Crystal-structure of the Complex Between Human Carbonic Anhydrase-ii and the Aromatic Inhibitor 1,2,4-triazole

The X-ray crystal structure of the complex between human carbonic anhydrase II and the inhibitor 1,2,4-triazole has been refined at 1·9 Å resolution to a final R-factor of 0·153. Triazole is an analogue of the competitive inhibitor imidazole, but the crystal structure shows a different type of binding to the enzyme, 1,2,4-Triazole is directly bound to the zinc(II) ion through the nitrogen in position 4, replacing the native water/hydroxyl (Wat263) in a distorted four-co-ordinated complex. The interaction of the inhibitor with the active site is completed by two hydrogen bonds to Oγ of Thr200 and to the amide nitrogen atom of Thr199 through the two adjacent N-1 and N-2 atoms. The binding site of triazole overlaps the proposed binding sites for the substrates, explaining the observed competitive behaviour of the inhibitor towards CO2/HCO-3 under equilibrium conditions.