title: Characterization of an oxygen-stable nitrogenase complex isolated from Azotobacter chroococcum.
abstract: 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.
mesh_terms: D001395:Azotobacter / Q000201:enzymology*; D002499:Centrifugation, Density Gradient; D004591:Electrophoresis, Polyacrylamide Gel; D008970:Molecular Weight; D009591:Nitrogenase / Q000037:antagonists & inhibitors / Q000302:isolation & purification*; D010100:Oxygen / Q000494:pharmacology; D013053:Spectrophotometry
chemical_list: D009591:Nitrogenase; D010100:Oxygen