inhibition patterns are compatible with both models, but hat discriminatory data, strongly favouring the model of Stêiner et al., are available in the literature.