The Effect of ATP upon the Oxygen Sensitivity of Nitrogenase from Azotobacter chroococcum

Crude preparations of nitrogenase (AS1) from Azotobacter chroococcum which were normally oxygen‐tolerant, became oxygen‐sensitive in the presence of either ATP, GTP, ITP, UTP, CTP or sodium pyrophosphate. MgCl 2 antagonised this effect of ATP. ATP‐induced oxygen damage was faster at 4°C than at 30°C. ATP‐induced oxygen damage to prepn AS1 affected fraction 2 (Ac2) of the nitrogenase; fraction 1 (Ac1) was unaffected. Purified fraction Ac2 became hypersensitive to oxygen in the presence of ATP. Pre‐treatment of prepn AS1 with “Chelex 100” or EDTA under anaerobic conditions made the nitrogenase oxygen‐sensitive. Oxygen‐tolerance was partly restored by metal ions. Similar anaerobic pre‐treatment of prepn AS1 with ATP followed by removing the ATP by dialysis did not induce oxygen‐sensitivity. Pure fraction Ac2 bound significantly more [ 14 C]ATP in the presence of MgCl 2 than did EDTA‐treated fraction Ac2 in the absence of MgCl 2 . Hill plots of the ATP requirement for acetylene reduction, for ATP‐induced oxygen damage, for the MgCl 2 , requirement for acetylene reduction and for the MgCl 2 , requirement for protection against oxygen damage to EDTA‐treated prepn AS1 all yielded n values between 1 and 2. These results are discussed in terms of a role for ATP in nitrogen fixation.