A Potent Pyridoxal Model Capable of Promoting Transamination and β-Elimination of Amino Acids
作者:Hiroki Kondo、Jun-ichi Kikuchi、Shinji Uchida、Tatsuya Kitamikado、Eiji Koyanagi、Junzo Sunamoto
DOI:10.1246/bcsj.58.675
日期:1985.2
N-Dodecyipyridoxal chloride (1a) has been synthesized as a potent pyridoxal 5′-phosphate model. The equilibrium constant for Schiff base formation of 1a with phenylalanine (21000 M−1) in 3 mM hexadecyltrimethylammonium chloride (CTACl) micelle is over a thousand times larger than that for the N-methyl counterpart (1b). The Schiff base of 1a and phenylalanine exists over a broad pH range of 5–11 in two ionic forms, HSB and SB−, that differ in the protonation state at the azomethine group. These Schiff bases undergo transamination at 30 °C in the absence of a metal ion at considerably different rates; 1.33×10−3 and 1.5×10−5 s−1 for HSB and SB−1, respectively. Likewise, the former species exhibited 11 times higher reactivity in the β-elimination of O-phosphonoserine catalyzed by 1a in 3 mM CTACl at 40 °C. These results indicate unambiguously the importance of a positive charge on the azomethine nitrogen as well as the one residing on the pyridine nitrogen in vitamin B6 catalysis. The kinetic α-deuterium isotope effect of 5.1 in the transamiantion of phenylalanine with 1a at pH 7.7 and 30 °C revealed that α-hydrogen abstraction takes place in the transition state.
N-十二烷基吡哆醛氯化物 (1a) 已被合成为有效的吡哆醛 5'-磷酸模型。在 3 mM 十六烷基三甲基氯化铵 (CTACl) 胶束中,与苯丙氨酸 (21000 M−1) 形成席夫碱 1a 的平衡常数比 N-甲基对应物 (1b) 大一千倍以上。 1a 和苯丙氨酸的希夫碱以两种离子形式 HSB 和 SB− 存在于 5-11 的宽 pH 范围内,它们在偶氮甲碱基团上的质子化状态不同。这些希夫碱在 30 °C 且不存在金属离子的情况下以相当不同的速率进行氨基转移; HSB 和 SB−1 分别为 1.33×10−3 和 1.5×10−5 s−1。同样,在 40 °C 3 mM CTACl 中 1a 催化的 O-膦酰丝氨酸 β-消除反应中,前一物种表现出高出 11 倍的反应性。这些结果明确表明偶氮甲碱氮上的正电荷以及吡啶氮上的正电荷在维生素 B6 催化中的重要性。在 pH 7.7 和 30 °C 下,5.1 在苯丙氨酸与 1a 转氨基作用中的动力学 α-氘同位素效应表明,α-氢夺取发生在过渡态。