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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.
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