英国谢菲尔德大学C. Neil Hunter 和 Matthew P. Johnson团队合作，揭示了菠菜细胞色素b6 f（cytb6 f）的冷冻电镜（cryo-EM）结构，分辨率达到3.6 Å。相关论文2019年11月13日在线发表在《自然》上。

他们介绍了菠菜中的二聚体cytb6 复合物f的3.6 Å分辨率的cryo-EM结构，揭示了喹诺酮（Q）循环及其氧化还原感知功能的结构基础。该复合物最多包含三个天然结合的质体醌（PQ）分子。第一个是PQ1，位于靠近血红素bp和叶绿素a的PQ氧化位点（Qp）附近的一个cytb6 f单体中。解决了两个构型的叶绿素a循环问题，一个阻止进入Qp位点，另一个允许它进入Qp位点，支持参与氧化还原感知的叶绿素a的门控功能。PQ2跨越单体腔，部分阻碍PQ1侧的PQ还原位点（Qn），并使相邻单体中占据Qn位点的电子转移网络进行周转。涉及丙酸血红素酯的构象转换促进了Qn位点的双电子、双质子的还原，并避免了反应性中间体半醌的形成。暂定分配的第三个PQ分子的位置与Q循环中相对单体中Qp和Qn位点之间的转换相一致。因此，菠菜的cytb6 f结构为复合物如何在光合作用中发挥催化和调节作用提供了新的见解。

研究人员表示，cytb6 f复合物在有氧光合作用中起着核心作用，将光系统I和II之间的电子转移联系起来，并将太阳能转化为跨膜质子梯度，用于ATP合成。 Cytb6 f内的电子转移通过Q循环发生，这通过两个电子分支催化质体喹诺醇（PQH2）的氧化以及质体蓝蛋白（PC）和PQ的还原。在高等植物中，cytb6 f也充当氧化还原传感中心，对调节光收集和循环电子传递至关重要，从而可防止代谢和环境胁迫。

附：英文原文

Title: Cryo-EM structure of the spinach cytochrome b 6  f ?complex at 3.6 Å resolution

Author: Lorna A. Malone, Pu Qian, Guy E. Mayneord, Andrew Hitchcock, David A. Farmer, Rebecca F. Thompson, David J. K. Swainsbury, Neil A. Ranson, C. Neil Hunter, Matthew P. Johnson

Issue&Volume: 2019-11-13

Abstract: The cytochrome b6f (cytb6f) complex has a central role in oxygenic photosynthesis, linking electron transfer between photosystems I and II and converting solar energy into a transmembrane proton gradient for ATP synthesis13. Electron transfer within cytb6foccurs via the quinol (Q) cycle, which catalyses the oxidation of plastoquinol (PQH2) and the reduction of both plastocyanin (PC) and plastoquinone (PQ) at two separate sites via electron bifurcation2. In higher plants, cytb6falso acts as a redox-sensing hub, pivotal to the regulation of light harvesting and cyclic electron transfer that protect against metabolic and environmental stresses3. Here we present a 3.6  resolution cryo-electron microscopy (cryo-EM) structure of the dimeric cytb6f complex from spinach, which reveals the structural basis for operation of the Q cycle and its redox-sensing function. The complex contains up to three natively bound PQ molecules. The first, PQ1, is located in one cytb6fmonomer near the PQ oxidation site (Qp) adjacent to haem bp and chlorophyll a. Two conformations of the chlorophyll a phytyl tail were resolved, one that prevents access to the Qp site and another that permits it, supporting a gating function for the chlorophyll a involved in redox sensing. PQ2 straddles the intermonomer cavity, partially obstructing the PQ reduction site (Qn) on the PQ1 side and committing the electron transfer network to turnover at the occupied Qn site in the neighbouring monomer. A conformational switch involving the haem cn propionate promotes two-electron, two-proton reduction at the Qn site and avoids formation of the reactive intermediate semiquinone. The location of a tentatively assigned third PQ molecule is consistent with a transition between the Qp and Qn sites in opposite monomers during the Q cycle. The spinach cytb6fstructure therefore provides new insights into how the complex fulfils its catalytic and regulatory roles in photosynthesis. A 3.6  resolution cryo-electron microscopy structure of the dimeric cytochrome b6f complex from spinach reveals the structural basis for operation of the quinol cycle and its redox-sensing function.

DOI: 10.1038/s41586-019-1746-6

Source:https://www.nature.com/articles/s41586-019-1746-6