Hydrolysis of Esters by Carboxypeptidase A Requires a Penta-coordinate Metal Ion

The catalytic role of the metal ion in bovine carboxypeptidase A (peptidyl-L-amino acid hydrolase; EC 3.4.12.2) is investigated by application of cryoenzymologic and electron paramagnetic resonance methods with use of the Co2+-reconstituted enzyme. Incorporation of 17O into oxygen-donor ligands induces a substantial change in the spin-lattice relaxation probability of the paramagnetic ion. While a change in spin-lattice relaxation is observed for the free Co2+-enzyme in 17O-enriched water, no change is observed for the enzyme complexed to glycyl-L-tyrosine. These results are consistent with x-ray crystallographic studies showing that the metal-bound water molecule in the active site is displaced upon binding of the peptide inhibitor. A change in spin-lattice relaxation of the Co2+ ion in the mixed anhydride, acyl-enzyme intermediate formed with the specific ester substrate O-(trans-p-chlorocinnamoyl)-L-beta-phenyllactate is observed when 17O is enriched either into water or into the carbonyl oxygen position of the scissile bond of the substrate. Since the protein supplies three amino acid side chains as ligands to the metal ion, these results indicate that the metal ion is altered from a tetra-coordinate species in the free enzyme to a penta-coordinate species in the acyl-enzyme reaction intermediate. In addition, the results provide structural support for our assignment of ionization of a metal-bound water molecule in rate-limiting deacylation (Makinen, M. W., Kuo, L. C., Dymowski, J. J., and Jaffer, S. (1979) J. Biol. Chem. 254, 356-366) and affirm that the metal-hydroxide species is the nucleophile responsible for the breakdown of the mixed anhydride reaction intermediate of carboxypeptidase A.