Kinetic and Magnetic Properties of Cobalt(III) Ion in the Active Site of Carbonic Anhydrase

Cobal(III) bovine carbonic anhydrase B was prepared by the oxidation of the cobalt(II) enzyme with hydrogen peroxide and was purified by affinity chromatography. The oxidation reaction is inhibited by specific inhibitors of carbonic anhydrase. The inhibition is explained by the fact that the Co(II)‐enzyme · inhibitor complex cannot be directly oxidized by hydrogen peroxide, but has to dissociate to give free Co(II) enzyme which is then oxidized. The Co(III) ion in Co(III) carbonic anhydrase cannot be directly substituted by zinc ions. It can be reduced by either dithionite or BH⁻₄ ions to give, first, their complexes with the Co(II) enzyme, and upon their removal, a fully active Co(II) enzyme. Cyanide and azide bind to cobalt(III) carbonic anhydrase with similar rate constants of 0.060 ± 0.005 and 0.070 ± 0.007 M⁻¹ s⁻¹ respectively. These rates are faster than those found for Co(III) inorganic complexes. The Co(III) ion in both Co(III) carbonic anhydrase and Co(III) carboxypeptidase A was found to be diamagnetic, indicating a near octahedral symmetry.