Crofts Antony R, Holland J Todd, Victoria Doreen, Kolling Derrick R J, Dikanov Sergei A, Gilbreth Ryan, Lhee Sangmoon, Kuras Richard, Kuras Mariana Guergova
Department of Biochemistry, University of Illinois at Urbana-Champaign, IL 61801, USA.
Biochim Biophys Acta. 2008 Jul-Aug;1777(7-8):1001-19. doi: 10.1016/j.bbabio.2008.04.037. Epub 2008 May 1.
Recent progress in understanding the Q-cycle mechanism of the bc(1) complex is reviewed. The data strongly support a mechanism in which the Q(o)-site operates through a reaction in which the first electron transfer from ubiquinol to the oxidized iron-sulfur protein is the rate-determining step for the overall process. The reaction involves a proton-coupled electron transfer down a hydrogen bond between the ubiquinol and a histidine ligand of the [2Fe-2S] cluster, in which the unfavorable protonic configuration contributes a substantial part of the activation barrier. The reaction is endergonic, and the products are an unstable ubisemiquinone at the Q(o)-site, and the reduced iron-sulfur protein, the extrinsic mobile domain of which is now free to dissociate and move away from the site to deliver an electron to cyt c(1) and liberate the H(+). When oxidation of the semiquinone is prevented, it participates in bypass reactions, including superoxide generation if O(2) is available. When the b-heme chain is available as an acceptor, the semiquinone is oxidized in a process in which the proton is passed to the glutamate of the conserved -PEWY- sequence, and the semiquinone anion passes its electron to heme b(L) to form the product ubiquinone. The rate is rapid compared to the limiting reaction, and would require movement of the semiquinone closer to heme b(L) to enhance the rate constant. The acceptor reactions at the Q(i)-site are still controversial, but likely involve a "two-electron gate" in which a stable semiquinone stores an electron. Possible mechanisms to explain the cyt b(150) phenomenon are discussed, and the information from pulsed-EPR studies about the structure of the intermediate state is reviewed. The mechanism discussed is applicable to a monomeric bc(1) complex. We discuss evidence in the literature that has been interpreted as shown that the dimeric structure participates in a more complicated mechanism involving electron transfer across the dimer interface. We show from myxothiazol titrations and mutational analysis of Tyr-199, which is at the interface between monomers, that no such inter-monomer electron transfer is detected at the level of the b(L) hemes. We show from analysis of strains with mutations at Asn-221 that there are coulombic interactions between the b-hemes in a monomer. The data can also be interpreted as showing similar coulombic interaction across the dimer interface, and we discuss mechanistic implications.
本文综述了对bc(1)复合物Q循环机制理解的最新进展。数据有力地支持了一种机制，即Q(o)位点通过泛醇向氧化态铁硫蛋白的首次电子转移为整个过程的速率决定步骤的反应来运作。该反应涉及质子耦合电子转移，沿着泛醇与[2Fe-2S]簇的组氨酸配体之间的氢键进行，其中不利的质子构型构成了活化能垒的很大一部分。该反应是吸能反应，产物是Q(o)位点的不稳定半醌，以及还原态铁硫蛋白，其外在可移动结构域现在可自由解离并离开该位点，将电子传递给细胞色素c(1)并释放H(+)。当半醌的氧化被阻止时，它会参与旁路反应，包括在有O(2)时生成超氧化物。当b型血红素链作为受体时，半醌在质子传递给保守的-PEWY-序列的谷氨酸且半醌阴离子将其电子传递给血红素b(L)以形成产物泛醌的过程中被氧化。与限速反应相比，该速率很快，并且需要半醌向血红素b(L)靠近以提高速率常数。Q(i)位点的受体反应仍存在争议，但可能涉及一个“双电子门”，其中稳定的半醌储存一个电子。讨论了解释细胞色素b(150)现象的可能机制，并综述了脉冲电子顺磁共振研究关于中间态结构的信息。所讨论的机制适用于单体bc(1)复合物。我们讨论了文献中的证据，这些证据被解释为表明二聚体结构参与了涉及跨二聚体界面电子转移的更复杂机制。我们通过对位于单体之间界面处的Tyr-199进行抗霉素A滴定和突变分析表明，在b(L)血红素水平未检测到这种单体间电子转移。我们通过对Asn-221处有突变的菌株进行分析表明，单体中的b型血红素之间存在库仑相互作用。这些数据也可解释为表明跨二聚体界面存在类似的库仑相互作用，我们讨论了其机制含义。
