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Coordination chemical studies on metalloenzymes. IV. Kinetic and thermodynamic studies of metal removal reaction from zinc- and cobalt-carbonic anhydrase with chelating agents.

The mechanism and rate of cobalt removal reaction from cobalt-enzyme with various chelating agents were investigated, in comparing with that of zinc-enzyme. The mechanism of cobalt removal reaction was the same as that of zinc-enzyme and the selectivity of chelating agents against zinc-enzyme and cobalt-enzyme was almost the same. The second-order rate constant of cobalt removal from cobalt-enzyme with various chelating agents (kapp) was much larger (13-30 times) than that of zinc-enzyme. The thermodynamic parameter of metal removal reaction was measured by the reaction of 5-methyl-1, 10-phenanthroline with cobalt- and zinc-enzyme. Activation enthalpy for breakdown of the ternary complex of cobalt- and zinc-enzyme was almost the same (28-29 kcal mol-1) but activation entropy (33 cal deg-1 mol-1) for breakdown of the ternary complex of cobalt-enzyme was much larger than that (22 cal deg-1 mol-1) of zinc-enzyme. This is the reason why kapp of the cobalt-enzyme is much larger than that of zinc enzyme.