Crofts Antony R, Shinkarev Vladimir P, Kolling Derrick R J, Hong Sangjin
Department of Biochemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
J Biol Chem. 2003 Sep 19;278(38):36191-201. doi: 10.1074/jbc.M305461200. Epub 2003 Jun 26.
Crystallographic structures of the bc1 complex from different sources have provided evidence that a movement of the Rieske iron-sulfur protein (ISP) extrinsic domain is essential for catalysis. This dynamic feature has opened up the question of what limits electron transfer, and several authors have suggested that movement of the ISP head, or gating of such movement, is rate-limiting. Measurements of the kinetics of cytochromes and of the electrochromic shift of carotenoids, following flash activation through the reaction center in chromatophore membranes from Rhodobacter sphaeroides, have allowed us to demonstrate that: (i) ubiquinol oxidation at the Qo-site of the bc1 complex has the same rate in the absence or presence of antimycin bound at the Qi-site, and is the reaction limiting turnover. (ii) Activation energies for transient processes to which movement of the ISP must contribute are much lower than that of the rate-limiting step. (iii) Comparison of experimental data with a simple mathematical model demonstrates that the kinetics of reduction of cytochromes c1 and bH are fully explained by the modified Q-cycle. (iv) All rates for processes associated with movement of the ISP are more rapid by at least an order of magnitude than the rate of ubiquinol oxidation. (v) Movement of the ISP head does not introduce a significant delay in reduction of the high potential chain by quinol, and it is not necessary to invoke such a delay to explain the kinetic disparity between the kinetics of reduction of cytochromes c1 and bH.
来自不同来源的bc1复合物的晶体结构提供了证据，表明 Rieske 铁硫蛋白（ISP）外在结构域的移动对于催化作用至关重要。这一动态特征引发了关于限制电子转移因素的问题，几位作者提出，ISP头部的移动或这种移动的门控是限速步骤。通过对球形红细菌的载色体膜中反应中心进行闪光激活后，对细胞色素的动力学以及类胡萝卜素的电致变色位移进行测量，使我们能够证明：（i）在bc1复合物的Qo位点上，泛醇氧化在Qi位点存在或不存在抗霉素的情况下具有相同的速率，并且是限制周转的反应。（ii）ISP移动必须参与的瞬态过程的活化能远低于限速步骤的活化能。（iii）将实验数据与一个简单的数学模型进行比较表明，细胞色素c1和bH还原的动力学可以通过修正的Q循环得到充分解释。（iv）与ISP移动相关的所有过程的速率至少比泛醇氧化速率快一个数量级。（v）ISP头部的移动不会在喹啉还原高电位链的过程中引入显著延迟，并且没有必要援引这样的延迟来解释细胞色素c1和bH还原动力学之间的差异。
