Structural Insights into the Iron Nitrogenase Complex

Nitrogenases are best known for catalysing the reduction of dinitrogen to ammonia at a complex metallic cofactor. Recently, nitrogenases were shown to reduce carbon dioxide (CO 2 ) and carbon monoxide to hydrocarbons, offering a pathway to recycle carbon waste into hydrocarbon products. Among the nitrogenase family the iron nitrogenase is the isozyme with the highest wildtype activity for the reduction of CO 2 , but the molecular architecture facilitating these activities remained unknown. Here, we report a 2.35-Å cryogenic electron microscopy structure of the Fe nitrogenase complex from Rhodobacter capsulatus, revealing an [Fe 8 S 9 C -(R )-homocitrate]-cluster in the active site. The enzyme complex suggests that the AnfG-subunit is involved in cluster stabilisation, substrate channelling and confers specificity between nitrogenase reductase and catalytic components. Moreover, the structure highlights a different interface between the two catalytic halves of the iron and the molybdenum nitrogenase, potentially influencing the intra-subunit ‘communication’ and thus the nitrogenase mechanism.