Comparison of solution and crystal properties of Co(II)-substituted human carbonic anhydrase II |
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Authors: | Balendu Sankara Avvaru Chingkuang Tu Robert McKenna David N Silverman |
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Institution: | a Department of Biochemistry and Molecular Biology, University of Florida, Gainesville, FL 32610, USA b Department of Physics, University of Florida, Gainesville, FL 32610, USA c Department of Pharmacology, University of Florida, Gainesville, FL 32610, USA |
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Abstract: | The visible absorption of crystals of Co(II)-substituted human carbonic anhydrase II (Co(II)-HCA II) were measured over a pH range of 6.0-11.0 giving an estimate of pKa 8.4 for the ionization of the metal-bound water in the crystal. This is higher by about 1.2 pKa units than the pKa near 7.2 for Co(II)-CA II in solution. This effect is attributed to a nonspecific ionic strength effect of 1.4 M citrate in the precipitant solution used in the crystal growth. A pKa of 8.3 for the aqueous ligand of the cobalt was measured for Co(II)-HCA II in solution containing 0.8 M citrate. Citrate is not an inhibitor of the catalytic activity of Co(II)-HCA II and was not observed in crystal structures. The X-ray structures at 1.5-1.6 Å resolution of Co(II)-HCA II were determined for crystals prepared at pH 6.0, 8.5 and 11.0 and revealed no conformational changes of amino-acid side chains as a result of the use of citrate. However, the studies of Co(II)-HCA II did reveal a change in metal coordination from tetrahedral at pH 11 to a coordination consistent with a mixed population of both tetrahedral and penta-coordinate at pH 8.5 to an octahedral geometry characteristic of the oxidized enzyme Co(III)-HCA II at pH 6.0. |
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Keywords: | Carbonic anhydrase Metalloenzyme Cobalt substitution Crystallography Carbon dioxide |
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