Chai J J, He C, Li M, Tang L, Luo M
Institute of Biophysics, Chinese Academy of Sciences, Beijing.
Protein Eng. 1998 Oct;11(10):841-5. doi: 10.1093/protein/11.10.841.
Two different crystal forms of carboxypeptidase A (CPA) complexed with an inactivator were obtained by the method of hanging drop vapor diffusion. The inactivator, 2-benzyl-3-iodo-propanoic acid (BIPA), binds covalently 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-II), 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 inactivator exhibits normal binding mode compared with other ligand complexes 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 forms 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 the inactivation of CPA and its implication for catalytic mechanism were discussed.
通过悬滴气相扩散法获得了两种与失活剂复合的羧肽酶A（CPA）不同晶体形式。失活剂2-苄基-3-碘丙酸（BIPA）与CPA的活性位点残基Glu270共价结合。复合物分别在空间群P2(1)（CPA-I）和P2(1)2(1)2(1)（CPA-II）中结晶。两种晶体形式的结构均通过分子置换法确定，使用天然CPA晶体结构作为搜索模型。CPA-I的最终晶体学残余因子为0.163，CPA-II为0.152。除了Glu270的修饰外，与CPA的其他配体复合物相比，失活剂表现出正常的结合模式。在最终的电子密度差图（2Fo-Fc，Fo-Fc）中，两种晶体形式均未发现碘离子的密度，而保守水分子仍像在天然CPA中一样与Zn2+配位。给出了CPA-BIPA复合物与天然CPA和CPA-D-苯丙氨酸复合物的比较。讨论了CPA失活的机制及其对催化机制的影响。
