Structure and function of carbonic anhydrases. Imidazole binding to human carbonic anhydrase B and the mechanism of action of carbonic anhydrases.

The isoenzymes carbonic anhydrases B and C (EC 4.2.1 .l. C02tH20 2 HCO; t H+) from human erythrocytes have been extensively investigated by different physico-chemical methods in order to understand the mechanism of action of these efficient enzymes [l-4]. These two enzymes differ in their catalytic rate by a factor of about 5 for a number of reactions investigated. The primary structures of these isoenzymes exhibit a high degree of homology and so do their tertiary structures [3]. Khalifah [5] has shown that imidazole is a competitive inhibitor of the hydration reaction of COZ by human carbonic anhydrase B. This is the only competitive inhibitor of this reaction reported for any carbonic anhydrase. It has been reported by a number of workers that aromatic or heterocyclic sulphonamides are competitive inhibitors for the reverse dehydration reaction [ 1,2]. The availability of a competetive inhibitor is of great help in the investigation of the enzyme mechanism by X-ray diffraction methods. We have located the binding site of imidazole in human carbonic anhydrase B (HCAB) crystals and arrived at the most probable COZ binding dte as well as the mechanism of action of the enzyme consistent with the existing data.