The Rnf complex is the primary respiratory enzyme of several anaerobic prokaryotes that transfers electrons from ferredoxin to NAD+ and pumps sodium ions (Na+) across a membrane, powering ATP synthesis. Rnf is widespread in primordial organisms and the evolutionary predecessor of the Na+-pumping NADH-quinone oxidoreductase (Nqr)1. By running in reverse, Rnf reduces ferredoxin with NADH as reductant at the expense of the transmembrane electrochemical ion gradient and provides low potential electrons for nitrogenases as well as CO2 reductases. Yet, the molecular principles that couple the long-range electron transfer to the Na+ translocation across the membrane remain elusive. Here we resolve key functional states along the electron transfer pathway using redox-controlled cryo-electron microscopy (cryo-EM) that, in combination with biochemical functional assays and atomistic molecular simulations, provide key insight into the redox-driven Na+ pumping mechanism. We show that the reduction of the unique membrane-embedded [2Fe2S] cluster in the vestibule between the RnfA/E subunits electrostatically attracts the sodium ions, and in turn, triggers an inward/outward transition with alternating membrane access driving the Na+ pump and the reduction of NAD+. Our study unveils an ancient mechanism for redox-driven ion pumping, and provides key understanding of the fundamental principles governing energy conversion in biological systems.

中文翻译：

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古代Rnf机械氧化还原耦合钠泵的分子原理


Rnf 复合物是几种厌氧原核生物的主要呼吸酶，它将电子从铁氧还蛋白转移到 NAD+ 并将钠离子 (Na+) 泵过膜，为 ATP 合成提供动力。 Rnf 广泛存在于原始生物体中，是 Na+ 泵 NADH-醌氧化还原酶 (Nqr)1 的进化前身。通过反向运行，Rnf 以 NADH 作为还原剂还原铁氧还蛋白，但会牺牲跨膜电化学离子梯度，并为固氮酶和 CO2 还原酶提供低电势电子。然而，将长程电子转移与跨膜 Na+ 易位结合起来的分子原理仍然难以捉摸。在这里，我们使用氧化还原控制的冷冻电子显微镜 (cryo-EM) 解析电子转移路径上的关键功能状态，结合生化功能测定和原子分子模拟，提供对氧化还原驱动的 Na+ 泵送机制的关键见解。我们发现，RnfA/E 亚基之间的前庭中独特的膜嵌入 [2Fe2S] 簇的还原会静电吸引钠离子，反过来，通过交替的膜访问驱动 Na+ 泵和钠离子，触发向内/向外的转变。 NAD+的减少。我们的研究揭示了氧化还原驱动离子泵的一种古老机制，并提供了对生物系统中控制能量转换的基本原理的关键理解。