Laboratory for Protein Crystallography, Institute for Protein Research, Osaka University, Suita, Osaka, 565-0871, Japan.
Department of Biological Sciences, Graduate School of Science, Osaka University, Toyonaka, Osaka, 560-0043, Japan.
Commun Biol. 2022 Sep 12;5(1):951. doi: 10.1038/s42003-022-03926-4.
Photosystem I (PSI) is a light driven electron pump transferring electrons from Cytochrome c (Cyt c) to Ferredoxin (Fd). An understanding of this electron transfer process is hampered by a paucity of structural detail concerning PSI:Fd interface and the possible binding sites of Cyt c. Here we describe the high resolution cryo-EM structure of Thermosynechococcus elongatus BP-1 PSI in complex with Fd and a loosely bound Cyt c. Side chain interactions at the PSI:Fd interface including bridging water molecules are visualized in detail. The structure explains the properties of mutants of PsaE and PsaC that affect kinetics of Fd binding and suggests a molecular switch for the dissociation of Fd upon reduction. Calorimetry-based thermodynamic analyses confirms a single binding site for Fd and demonstrates that PSI:Fd complexation is purely driven by entropy. A possible reaction cycle for the efficient transfer of electrons from Cyt c to Fd via PSI is proposed.
光系统 I（PSI）是一种光驱动的电子泵，它将电子从细胞色素 c（Cyt c）转移到铁氧还蛋白（Fd）。PSI:Fd 界面的结构细节以及 Cyt c 的可能结合位点的缺乏，阻碍了对这种电子转移过程的理解。在这里，我们描述了 Thermosynechococcus elongatus BP-1 PSI 与 Fd 和一个松散结合的 Cyt c 复合物的高分辨率冷冻电镜结构。PSI:Fd 界面处的侧链相互作用，包括桥接水分子，都被详细地可视化了。该结构解释了影响 Fd 结合动力学的 PsaE 和 PsaC 突变体的性质，并提出了 Fd 还原时解离的分子开关。基于量热法的热力学分析证实了 Fd 的单一结合位点，并表明 PSI:Fd 复合物的形成纯粹是由熵驱动的。提出了一个可能的反应循环，用于通过 PSI 从 Cyt c 到 Fd 高效传递电子。
