Binding of D-phenylalanine and D-tyrosine to carboxypeptidase A

The structures of the complexes of carboxypeptidase A with the amino acids D-phenylalanine and D-tyrosine are reported as determined by x-ray crystallographic methods to a resolution of 2.0 Å. In each individual study one molecule of amino acids binds to the enzyme in the COOH-terminal hydrophobic pocket: the carboxylate of the bound ligand salt links with Arg-145, and the α-amino group salt links with Glu-270. The carboxylate of Glu-270 must break its hydrogen bond with the native zinc-bound water molecule in order to exploit the latter interaction. This result is in accord with spectroscopic studies which indicate that the binding of D or L amino acids (or analogues thereof) allows for more facile displacement of the metal-bound water by anions (Bicknell, R., Schaffer, A., Bertini, I., Luchinat, C., Vallee, B.L., and Auld, D.S. (1988) Biochemistry 27, 1050-1057). Additionally, we observe a significant movement of the zinc-bound water molecule (~ 1 Å) upon the binding of D-ligands. We propose that this unanticipated movement also contributes to anion sensitivity. The structural results of the current x-ray study correct predictions made in an early model building study regarding the binding of D-phenylalanine (Lipscomb, W.N., Hartsuck, J.A., Reeke, G.N., Jr., Quiocho, F.A., Bethge, P.H., Ludwig, M.L., Steitz, T.A., Muirhead, H., and Coppola, J.C. (1968) Brookhaven Symp. Biol. 21, 24-90).