Effects of pH on the structure and function of carboxypeptidase A: crystallographic studies.
High-resolution crystal structures are described for carboxypeptidase A (EC 3.4.17.1) in crystals grown at pH 8.5, 9.0, and 9.5 and compared with the structure at pH 7.5. The comparison shows that in the pH range of 7.5-9.5 the enzyme structure is practically unchanged, and, most importantly, that the flexible side chain of Tyr-248 remains exclusively in the "up" position, away from the Zn atom, throughout the pH range. There is no evidence for binding of Tyr-248 to Zn at any of these pH values. We conclude that the interaction of Tyr-248 with Zn is not an essential part of the mechanism of carboxypeptidase A and that its occurrence is an artifact of chemical modification of Tyr-248. It is also suggested that Tyr-248 is not uniquely associated with the observed high pK of the enzymatic hydrolysis.
D000818:Animals; D002268:Carboxypeptidases / Q000378:metabolism*; D043422:Carboxypeptidases A; D002417:Cattle; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008958:Models, Molecular; D011487:Protein Conformation; D014443:Tyrosine; D014961:X-Ray Diffraction; D015032:Zinc
D014443:Tyrosine; D002268:Carboxypeptidases; D043422:Carboxypeptidases A; D015032:Zinc