Kinetics of interaction of ligands with carboxypeptidase A.

Rate constants for the interaction of a number of ligands with the active site zinc ion of carboxypeptidase A have been measured at pH 7.0, 25 degrees, 1.0 M NaCl. Polydentate ligands such as EDTA, NTA or CyDta do not accelerate the rate at which the zinc ion dissociates from the protein. Bidentate or tridentate ligands on the other hand are able to attack the zinc ion directly; the rates are first order in enzyme and first order in ligand. A mechanism for the reaction is proposed, in which a ternary complex LZnCPA is formed which rapidly dissociates into ZnL and apo CPA. Comparison of results for a variety of ligands leads to the conclusion that in the ternary complex tridentate ligands bind to the zinc ion through only two donor groups. The reaction of 1.10-phenanthroline with ZnCPA has been studied from pH 6 to 9, and a mechanism proposed which accounts for the pH profile of the reaction.