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Index >>Nitrogen Fixation >>Factors Affecting Nitrogenase Activity

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Nitrogen Fixation Nitrogen Fixation Part 1 Structure of Nitrogenase Components Fe Protein Mo-Fe Protein Mechanism of Nitrogenase Action Hydrogen Evolution Factors Affecting Nitrogenase Activity Nitrogen Fixation Genes Nitrification Nitrate Reduction Glutamate Synthase Pathway

Factors Affecting Nitrogenase Activity

	Factors Affecting Nitrogenase Activity - The factors affecting nitrogenase activity include oxygen sensitivity, temperature, pH, and soil moisture. (i) Oxygen Sensitivity One of the most striking properties of nitrogenase is its sensitivity to oxygen. The Fe protein is irreversibly damaged by oxygen, while the MoFe protein is relatively insensitive. How does the enzyme function in such highly aerobic species as Azotobacter? There are two protective mechanisms operating, conformational protection and respiratory protection. In conformational protection, the enzyme undergoes a conformational change by which it becomes insensitive to oxygen. In this condition it is unable to catalyze dinitrogen fixation. In respiratory protection there is an oxygen scavenging process operating as a result of high respiratory activity. This permits the enzyme to maintain a catalytically active form.
Azospirillum spp are aerobic bacteria which require low partial pressure of oxygen (pO2) for expression of hydrogenase activity. Under fully aerobic conditions, the fixation of nitrogen is not possible unless nitrogenase is protected. Azospirillum chroococcum and Azotobacter vinelandii have been used for nitrogen fixation studies under conditions of high pO2. Azotobacter can carry out nitrogen fixation in air because of respiratory and conformational protection of its nitrogenase, and because of the location of the enzyme within the cell. Another mechanism by which certain bacteria can express nitrogenase activity is microaerophily. Azospirillum requires microaerophilic conditions for nitrogen fixation. A. brasilense does not grow with N2 as the sole source of nitrogen under aerobic conditions. In Azospirillum, nitrogen fixation takes place at optimal rate between 0.005-0.007 atmospheres (0.507 and 0.709 k Pa).
(ii) Temperature and pH In Azospirillum, the optimum temperature for H2-depentent growth is 32-40°C. This is similar to the optima of tropical nitrogen fixing bacteria. Nitrogenase activity of Azospirillum is sensitive to temperatures below IS°C. This accounts for the higher incidence of these bacteria in tropical countries For Azospirillum, the optimal pH for N2 dependent growth is 6.8-7.8. These requirements are met with at the surface or within the cells of roots. Roots of grasses provide optimal pH conditions to bacteria even when the soil pH is low.
(iii) Soil Moisture In the Broadbalk experiments it was observed that soil cores from wet areas showed high nitrogenase activity. It was thus inferred that nitrogenase activity is correlated with soil moisture (Day et al, 1975). The rate of acetylene reduction was found to increase exponentially with linear increase in soil moisture. A similar correlation has been found between nitrogen fixation and soil core moisture of grasslands. Day et al., suggested that increased soil moisture resulted in a higher level of anaerobiosis in the rhizosphere.
	The changed pO₂ in turn affects nitrogenase activity this view is supported by the fact that higher rates of acetylene reduction are observed in soil cores incubated under N₂ compared with incubation under air. These observations support the hypothesis that nitrogen fixation in non nodular plants may be increased in wet soils. Nitrogenase activity in wetland grasses has been reported to be much higher than that of plants growing in mesic or dry soils. The much higher rates of acetylene reduction in soil cores of marsh plants and paddy rice, as compared with those of agriculturally important cereal grasses, may be explained by the leaching hypothesis. High levels of combined nitrogen are inhibitory to nitrogen fixation. Because of denitrification and leaching of nitrates, the available combined nitrogen content of wet soils is low. The lowered combined nitrogen content of the soil increases nitrogen fixation.

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Nitrogen Fixation Part 1 Ammonification Denitrification Denitrifying Bacteria Ecological Importance of Denitrification Nitrogen Assimilation Transamination Glutamate Dehydrogenase (GDH) Glutamate Synthase Pathway Glutamine Synthetase (GS) Reaction Glutamate Synthase (GOGAT) Reaction Transaminase Reactions Glutamine Synthtase Adenylylation and Deadenylylation

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