Biological nitrogen fixation requires substantial metabolic energy in form of ATP as well as low-potential electrons that must derive from central metabolism. During aerobic growth, the free-living soil diazotroph Azotobacter vinelandii transfers electrons from the key metabolite NADH to the low-potential ferredoxin FdxA that serves as a direct electron donor to the dinitrogenase reductases. This process is mediated by the RNF complex that exploits the proton motive force over the cytoplasmic membrane to lower the midpoint potential of the transferred electron. Here we report the cryogenic electron microscopy structure of the nitrogenase-associated RNF complex of A. vinelandii, a seven-subunit membrane protein assembly that contains four flavin cofactors and six iron–sulfur centers. Its function requires the strict coupling of electron and proton transfer but also involves major conformational changes within the assembly that can be traced with a combination of electron microscopy and modeling.

生物固氮需要大量 ATP 形式的代谢能量以及必须来自中央代谢的低电势电子。在有氧生长过程中，自由生活的土壤固氮菌维氏固氮菌将电子从关键代谢物 NADH 转移到低电位铁氧还蛋白 FdxA，后者充当二固氮酶还原酶的直接电子供体。该过程由 RNF 复合物介导，RNF 复合物利用细胞质膜上的质子动力来降低转移电子的中点电位。在这里，我们报告了 A. vinelandii 固氮酶相关 RNF 复合物的低温电子显微镜结构，这是一种七亚基膜蛋白组装体，包含四个黄素辅因子和六个铁硫中心。其功能需要电子和质子转移的严格耦合，但也涉及组件内的主要构象变化，这些变化可以通过电子显微镜和建模的组合来追踪。