[摘要]：The visible absorption of crystals of Co(II)-substituted human carbonic anhydrase II (Co(II)-HCA II) were measured over a pH range of 60-11 0 giving an estimate of pK(a) 8 4 for the ionization of the metal-bound water in the crystal This is higher by about 1 2 pK(a) units than the pK(a) 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 pK(a) of 8 3 for the aqueous ligand of the cobalt was measured for Co(II)-HCA II in solution containing 08 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 6A resolution of Co(II)-HCA II were determined for crystals prepared at pH 60. 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 (C) 2010 Elsevier Inc All rights reserved.
