EXAFS studies of human carbonic anhydrase

Human Carbonic Anhydrase (HCA) is a zinc metalloenzyme which catalyzes the hydration of carbon dioxide. It is known to exist in at least nine distinct isoforms, which have widely different catalytic activities. HCA II is one of the fastest enzymes known. Structural data from X-ray diffraction is available for HCA I and II only. We have obtained high-quality EXAFS data from the zinc K edge for active HCA I, II and III in aqueous solution and analyzed it using restrained refinement with inclusion of multiple-scattering effects. We find the zinc environment to comprise three imidazole rings from histidine residues and one oxygen atom, presumably from a water molecule. The orientation of the rings is well-defined. Despite the variation in catalytic activity between the three forms we find the zinc environment to be essentially identical in them all.