Soda K, Yoshimura T, Esaki N
Department of Biotechnology, Faculty of Engineering, Kansai University, Suita, Osaka 564-8680, Japan.
Chem Rec. 2001;1(5):373-84. doi: 10.1002/tcr.1021.
We have studied the stereospecificities of various pyridoxal 5'-phosphate dependent enzymes for the hydrogen transfer between the C-4' of a bound coenzyme and the C-2 of a substrate in the transamination catalyzed by the enzymes. Prior to our studies, pyridoxal enzymes so far studied were reported to catalyze the hydrogen transfer only on the si-face of the planar imine intermediate formed from substrate and coenzyme. This finding had been considered as the evidence that pyridoxal enzymes have evolved divergently from a common ancestral protein, because identity in the stereospecificity reflects the similarity in the active-site structure, in particular in the geometrical relationship between the coenzyme and the active site base participating in the hydrogen transfer. However, we found that D-amino acid aminotransferase, branched-chain L-amino acid aminotransferase, and 4-amino-4-deoxychorismate lyase catalyze the re-face specific hydrogen transfer, and that amino acid racemases catalyze the nonstereospecific hydrogen transfer. These findings suggest the convergent evolution of pyridoxal enzymes. Crystallographical studies have shown that the stereospecificity reflects the active-site structure of the enzymes, and that the enzymes with the same fold exhibit the same stereospecificity. The active site structure with the catalytic base being situated on the specific face of the cofactor has been conserved during the evolution among the pyridoxal enzymes of the same family.
我们研究了多种依赖磷酸吡哆醛的酶在其催化的转氨反应中，辅酶结合位点的C-4'与底物的C-2之间氢转移的立体特异性。在我们的研究之前，据报道，迄今为止所研究的吡哆醛酶仅在由底物和辅酶形成的平面亚胺中间体的si面上催化氢转移。这一发现被认为是吡哆醛酶从共同祖先蛋白发散进化的证据，因为立体特异性的一致性反映了活性位点结构的相似性，特别是参与氢转移的辅酶与活性位点碱基之间的几何关系。然而，我们发现D-氨基酸转氨酶、支链L-氨基酸转氨酶和4-氨基-4-脱氧分支酸裂解酶催化re面特异性氢转移，而氨基酸消旋酶催化非立体特异性氢转移。这些发现表明吡哆醛酶的趋同进化。晶体学研究表明，立体特异性反映了酶的活性位点结构，具有相同折叠的酶表现出相同的立体特异性。在同一家族的吡哆醛酶进化过程中，催化碱基位于辅因子特定面上的活性位点结构得以保留。
