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Vanadium nitrogenase
Vanadium nitrogenase is a key enzyme for nitrogen fixation found in nitrogen-fixing bacteria, and is used as an alternative to molybdenum nitrogenase when molybdenum is unavailable.〔 An important component of the nitrogen cycle, vanadium nitrogenase converts nitrogen gas to ammonia, thereby making otherwise inaccessible nitrogen available to plants. Unlike molybdenum nitrogenase, vanadium nitrogenase can also reduce carbon monoxide to ethylene, ethane and propane but both enzymes can reduce protons to hydrogen gas and acetylene to ethylene.
==Biological functions==
Vanadium nitrogenases are found in members of the bacterial genus ''Azotobacter'' as well as the species ''Rhodopseudomonas palustris'' and ''Anabaena variabilis''.〔〔 Most of the functions of vanadium nitrogenase match those of the more common molybdenum nitrogenases and serve as an alternative pathway for nitrogen fixation in molybdenum deficient conditions. Like molybdenum nitrogenase, dihydrogen functions as a competitive inhibitor and carbon monoxide functions as a non-competitive inhibitor of nitrogen fixation. Vanadium nitrogenase has an α2β2Ύ2 subunit structure while molybdenum nitrogenase has an α2β2 structure. Though the structural genes encoding vanadium nitrogenase show only about 15% conservation with molybdenum nitrogenases, the two nitrogenases share the same type of redox centers. At room temperature, vanadium nitrogenase is less efficient at fixing nitrogen than moldybdenum nitrogenases because it converts more H+ to H2 as a side reaction.〔 However, at low temperatures vanadium nitrogenases have been found to be more active than the molybdenum type, and at temperatures as low as 5 °C its nitrogen-fixing activity is 10 times higher than that of molybdenum nitrogenase. Like molybdenum nitrogenase, vanadium nitrogenase is easily oxidized and is thus only active under anaerobic conditions. Various bacteria employ complex protection mechanisms to avoid oxygen.〔
The overall stoichiometry of nitrogen fixation catalyzed by vanadium nitrogenase can be summarized as follows:〔
:N2 + 12e− + 14H+ + 24MgATP → 2NH4+ + 3H2 + 24MgADP + 24HPO42−
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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