198 - Oxidation of nitrogenase iron protein by dioxygen without inactivation could contribute to high respiration rates of Azotobacter species and facilitate nitrogen fixation in other aerobic environments

This study looks at oxygen interactions with dinitrogenase reductase (DNR) in Azotobacter chroococcum (and Klebsiella pneumoniae) to see if it's possible that the enzyme can be oxidized without being totally inactivated.

When the DNR was present in high enough levels relative to the amount of oxygen (4-fold molar excess), it was protected from inactivation by oxygen: it seemed to reduce superoxides to peroxide (and then water if catalase doesn't get to it first) before the reactive oxygen species could harm it. Superoxide seems to be the harmful form of oxygen for DNR. They suggest this helps with high respiration rates (i.e. consumption of oxygen). They calculated that if 10% of the cell's protein were DNR, it could account for pretty much all of the oxygen consumption. This is unlikely, but it could still be a significant part of respiratory protection. They call this "autoprotection."

Reference:

Thorneley, R. N. & Ashby, G. A. Oxidation of nitrogenase iron protein by dioxygen without inactivation could contribute to high respiration rates of Azotobacter species and facilitate nitrogen fixation in other aerobic environments. Biochem J 261, 181–187 (1989).