Native carboxypeptidase a in a new crystal environment reveals a differentconformation of the important tyrosine 248

Native carboxypeptidase A has been crystallized in a new crystal form, and the structure has been refined with X-ray data to 2.0 Angstrom resolution. In contrast to the previously published structure [Rees, D. C., Lewis, M., and Lipscomb, W. N, (1983) J. Mol. Biol. 168, 367-387], no active-site amin o acids are involved in the crystal packing, The important Tyr248 is stabil ized inside the active site by a hydrogen bond and by interactions with Ile 247. The proposed role of Tyr248 in the induced fit mechanism is therefore not supported by the findings in this structure of native carboxypeptidase A. The structure has a partly populated inhibitory Zn2+ site in close proxi mity to the catalytic Zn2+ as evident from X-ray anomalous dispersion data. A hydroxo bridge is found between the catalytic Zn2+ and the inhibitory Zn 2+ with a Zn2+-Zn2+ distance of 3.48 Angstrom. In addition, the inhibitory Zn2+ has Glu270 as a monodentate ligand, No other protein ligands to the in hibitory Zn2+ are seen. The crystals were grown at 0.3 M LiCl and weak evid ence for a binding site for partly competitive inhibitory anions is observe d.