Faleev Nicolai G, Demidkina Tatyana V, Tsvetikova Marina A, Phillips Robert S, Yamskov Igor A
Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow, Russia.
Eur J Biochem. 2004 Nov;271(22):4565-71. doi: 10.1111/j.1432-1033.2004.04428.x.
To shed light on the mechanism of isotopic exchange of alpha-protons in amino acids catalyzed by pyridoxal phosphate (PLP)-dependent enzymes, we studied the kinetics of quinonoid intermediate formation for the reactions of tyrosine phenol-lyase with L-phenylalanine, L-methionine, and their alpha-deuterated analogues in D2O, and we compared the results with the rates of the isotopic exchange under the same conditions. We have found that, in the L-phenylalanine reaction, the internal return of the alpha-proton is operative, and allowing for its effect, the exchange rate is accounted for satisfactorily. Surprisingly, for the reaction with L-methionine, the enzymatic isotope exchange went much faster than might be predicted from the kinetic data for quinonoid intermediate formation. This result allows us to suggest the existence of an alternative, possibly concerted, mechanism of alpha-proton exchange.
为了阐明磷酸吡哆醛（PLP）依赖性酶催化氨基酸中α-质子的同位素交换机制，我们研究了酪氨酸酚裂解酶与L-苯丙氨酸、L-甲硫氨酸及其在D2O中的α-氘代类似物反应生成醌型中间体的动力学，并将结果与相同条件下的同位素交换速率进行了比较。我们发现，在L-苯丙氨酸反应中，α-质子的内部返回起作用，考虑到其影响，交换速率得到了令人满意的解释。令人惊讶的是，对于与L-甲硫氨酸的反应，酶促同位素交换比根据醌型中间体形成的动力学数据预测的要快得多。这一结果使我们能够推测存在一种替代的、可能协同的α-质子交换机制。
