Davidson V L, Jones L H, Graichen M E
Department of Biochemistry, University of Mississippi Medical Center, Jackson 39216-4505.
Biochemistry. 1992 Apr 7;31(13):3385-90. doi: 10.1021/bi00128a012.
It had been previously reported that aromatic amines were not substrates for the bacterial quinoprotein methylamine dehydrogenase. In this study, benzylamine-dependent activity was also not observed in the steady-state assay of this enzyme with the artificial electron acceptor phenazine ethosulfate (PES). Benzylamines did, however, stoichiometrically reduce the protein-bound tryptophan tryptophylquinone (TTQ) prosthetic group and acted as reversible competitive inhibitors of methylamine oxidation when the enzyme was assayed with PES. When methylamine dehydrogenase activity was monitored using a steady-state assay which employed its physiological electron acceptor amicyanin instead of PES, very low but detectable benzylamine-dependent activity was observed. The reactions of a series of para-substituted benzylamines with methylamine dehydrogenase were examined. A Hammett plot of the log of Ki values for the competitive inhibition by these amines against sigma p exhibited a negative slope. Rapid kinetic measurements allowed the determination of values of k3 and Ks for the reduction of TTQ by each of these amines. A Hammett plot of log k3 versus sigma p exhibited a positive slope, which suggests that the oxidation of these amines by methylamine dehydrogenase proceeds through a carbanionic reaction intermediate. A negative slope was observed for the correlation between log Ks and sigma p. Plots of log k3 and log Ks against substituent constants which reflected either resonance or field/inductive parameters for each para substituent indicated that the magnitude of k3 was primarily influenced by field/inductive effects while Ks was primarily influenced by resonance effects. No correlation was observed between either k3 or Ks and the relative hydrophobicity of the para-substituted benzylamines or steric parameters.(ABSTRACT TRUNCATED AT 250 WORDS)
先前已有报道称，芳香胺并非细菌醌蛋白甲胺脱氢酶的底物。在本研究中，在该酶与人工电子受体吩嗪硫酸乙酯（PES）的稳态测定中，也未观察到苄胺依赖性活性。然而，苄胺确实能化学计量地还原蛋白质结合的色氨酸色氨酰醌（TTQ）辅基，并且当用PES测定该酶时，它可作为甲胺氧化的可逆竞争性抑制剂。当使用其生理电子受体蓝铜蛋白而非PES的稳态测定来监测甲胺脱氢酶活性时，观察到了非常低但可检测到的苄胺依赖性活性。研究了一系列对取代苄胺与甲胺脱氢酶的反应。这些胺竞争性抑制的Ki值的对数对σp的哈米特图呈现负斜率。快速动力学测量使得能够确定每种胺还原TTQ的k3和Ks值。log k3对σp的哈米特图呈现正斜率，这表明甲胺脱氢酶对这些胺的氧化通过碳负离子反应中间体进行。log Ks与σp之间的相关性观察到负斜率。log k3和log Ks对反映每个对位取代基的共振或场/诱导参数的取代基常数的图表明，k3的大小主要受场/诱导效应影响，而Ks主要受共振效应影响。在k3或Ks与对位取代苄胺的相对疏水性或空间参数之间未观察到相关性。（摘要截短于250字）
