固氮作用
Nitrogenase is the most important enzyme involved in nitrogen fixation. "Azotobacter" species have several types of nitrogenase. The basic one is molybdenum-iron nitrogenase. An alternative type contains vanadium; it is independent of molybdenum ions and is more active than the Mo-Fe nitrogenase at low temperatures. So it can fix nitrogen at temperatures as low as 5 °C, and its low-temperature activity is 10 times higher than that of Mo-Fe nitrogenase. An important role in maturation of Mo-Fe nitrogenase plays the so-called P-cluster. Synthesis of nitrogenase is controlled by the "nif" genes. Nitrogen fixation is regulated by the enhancer protein NifA and the "sensor" flavoprotein NifL which modulates the activation of gene transcription of nitrogen fixation by redox-dependent switching. This regulatory mechanism, relying on two proteins forming complexes with each other, is uncommon for other systems.