Tikhonov Alexander N
Faculty of Physics, Moscow State University, Moscow, Russia.
Subcell Biochem. 2018;87:287-328. doi: 10.1007/978-981-10-7757-9_10.
This chapter presents an overview of structural properties of the cytochrome (Cyt) b f complex and its functioning in chloroplasts. The Cyt b f complex stands at the crossroad of photosynthetic electron transport pathways, providing connectivity between Photosystem (PSI) and Photosysten II (PSII) and pumping protons across the membrane into the thylakoid lumen. After a brief review of the chloroplast electron transport chain, the consideration is focused on the structural organization of the Cyt b f complex and its interaction with plastoquinol (PQH, reduced form of plastoquinone), a mediator of electron transfer from PSII to the Cyt b f complex. The processes of PQH oxidation by the Cyt b f complex have been considered within the framework of the Mitchell's Q-cycle. The overall rate of the intersystem electron transport is determined by PQH turnover at the quinone-binding site Q of the Cyt b f complex. The rate of PQH oxidation is controlled by the intrathylakoid pH, which value determines the protonation/deprotonation events in the Q-center. Two other regulatory mechanisms associated with the Cyt b f complex are briefly overviewed: (i) redistribution of electron fluxes between alternative (linear and cyclic) pathways, and (ii) "state transitions" related to redistribution of solar energy between PSI and PSII.
本章概述了细胞色素（Cyt）b6f复合体的结构特性及其在叶绿体中的功能。细胞色素b6f复合体处于光合电子传递途径的交叉点，在光系统（PSI）和光系统II（PSII）之间建立连接，并将质子跨膜泵入类囊体腔。在简要回顾叶绿体电子传递链之后，重点讨论细胞色素b6f复合体的结构组织及其与质体醌醇（PQH2，质体醌的还原形式）的相互作用，质体醌醇是从PSII到细胞色素b6f复合体的电子传递介质。细胞色素b6f复合体对PQH2的氧化过程已在米切尔的Q循环框架内进行了探讨。系统间电子传递的总体速率由细胞色素b6f复合体醌结合位点Q处的PQH2周转决定。PQH2的氧化速率受类囊体腔内pH值控制，该值决定了Q中心的质子化/去质子化事件。还简要概述了与细胞色素b6f复合体相关的另外两种调节机制：（i）电子通量在替代（线性和循环）途径之间的重新分配，以及（ii）与PSI和PSII之间太阳能重新分配相关的“状态转换”。
