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Studies on the Octameric and Tetrameric Molybdenum-iron Proteins of Nitrogenase From Azotobacter Vinelandii

We have found  and purified the octamer of molybdenum-iron protein of nitrogenase from Azotobacter vinelandii 230. The activity of the octamer is lower than that of the tetramer. The octamer formation in vivo is related to cell growth process. During the log-phase of cell growth, the specific activity of nitrogenase is increasing, and the tetramer is the major form of molybdenum-iron protein. After entering into the plateau-phase, the energy activity decreases from maximum quickly and tends to stabilize at a low value, and at the same time the octamer becomes the major form of molybdenum-iron protein. Adding ammonium to the culture medium during nitrogen-fixing growth stage of Azotobacter vinelandii induces the formation of the octamer obviously.

We suggest that, the octamer of molybdenum-iron protein is a possible metablite in regulation of nitrogenase activity, the conversion from the tetramer to octamer will cause the decrease of nitrogenase specific activity in vivo. The octamer converts to tetramer at 4℃ in vitro, and the direction of conversion is reversed at 30℃.

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