Crystallographic studies of azide binding to human carbonic anhydrase II

The crystal structures of human carbonic anhydrase II (CAII) at pH 5.7 and 8.0 have been determined at 0.21-nm resolution in the presence of 20 mM azide, which is a noncompetitive inhibitor of the CAII-catalyzed CO2 hydration reaction. Although azide often facilitates the crystallization of CAII and its variants, this small anion does not cause any significant structural changes in the enzyme active site or in the overall protein structure, and zinc coordination remains tetrahedral over the pH range 5.7-8.0. Importantly, the binding of azide at pH 8.0 has implications for the zinc-binding mode of the catalytic product, bicarbonate ion. Since azide is a competitive inhibitor of the reverse reaction of bicarbonate dehydration, and since the zinc-bound azide nitrogen makes a non-hydrogen-bonded van der Waals contact with the hydroxyl group of Thr199, it is possible that a zinc-bound bicarbonate oxyanion could likewise make a non-hydrogen-bonded, van der Waals contact with the hydroxyl group of Thr199. Therefore, the donation of a hydrogen bond to Thr199 is not absolutely required for anion binding to tetracoordinate zinc.