Native Carboxypeptidase A in a New Crystal Environment Reveals a Different Conformation of the Important Tyrosine 248,

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Abstract
Native carboxypeptidase A has been crystallized in a new crystal form, and the structure has been refined with X-ray data to 2.0 A 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 amino acids are involved in the crystal packing. The important Tyr248 is stabilized inside the active site by a hydrogen bond and by interactions with Ile247. 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 proximity to the catalytic Zn2+ as evident from X-ray anomalous dispersion data. A hydroxo bridge is found between the catalytic Zn2+ and the inhibitory Zn2+ with a Zn2+-Zn2+ distance of 3.48 A. In addition, the inhibitory Zn2+ has Glu270 as a monodentate ligand. No other protein ligands to the inhibitory Zn2+ are seen. The crystals were grown at 0.3 M LiCl and weak evidence for a binding site for partly competitive inhibitory anions is observed.