title: Kinetics and mechanism of anionic ligand binding to carbonic anhydrase.
abstract: The kinetics of complex formation between Co(II)-carbonic anhydrase B and the anions cyanate, thiocyanate and cyanide has been studied at different pH values employing temperature-jump relaxation spectrometry. Formation of the 1:1 complex occurs via binding of the deprotonated state of the anion to an acidic state of the enzyme. The determined formation rate constants range from 10(8) to 3 X 10(9) M-1 s-1 and are two to three orders of magnitude higher than the value estimated for a ligand coordination to the central Co2+, based on a solvate substitution mechanism. These kinetic results strongly indicate that the deprotonated anion binds to an unoccupied coordination position of the protein-bound heavy metal ion in the form of an addition reaction. Upon binding of the anion, the coordination number of the Co2+ in the acidic state of the enzyme is increased from four to five. In the case of cyanide, a 2:1 anion complex is also formed. The formation rate constant is 5 X 10(5) M-1 s-1 which provides good evidence that this binding process is controlled by a solvate substitution mechanism.
mesh_terms: D000818:Animals; D000838:Anions; D002256:Carbonic Anhydrases / Q000378:metabolism*; D002417:Cattle; D003035:Cobalt / Q000378:metabolism; D007700:Kinetics; D008024:Ligands; D008433:Mathematics; D013053:Spectrophotometry; D013816:Thermodynamics
chemical_list: D000838:Anions; D008024:Ligands; D003035:Cobalt; D002256:Carbonic Anhydrases