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Structural basis for the conformational protection of nitrogenase from O2 | Nature The low reduction potentials required for the reduction of dinitrogen (N2) render metal-based nitrogen-fixation catalysts vulnerable to irreversible damage by dioxygen (O2)1–3. Such O2 sensitivity represents a major conundrum for the enzyme nitrogenase, as a large fraction of nitrogen-fixing organisms are either obligate aerobes or closely associated with O2-respiring organisms to support the high energy demand of catalytic N2 reduction4. To counter O2 damage to nitrogenase, diazotrophs use O2 scavengers, exploit compartmentalization or maintain high respiration rates to minimize intracellular O2 concentrations4–8. A last line of damage control is provided by the ‘conformational protection’ mechanism9, in which a [2Fe:2S] ferredoxin-family protein termed FeSII (ref. 10) is activated under O2 stress to form an O2-resistant complex with the nitrogenase component proteins11,12. Despite previous insights, the molecular basis for the conformational O2 protection of nitrogenase and the mecha https://doi.org/10.1038/s41586-024-08311-1