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Index >> Nitrogen Fixation Free Living and Associative Symbiotic Bacteria >>Nitrogenase

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Nitrogen Fixation in Free-Living and Associative Symbiotic Bacteria Nitrogen Fixing Bacteria Nitrogenase Genetics Mechanism of Nitrogen Fixation Response of Plants to Azotobacter Inoculation Nitrogen Fixation in the Root Zone of Rice

Nitrogenase

	Nitrogenase One of the significant advances made in our knowledge of biological nitrogen fixation was the discovery that cell-free extracts of Azotobacter and Clostridium could fix nitrogen in the same way as the free-living intact bacterial cells. The finding led to the initial isolation of the enzyme nitrogenase from C. pasteurianum and A. vinelandii and the subsequent find ing that the enzyme is responsible for the adsorption and reduction of N₂ gas.
Cell-free extracts which retain the capacity to fix nitrogen have also been obtained by various workers from Mycobacterium flavum, Bacillus polymyxa, Klebsiella pneumoniae, Rhodospirillum rub rum, Chromatium and Chloropseudomonas ethylicum. Nitrogenase has been isolated from the following genera of free-living nitrogen-fixing microorganisms: Clostridium, Bacillus, Klebsiella, Chiaropseudomonas, Chromatium, Rhodospirillum, Anabaena, Gloeocapsa, Plectonema, Azotobacter and Mycobacterium. The enzyme consists of two protein fractions-the Mo-Fe containing protein (mol. weight 220,000-270,000) and the Fe containing protein (mol. weight 55,000-66,800).
Active nitrogenase can be reconstituted by the addition of purified Mo-Fe and Fe proteins of different microorganisms. For examples, proteins of Klebsiella pneumoniae and Bacillus polymyxa and those of blue-green algae and photosynthetic bacteria have been combined to reconstitute active nitrogenases, capable of reducing acetylene to ethylene. The Mo-Fe protein has been designated as dinitrogenase because nitrogen binds to the protein moiety whereas the Fe protein has been referred to as the dinitrogen reductase since the second moiety serves the specific function of reducing the Mo-Fe protein.
During catalysis by nitrogenase, protons and nitrogen compete for electrons. Therefore, in an atmosphere containing nitrogen, hydrogen evolution occurs simultaneously with ammonia formation. This evolution of hydrogen diverts 25-35 per cent of the total reductants available for the nitrogenase reaction, which is regarded as an intracellular wastage of energy in the over all process of nitrogen fixation, and the reaction can be summarized as follows: Nitrogenase
	In Azotobacter vinelandii, two additional nitrogenases have been recognized, in addition to the nitrogenase described earlier. These nitrogenases have been studied in mutants of A vinelandii One of these contains vanadium instead of molybdenum and the other has neither molybdenum nor vanadium. The characterization of these nitrogenases, has posed fresh problems in pinpointing evidences to demonstrate the essentiality of molybdenum for nitrogen fixation and characterization of the site at which nitrogen bind to nitrogenase.

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Nitrogen Fixing Activity of Paddy Soil in Pots under Various Treatments Nitrogen Fixation in the Rhizosphere of Grasses and Weeds Associative Symbiosis Some New Nitrogen Fixing Bacterial Associations Several Nitrogen Fixing Bacteria Response of Plants Azospirillum Inoculation The Concept of C4 Dicarboxylic Acid Pathway Nitrogen Fixation by Rhizobium in a Free Living State

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