Coordination chemical studies on metalloenzymes. IX. Properties of the ternary complex between cobalt(II)-bovine carbonic anhydrase and bidentate ligands.

The spectrum, thermodynamic parameters, and proton longitudinal relaxation time of the ternary complex between various bidentate ligands (2-pyridinecarboxylate, 2-quinolincarboxylate, 8-quinolinecarboxylate, and 2-pyridylacetate) and cobalt(II)-bovine carbonic anhydrase were measured to clarify the nature of the ternary complex. The formation constants of the ternary complexes of bidentate ligands are in the order of (2-pyridinecarboxylate approximately greater than 8-quinolinecarboxylate much greater than 2-quinolinecarboxylate approximately equal to 2-pyridylacetate). The degree of the shift of the band characteristic of five-coordinate species at 13-15 kcm-1 (cm-1 X 10(-3)) and that of the higher energy band at 21-22 kcm-1 decrease almost in the same order. These results are explained on the basis of the contribution of the bond formation between the nitrogen atom of the heterocyclic ring of ligands and cobalt ion. The formation constants of the ternary complex of bidentate ligands were compared to the stability constants of various ligands with a cobalt ion but there is no correlation in these values. The rate constant of break-up of the ternary complex was discussed on the coordination geometry of the ternary complex on the basis of the degree of the distortion.