Coordination Chemical Studies on MetalloenzymesII. Kinetic Behavior of Various Types of Chelating Agents towards Bovine Carbonic Anhydrase

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Abstract
In order to investigate the kinetics and mechanism of the removal of zinc ions from bovine carbonic anhydrase [EC 4.2.1.1] (BCA), several chelating agents with various stability constants were used to remove zinc from BCA. The second-order rate constants (kapp) of zinc- removal from BCA were found to be in the following order; 2,6-pyridinedicarboxylic acid≫2-pyridinecarboxylic acid> 2,4-pyridinedicarboxylic acid> 2,3-pyridinedicarboxylic acid ≥1,10-phenanthroline≥5-methyl-1,10-phenanthroline≫2,2′-bipyridine. With similar chelating agents. the greater the stability constant, the faster was the rate of removal of zinc ions from BCA. With EDTA, trans-1,2-cyclohexanediaminetetraacetic acid, and nitrilotriacetic acid, the rate of zinc ion removal from the native enzyme was governed by the rate of spontaneous dissociation of the zinc enzyme. The rate constants for the removal of zinc ions from BCA were governed by the affinity of the chelating agents for the metal ion and the conformation of the chelating agents. Based on these findings, reaction pathways for various chelating agents are proposed.