Abstact

The X-ray structures of native carboxypeptidase A and of the enzyme-inhibitor complex with L-phenyl lactate have been refined at 1.54 and 1.45 A resolution to R factors of 0.151 and 0.161, respectively. Crystals of the complex were isomorphous with the native crystals (space group P2(1), a = 51.60, b = 60.27, c = 47.25 A, beta = 97.27 degrees). The high-resolution electron density allowed correction of many side-chain positions in the classical carboxypeptidase A model. This reflects the advantages of the high-quality complete synchrotron data collected with an imaging plate detector. The conformational changes in the active centre of the enzyme upon binding of the inhibitor are restricted to only two residues, Tyr248 and Arg145. L-Phenyl lactate is bound in the S1' pocket and forms hydrogen bonds to Arg145, Glu270 and to the zinc-bound water molecule. The present structure provides an explanation for the higher stability of the complexes with the products of esterolysis in comparison with those of amidolysis. This is consistent with the finding that product release is rate limiting for esters but not for peptides.