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Nitrogenases are the only known enzymes that reduce molecular nitrogen (N 2 ) to ammonia. Recent findings have demonstrated that nitrogenases also reduce the greenhouse gas carbon dioxide (CO 2 ), suggesting CO 2 to be a competitor of N 2 . However, the impact of omnipresent CO 2 on N 2 fixation has not been investigated to date. Here, we study the competing reduction of CO 2 and N 2 by the two nitrogenases of Rhodobacter capsulatus , the molybdenum and the iron nitrogenase. The iron nitrogenase is almost threefold more efficient in CO 2 reduction and profoundly less selective for N 2 than the molybdenum isoform under mixtures of N 2 and CO 2 . Correspondingly, the growth rate of diazotrophically grown R. capsulatus strains relying on the iron nitrogenase notably decreased after adding CO 2 . The in vivo CO 2 activity of the iron nitrogenase facilitates the light-driven extracellular accumulation of formate and methane, one-carbon substrates for other microbes, and feedstock chemicals for a circular economy.

Nitrogenases are best known for catalyzing 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 three nitrogenase isozymes, the iron nitrogenase has the highest wild-type activity for the reduction of CO 2 , but the molecular architecture facilitating these activities has remained unknown. Here, we report a 2.35-Å cryogenic electron microscopy structure of the ADP·AlF 3 -stabilized iron 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 iron nitrogenase G subunit is involved in cluster stabilization and substrate channeling and confers specificity between nitrogenase reductase and catalytic component proteins. Moreover, the structure highlights a different interface between the two catalytic halves of the iron and the molybdenum nitrogenase, potentially influencing the intrasubunit 'communication' and thus the nitrogenase mechanism., (© 2023. The Author(s).)