Cross-Coupling of Mo- and V-Nitrogenases Permits Protein-Mediated Protection from Oxygen Deactivation.

Nitrogenases catalyze dinitrogen (N₂) fixation to ammonia (NH₃). While these enzymes are highly sensitive to deactivation by molecular oxygen (O₂) they can be produced by obligate aerobes for diazotrophy, necessitating a mechanism by which nitrogenase can be protected from deactivation. In the bacterium Azotobacter vinelandii, one mode of such protection involves an O₂-responsive ferredoxin-type protein ("Shethna protein II", or "FeSII") which is thought to bind with Mo-dependent nitrogenase's two component proteins (NifH and NifDK) to form a catalytically stalled yet O₂-tolerant tripartite protein complex. This protection mechanism has been reported for Mo-nitrogenase, however, in vitro assays with V-nitrogenase suggest that this mechanism is not universal to the three known nitrogenase isoforms. Here we report that the reductase of the V-nitrogenase (VnfH) can engage in this FeSII-mediated protection mechanism when cross-coupled with Mo-nitrogenase NifDK. Interestingly, the cross-coupling of the Mo-nitrogenase reductase NifH with the V-nitrogenase VnfDGK protein does not yield such protection.