Catalytic role of monovalent cations in the mechanism of proton transfer which gates an interprotein electron transfer reaction
Catalytic role of monovalent cations in the mechanism of proton transfer which gates an interprotein electron transfer reaction
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DOI：
10.1021/bi970586a
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发表时间：
1997-11-04
期刊：
影响因子：
2.9
通讯作者：
Davidson, VL
中科院分区：
文献类型：
--
作者：
Bishop, GR;Davidson, VL
在甲胺脱氢酶（MADH）-amicyanin蛋白复合物中，MADH的色氨酸-蒽醌（TTQ）和amicyanin的I型铜之间发生长距离分子间电子转移（ET）。研究了TTQ的两种化学上不同的还原形式的再氧化，通过连二亚硫酸盐还原产生的醌醇（O-醌醇）和通过甲胺还原产生的生理上相关的氨基醌醇（N-醌醇）。后者含有底物衍生的氨基，其取代TTQ上的C6羰基氧。ET从N-醌醇MADH到amicyanin是通过溶剂可交换质子的转移门控的[Bishop，G. R.，& Davidson，V. L.（1995）Biochemistry 34，12082-12086]。研究了影响质子转移反应的因素。PT的速率随着pH的增加和盐浓度的增加而增加。盐效应是由于特定的一价阳离子，而不是一般的离子强度效应。通过pH和阳离子的速率增强并不反映门控ET的PT步骤的消除。在pH 5.5至9.0的范围内，阳离子浓度为0至200 mM，观察到的氧化还原反应速率仍然是PT的速率。动力学溶剂同位素效应研究证明了这一点，该研究表明，即使在最高的pH值和阳离子浓度下，初级同位素效应也会持续存在。提出了一个模型来解释如何特定的阳离子有助于催化和影响PT在这个反应中的速率。pH依赖性归因于参与阳离子结合的可电离基团。阳离子的作用是稳定带负电荷的反应中间体，该反应中间体在N-醌醇的去质子化期间形成，并且从该反应中间体快速ET至amicyanin的铜。这些研究结果的相关性表现出的反应速率是由一价阳离子的影响，其他酶也进行了讨论。
Within the methylamine dehydrogenase (MADH)-amicyanin protein complex, long range intermolecular electron transfer (ET) occurs between tryptophan tryptophylquinone (TTQ) of MADH and the type I copper of amicyanin. The reoxidations of two chemically distinct reduced forms of TTQ were studied, a quinol (O-quinol) generated by reduction by dithionite and the physiologically relevant aminoquinol (N-quinol) generated by reduction by methylamine. The latter contains a substrate-derived amino group which displaces the C6 carbonyl oxygen on TTQ. ET from N-quinol MADH to amicyanin is gated by the transfer of a solvent exchangeable proton [Bishop, G. R., & Davidson, V. L. (1995) Biochemistry 34, 12082-12086]. The factors which influence this proton transfer (PT) reaction have been examined. The rate of PT increases with increasing pH and with increasing salt concentration. The salt effect is due to specific monovalent cations and is not a general ionic strength effect. The rate enhancements by pH and cations do not reflect an elimination of the PT step that gates ET. Over the range of pH from 5.5 to 9.0 and with cation concentrations from 0 to 200 mM, the observed rate of the redox reaction is still that of PT. This is proven by kinetic solvent isotope effect studies which show that a primary isotope effect persists even at the highest values of pH and cation concentration. A model is presented to explain how specific cations contribute to catalysis and influence the rate of PT in this reaction. The pH dependence is attributed to an ionizable group that is involved in cation binding. The effect of the cation is stabilization of a negatively charged reaction intermediate that is formed during the deprotonation of the N-quinol, and from which rapid ET to the copper of amicyanin occurs. The relevance of these findings to other enzymes which exhibit reaction rates that are influenced by monovalent cations is also discussed.