The inhibition by anion binding of reactions of inorganic radical anions with bovine carbonic anhydrase B

Reactions of the inorganic radical anions, Br₂^$\text{•}$ and (SCN)₂^$\text{•}$, with bovine carbonic anhydrase (carbonate hydrolyase, EC 4.2.1.1) have been studied by pulse radiolysis. Reaction is almost completely inhibited by the binding of Br⁻, SCN⁻ and ClO₄⁻ to an electrophilic site at the active centre of the enzyme. Dissociation constants for anion binding calculated from the reduction in free radical reactivity agree well with inhibition constants for these anions. The anions OCN⁻ and CN⁻, although potent inhibitors of carbonic anhydrase activity, have relatively little effect on the reactivity of radical anions with the enzyme. Reaction of radical anions occurs mainly with tryptophan and tyrosine residues in the hydrophobic core of the enzyme, through a channel at the active site. This channel is closed by the anions in accord with their position in the lyotropic series.