Graduate Program in Bioorganic Chemistry, Graduate Program in Biophysics and Biochemistry, Department of Biochemistry, Brandeis University, Waltham, Massachusetts 02454-9110, USA.
Biochemistry. 2010 Apr 13;49(14):3138-47. doi: 10.1021/bi902052x.
Mechanism-based inhibitors such as cycloserine and gabaculine can inactivate aminotransferases via reactions of the compounds with the pyridoxal phosphate cofactor forming an irreversible adduct. The reaction is chirally specific in that any one enzyme usually only recognizes one enantiomer of the inactivator. For instance, l-aspartate aminotransferase (l-AspAT) is inactivated by 4-amino-4,5-dihydro-2-thiophenecarboxylic acid (ADTA), however, only by the S-isomer. We have now shown that d-amino acid aminotransferase (d-a-AT) is irreversibly inactivated by the R-isomer of the same compound. The X-ray crystal structure (PDB code: 3LQS ) of the inactivated enzyme shows that in the product the enzyme no longer makes a Schiff base linkage to the pyridoxal 5'-phosphate (PLP) cofactor, and instead the compound has formed a derivative of the cofactor. The adduct is similar to that formed between d-cycloserine and d-a-AT or alanine racemase (Ala-Rac) in that the thiophene ring of R-ADTA is intact and seems to be aromatic. The plane of the ring is rotated by nearly 90 degrees with respect to the plane of the pyridine ring of the cofactor, in comparison with the enzyme inactivated by cycloserine. Based on the structure of the product, the mechanism of inactivation most probably involves a transamination followed by aromatization to form an aromatic thiophene ring.
基于机制的抑制剂，如环丝氨酸和加巴喷丁，可通过化合物与吡哆醛 5′-磷酸辅因子反应形成不可逆加合物，使氨基转移酶失活。该反应在立体上具有特异性，即任何一种酶通常只识别抑制剂的一个对映体。例如，l-天冬氨酸氨基转移酶（l-AspAT）被 4-氨基-4,5-二氢-2-噻吩羧酸（ADTA）失活，然而，只有 S-异构体可以。我们现在表明，d-氨基酸氨基转移酶（d-a-AT）被同一化合物的 R-异构体不可逆失活。失活酶的 X 射线晶体结构（PDB 代码：3LQS）表明，在产物中，酶不再与吡哆醛 5′-磷酸（PLP）辅因子形成希夫碱键，而是形成了辅因子的衍生物。加合物类似于 d-环丝氨酸与 d-a-AT 或丙氨酸消旋酶（Ala-Rac）形成的加合物，即 R-ADTA 的噻吩环保持完整，似乎具有芳香性。与被环丝氨酸失活的酶相比，噻吩环的平面相对于辅因子的吡啶环旋转了近 90 度。基于产物的结构，失活机制很可能涉及转氨作用，然后芳构化形成芳香噻吩环。
