CRYSTAL-STRUCTURE OF CARBOXYPEPTIDASE-A COMPLEXED WITH AN INACTIVATORIN 2 CRYSTAL FORMS

Two different crystal forms of carboxypeptidase A (CPA) complexed with an inactivator were obtained by the method of hanging drop vapor diff usion, The inactivator, 2-benzyl-3-iodo-propanoic acid (BIPA), binds c ovalently to an active site residue Glu270 of CPA. The complexes were crystallized in the space group P2(1) (CPA-I) and P2(1)2(1)2(1) (CPA-H ), respectively. The structures of both crystal forms were determined by molecular replacement using the native CPA crystal structure as the search model. The final crystallographic residuals are 0.163 for CPA- I and 0.152 for CPA-II, Except for the modification of Glu270, the ina ctivator exhibits normal binding mode compared with other ligand compl exes of CPA. In the final electron density difference maps (2Fo-Fc, Fo -Fc), the density of the iodo ion could not be found in both crystal f orms while the conserved water molecule remains coordinated to Zn2+ as in the native CPA. Comparisons of the complexes of CPA-BIPA with the native CPA and the CPA-D-Phe complex are presented. The mechanism of t he inactivation of CPA and its implication for catalytic mechanism wer e discussed.