Enzyme-substrate interactions: structure of human carbonic anhydrase I complexed with bicarbonate

Abstract

The structure of HCAI-HCO^-₃ complex has been refined with 10-1.6 Å X-ray diffraction data to an R-value of 17.7%. The structure reveals monodentate binding of the HCO^-₃ anion at an apical tetrahedral position to the zinc ion. The binding mode and interactions of HCO^-₃ in HCAI differ from that in HCAII. The activity linked H₂O/OH^- group in the free HCAI is replaced by the hydroxyl group of the bicarbonate anion. This result rules out the rearrangement of the bound HCO^-₃ advocated earlier to explain the microscopic reversibility of the catalysed reaction. From the geometry of the H-bonds between Glu106-Thr199 pair and Glu117-His119 couple, the glutamic acids are expected to be ionized and accept H-bonds from their partners. These product-inhibition by HCO^-₃ anion is explained on the basis of proton localization on His119 in the Glu117-His119 couple. These results are consistent with the hypothesis that Glu117-His119 tunes the ionicity of the Zn²⁺ and the binding strength of HCO^-₃ anion. A π hydrogen bond is observed between a water and phenyl ring of the Tyr114 residue.