C-13 AND H-1-NMR STUDIES OF IMIDAZOLE BINDING TO NATIVE AND CO(II)-SUBSTITUTED HUMAN CARBONIC ANHYDRASE-I

The inhibition constant of imidazole toward human carbonic anhydrase I has been directly determined at pH 6.8 and 8.8 through C-13 NMR. The data are in agreement with those obtained by indirect methods and confirm that the binding affinity of imidazole is substantially constant in the pH range investigated. Analysis of the isotropically-shifted signals in the H-1 NMR spectra of the Co(II)-substituted enzyme interacting with imidazole at high and low pH in the presence of sulfate as counterion indicates the likely formation of a five-coordinate HCAI-imidazole-sulfate adduct at low pH. The binding mode of imidazole in the active site of the enzyme is discussed.