Metal Ion Dependent Binding of Sulphonamide to Carbonic Anhydrase

Abstract

ACETAZOLAMIDE (2-acetylamino-1,3,4-thiadiazole-5-sulphonamide, ‘Diamox’) is the most potent known inhibitor of the zinc enzyme carbonic anhydrase. This communication reports the direct demonstration that binding of acetazolamide to human carbonic anhydrase requires the presence of a metal ion at the active site and that binding depends on the species of divalent metal ion present. Zinc (II) and cobalt (II) ions are the only ions which induce the formation of very stable acetazolamide carbonic anhydrase complexes and are also the ions which most effectively catalyse the hydration of carbon dioxide and the hydrolysis of p-nitrophenyl acetate. Metal-binding monodentate ions, CN⁻, HS⁻, OCN⁻, and N₃⁻, known as effective carbonic anhydrase inhibitors, compete for the acetazolamide binding site of the zinc enzyme.