Characterization of an oxygen-stable nitrogenase complex isolated from Azotobacter chroococcum

In crude cell-free extracts of Azotobacter chroococcum, nitrogenase was much less sensitive to irreversible inactivation by O2 than was the purified enzyme. When nitrogenase was partially purified by anaerobic discontinuous sucrose-density-gradient centrifugation, O2-tolerance was retained. This preparation was considerably enriched in four polypeptides, three of which were derived from the Mo-Fe(molybdenum-iron) protein and Fe (iron) protein of nitrogenase. The fourth was purified to homogeneity and shown to be an iron-sulphur protein (mol.wt. 14000) probably containing a 2Fe--2S centre. When this protein was added to purified nitrogenase, the enzyme was rendered O2-tolerant, through stabilization was Mg2+-dependent. The isolated O2-tolerant nitrogenase was an equimolar stoicheiometric complex between the MO--Fe, Fe and protective proteins. It is likely that the formation of this complex in vivo is the mechanism of 'conformational protection' in this organism.