Reaction intermediates of quinol oxidation in a photoactivatable system that mimics electron transfer in the cytochrome bc1 complex
Reaction intermediates of quinol oxidation in a photoactivatable system that mimics electron transfer in the cytochrome bc1 complex
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DOI：
10.1021/ja043955g
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发表时间：
2005-03-30
影响因子：
15
通讯作者：
Kramer, DM
中科院分区：
文献类型：
--
作者：
Cape, JL;Bowman, MK;Kramer, DM
目前与之竞争的细胞色素b（C1）、络合物和相关络合物双电子氧化喹啉（QH(2)）的模型对反应中间体有不同的要求。目前，酶氧化过程的中间物质尚未被观察到或表征，可能是由于它们的瞬态性质。在这里，我们使用一种仿生氧化剂，激发态Ru(bpy)(2)(pbim)(+) （bpy = 2,2'-二吡啶基，pbim = 2-（2-吡啶基）苯并咪唑盐）在非质子介质中探测泛醇类似物2,3-二甲氧基-5-甲基-1,4-苯并喹啉（UQH(2)-0）和塑料喹啉类似物三甲基-1,4-苯并喹啉（TMQH(2)-0）的氧化，使用时间分辨和稳态光谱技术。尽管简单，该系统定性再现了线粒体细胞色素b（C1）复合物氧化泛醇过程中观察到的关键特征。通过对天然体系和合成体系中同位素依赖性活化特性的比较，以及对合成体系中时间分辨直接探测电子顺磁共振信号的分析，我们得出以下结论：(1)在生物体系和仿生体系中，喹啉氧化的初始和限速步骤都涉及电子和质子转移，可能是通过质子耦合的电子转移机制进行的；(2)在仿生体系中形成中性半醌中间体；(3)QH中心点/QH(2)对UQH(2)-0而不是TMQH(2)-0的氧化，对其Arrhenius活化能（δ G(TS)）表现出不同寻常和意想不到的初等氘动力学同位素效应，其中质子化形式的δ G（TS）大于氘化形式。在使用泛醇（而不是塑喹啉）作为底物的cyt b（C1）复合物的稳态转换过程中观察到相同的行为，从而得出结论，这两个系统中涉及类似的化学途径。合成系统是一个明确的n = 1电子受体，因此可以推断，泛醇的连续氧化（通过两个连续的n = 1过程）比cyt b（C1）络合物中真正一致的（n = 2）氧化更快。
Current competing models for the two-electron oxidation of quinol (QH(2)) at the cytochrome b(C1), complex and related complexes impose distinct requirements for the reaction intermediate. At present, the intermediate species of the enzymatic oxidation process have not been observed or characterized, probably due to their transient nature. Here, we use a biomimetic oxidant, excited-state Ru(bpy)(2)(pbim)(+) (bpy = 2,2'-dipyridyl, pbim = 2-(2-pyridyl)benzimidazolate) in an aprotic medium to probe the oxidation of the ubiquinol analogue, 2,3-dimethoxy-5-methyl-1,4-benzoquinol (UQH(2)-0), and the plastoquinol analogue, trimethyl-1,4-benzoquinol (TMQH(2)-0), using time-resolved and steady-state spectroscopic techniques. Despite its simplicity, this system qualitatively reproduces key features observed during ubiquinol oxidation by the mitochondrial cytochrome b(C1) complex. Comparison of isotope-dependent activation properties in the native and synthetic systems as well as analysis of the time-resolved direct-detection electron paramagnetic resonance signals in the synthetic system allows us to conclude that (1) the initial and rate-limiting step in quinol oxidation, both in the biological and biomimetic systems, involves electron and proton transfer, probably via a proton-coupled electron-transfer mechanism, (2) a neutral semiquinone intermediate is formed in the biomimetic system, and (3) oxidation of the QH center dot/QH(2) couple for UQH(2)-0, but not TMQH(2)-0, exhibits an unusual and unexpected primary deuterium kinetic isotope effect on its Arrhenius activation energy (Delta G(TS)), where Delta G(TS) for the protiated form is larger than that for the deuterated form. The same behavior is observed during steady-state turnover of the cyt b(C1) complex using ubiquinol, but not plastoquinol, as a substrate, leading to the conclusion that similar chemical pathways are involved in both systems. The synthetic system is an unambiguous n = 1 electron acceptor, and it is thus inferred that sequential oxidation of ubiquinol (by two sequential n = 1 processes) is more rapid than a truly concerted (n = 2) oxidation in the cyt b(C1) complex.