Multiple Q-cycle bypass reactions at the Qo site of the cytochiome bc1 complex
Multiple Q-cycle bypass reactions at the Qo site of the cytochiome bc1 complex
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DOI：
10.1021/bi025581e
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发表时间：
2002-06-25
期刊：
影响因子：
2.9
通讯作者：
Kramer, DM
中科院分区：
文献类型：
--
作者：
Muller, F;Crofts, AR;Kramer, DM
细胞色素 (cyt) bc(1) 复合物是许多物种能量转导的核心。大多数研究人员现在接受改良的 Q 循环作为该酶的催化机制。为了 Q 循环的有效运行，必须尽量减少一些热力学上有利的副反应。其中，Q(o)位半醌还原氧产生超氧化物具有特殊的病理生物学意义。这些产生超氧化物的旁路反应最引人注目的是细胞色素 c 的抗霉素 A 或粘噻唑耐药性减少。在这项工作中，我们证明这些抑制剂抗性细胞色素 c 还原酶活性在很大程度上不受分离的酵母细胞色素 bc(1) 复合物中 O-2 去除的影响。此外，将 O-2 张力增加 5 倍会刺激抗霉素 A 耐药性少量减少（大约 25%），同时保持粘噻唑耐药性减少不变。这很可能表明超氧化物产生中的限速步骤是能够快速还原O-2的活性物质（可能是半醌）的形成，并且在不存在O-2的情况下，该物质可以通过一些其他途径还原细胞色素c。我们建议一种可能性是半醌从 Qo 位点逃逸并直接还原 O-2 或细胞色素 c。高 O-2 条件下抗霉素 A 细胞色素 c 减少率的小幅增加可以通过 Q(o) 位点内低浓度半醌的积累来解释。在有氧条件下，添加饱和水平的超氧化物歧化酶 (SOD) 在 myxothiazol 存在下抑制了 50% 的细胞色素 c 还原，这意味着基本上所有旁路反应都伴随着超氧化物的产生而发生。然而，SOD 仅抑制 35% 的抗霉素 A 抗性细胞色素 c 还原，表明存在第二个较慢的旁路反应，该反应不会还原 O-2。鉴于 myxothiazol 阻止细胞色素 b 还原，而抗霉素 A 促进细胞色素 b 还原，我们提出第二个旁路是通过预还原的细胞色素 b(L) 还原 Q(o) 位点半醌而发生的。
The cytochrome (cyt) bc(1) complex is central to energy transduction in many species. Most investigators now accept a modified Q-cycle as the catalytic mechanism of this enzyme. Several thermodynamically favorable side reactions must be minimized for efficient functioning of the Q-cycle. Among these, reduction of oxygen by the Q(o) site semiquinone to produce superoxide is of special pathobiological interest. These superoxide-producing bypass reactions are most notably observed as the antimycin A- or myxothiazol-resistant reduction of cyt c. In this work, we demonstrate that these inhibitor-resistant cyt c reductase activities are largely unaffected by removal of O-2 in the isolated yeast cyt bc(1) complex. further, increasing O-2 tension 5-fold stimulated the antimycin A-resistant reduction by a small amount (similar to25%), while leaving the myxothiazol-resistant reduction unchanged. This most likely indicates that the rate-limiting step in superoxide production is the formation of a reactive species (probably a semiquinone), capable of rapid O-2 reduction, and that in the absence Of 02 this species can reduce cyt c by some other pathway. We suggest as one possibility that a semiquinone escapes from the Qo site and reduces either O-2 or cyt c directly. The small increase in antimycin A-resistant cyt c reduction rate at high O-2 can be explained by the accumulation of a low concentration of a semiquinone inside the Q(o) site. Under aerobic conditions, addition of saturating levels of superoxide dismutase (SOD) inhibited 50% of cyt c reduction in the presence of myxothiazol, implying that essentially all bypass reactions occur with the production of superoxide. However, SOD inhibited only 35% of antimycin A-resistant cyt c reduction, suggesting the presence of a second, slower bypass reaction that does not reduce O-2. Given that myxothiazol blocks cyt b reduction whereas antimycin A promotes it, we propose that this second bypass occurs by reduction of the Q(o) site semiquinone by prereduced cyt b(L).