Proton magnetic resonance studies of carbonic anhydrase. III. Binding of sulfonamides

Resonances of the histidine region of human carbonic anhydrase B have been studied by proton magnetic resonance spectroscopy in the presence of seven sulfonamide inhibitors. Results of difference spectroscopy and observation of the C-2 resonance of an additional titratable histidine in some of these spectra suggest a conformational change in the enzyme, while the large number of unaltered resonances indicates involvement of only a few residues. Inhibition of carbonic anhydrase by sulfonamides appears to involve: stabilization of an appropriately oriented initial complex by hydrophobic binding of the aromatic ring of the inhibitor to residues of the cavity forming the active site; ionization of the sulfonamido group, facilitated by its proximity to zinc; protonation and displacement of the high pH ligand to the metal controlling catalytic activity, thought here to be a histidine residue; and formation by the sulfonamido group of an ionic bond to zinc and a hydrogen bond to the hydroxyl group of serine or threonine. Diversity of spectra produced with various sulfonamides suggests that substituents on the ring and heteroatoms within the ring interact with additional groups at the active site. Increase in inhibitory potency appears to involve optimizing the number as well as the strength of these interactions. An upper limit for the dissociation rate of these complexes of 10 sec-1 was obtained.