Binds to and protects nitrogenase from irreversible exposure to O(2), in what is known as 'conformational protection' (PubMed:7830548, PubMed:7548055, PubMed:10220344, PubMed:26654855). Shifts nitrogenase into an inactive, O(2)-tolerant state. Exists in 2 states, an open oxidized state that binds and protects nitrogenase, and a closed reduced state that probably does not bind nitrogenase; cooperative oxidation of the 2Fe-2S clusters causes the conformation change (PubMed:26654855). Does not protect nitrogenase with the vanadium-iron subunit, not clear if it protects the iron-only nitrogenase (PubMed:7830548). Binds 1 [2Fe-2S] cluster per subunit. Homodimer (PubMed:7548055, PubMed:26654855). Forms a complex with nitrogenase consisting of 1 MoFe heterotetramer, 1 Fe protein dimer and 1 FeSII dimer; the interaction is electrostatic and disurpted by increasing NaCl levels (PubMed:26654855). Expressed in nitrogen-fixing and non-nitrogen-fixing cells, although transcript levels are higher in nitrogen-fixing cells. Expressed during log and stationary phase growth (at protein level). The gene is monocistronic (PubMed:7830548). Has mobile N-loop (residues 59-96) that changes position in a redox-dependent manner being folded against the protein in the closed reduced state, protecting its [2Fe-2S] cluster, or away from the protein in the open oxidized state. The open form complexes with the MoFe and Fe subunits of nitrogenase protecting them from O(2) (PubMed:26654855). No visible growth phenotype, but shortly after entry into stationary phase the nitrogenase subunitsare destabilized and are no longer detectable (PubMed:7830548). In vitro nitrogenase is highly sensitive to O(2) (PubMed:7830548, PubMed:10220344). Belongs to the adrenodoxin/putidaredoxin family.