3KOK: Crystal Structure Of Cobalt (Ii) Human Carbonic Anhydrase Ii At Ph 8.5

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 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 1.4M 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 0.8M 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 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.
PDB ID: 3KOKDownload
MMDB ID: 84365
PDB Deposition Date: 2009/11/13
Updated in MMDB: 2010/09
Experimental Method:
x-ray diffraction
Resolution: 1.5  Å
Source Organism:
Similar Structures:
Biological Unit for 3KOK: monomeric; determined by author and by software (PISA)
Molecular Components in 3KOK
Label Count Molecule
Protein (1 molecule)
Carbonic Anhydrase 2(Gene symbol: CA2)
Molecule annotation
Chemical (1 molecule)
* Click molecule labels to explore molecular sequence information.

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