Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland.
Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland.
Biochim Biophys Acta Bioenerg. 2018 Jun;1859(6):459-469. doi: 10.1016/j.bbabio.2018.03.010. Epub 2018 Mar 27.
Dimeric cytochromes bc are central components of photosynthetic and respiratory electron transport chains. In their catalytic core, four hemes b connect four quinone (Q) binding sites. Two of these sites, Q sites, reduce quinone to quinol (QH) in a step-wise reaction, involving a stable semiquinone intermediate (SQ). However, the interaction of the SQ with the adjacent hemes remains largely unexplored. Here, by revealing the existence of two populations of SQ differing in paramagnetic relaxation, we present a new mechanistic insight into this interaction. Benefiting from a clear separation of these SQ species in mutants with a changed redox midpoint potential of hemes b, we identified that the fast-relaxing SQ (SQ) corresponds to the form magnetically coupled with the oxidized heme b (the heme b adjacent to the Q site), while the slow-relaxing SQ (SQ) reflects the form present alongside the reduced (and diamagnetic) heme b. This so far unreported SQ calls for a reinvestigation of the thermodynamic properties of SQ and the Q site. The existence of SQ in the native enzyme reveals a possibility of an extended electron equilibration within the dimer, involving all four hemes b and both Q sites. This substantiates the predicted earlier electron transfer acting to sweep the b-chain of reduced hemes b to diminish generation of reactive oxygen species by cytochrome bc. In analogy to the Q site, we anticipate that the quinone binding sites in other enzymes may contain yet undetected semiquinones which interact magnetically with oxidized hemes upon progress of catalytic reactions.
二聚细胞色素 bc 是光合和呼吸电子传递链的核心组成部分。在其催化核心中，四个细胞色素 b 连接四个醌（Q）结合位点。其中两个位点，Q 位点，通过涉及稳定半醌中间体（SQ）的逐步反应将醌还原为氢醌（QH）。然而，SQ 与相邻细胞色素之间的相互作用在很大程度上仍未被探索。在这里，通过揭示在细胞色素 b 氧化还原中点电位发生变化的突变体中存在两种具有不同顺磁弛豫的 SQ 群体，我们提出了对此相互作用的新的机制见解。得益于这些 SQ 物种在具有改变的细胞色素 b 氧化还原中点电位的突变体中的清晰分离，我们确定了快速弛豫的 SQ（SQ）对应于与氧化细胞色素 b（与 Q 位点相邻的细胞色素 b）磁耦合的形式，而缓慢弛豫的 SQ（SQ）反映了与还原（和抗磁性）细胞色素 b 共存的形式。这种迄今为止未报道的 SQ 呼吁重新研究 SQ 和 Q 位点的热力学性质。天然酶中 SQ 的存在揭示了在二聚体中涉及所有四个细胞色素 b 和两个 Q 位点的电子平衡的可能性。这证实了先前预测的电子转移作用，该作用可将还原的细胞色素 b 的 b-链上的电子转移，以减少细胞色素 bc 产生的活性氧。与 Q 位点类似，我们预计其他酶的醌结合位点可能还包含尚未检测到的半醌，这些半醌在催化反应进行时与氧化的细胞色素相互作用。
