Drug-biomolecule interactions: mechanism of ligand interactions with carbonic anhydrase studied by magnetic resonance relaxation and rapid reaction methods.

Kinetcs of interaction between the metalloenzyme carbonic anhydrase and either monovalent anions or aromatic sulfonamides were examined by three distinct fast reaction techniques: stopped flow, equilibrium perturbation, and magentic resonance relaxation. By correlating spectroscopic data on conformational and ionization equilibria of the complex and free species with the reaction kinetics, a relatively complete description of the mechanism can be presented. A proton-dependent equilibrium between two coordination forms of the free enzyme can be demonstrated spectroscopically. Anions selectively combine with the form predominating at low pH. For a series of carboxylate ligands, formate and substituted acetates, anion association is found to be three orders of magnitude greater than similar ligand substitution processes known in inorganic chemistry. For sulfonamide association, the attacking species are the form of carbonic anhydrase predominating at high pH and the neutral sulfonamide. Combination involving the neutral species is followed by loss of a proton to form the sulfonamido anion in the stabilized complex. This obligate proton transfer offers a probable explanation for the unique specificity of sulfonamides in inhibiting this enzyme. The anionic sulfonamido moiety can be shown to resemble closely a transition intermediate in the catalytic step of substrate hydration.