Biological Nitrogen fixation mechanism

Following reactions take place during biological nitrogen fixation:

  1. Formation of ammonia: Nitrogen is reduced to ammonia in the bacterioids as follows:

N2 + 81 + 16ATP + SH ________________ 2N1-13 +H2 + 16ADP

The enzyme nitrogenase controls this reaction. This enzyme contains two metallic components molybdenum and iron. This reaction is not so simple. Several intermediate compounds are formed. But such compounds are not stable. Therefore, they are not clearly identified. Diamide (NH = NH) and hydrazine (N112—N142) and NH3 — NH3 are the intermediate compounds. They are formed during formation of ammonia. Hydrogen for the reaction is donated by NADH,. Respiration provides energy.




  1. Synthesis of amino acids: One molecule of ammonia is formed. It is incorporated into organic acids like a- ketoglutaric acid and form glutamic acid. The- ketoglutaric acid is an intermediate of respiratory cycle. Glutamic acid is amino acids. Other amino acids are synthesized by transamination. Some of the amino acids are utiliz,ed by the bacteria for their growth. Other amino acids are translocated to the host plant.Action and significance of nitrogenaseThe enzyme nitrogenase inactivated in the presence of oxygen. Therefore, enzymatic reduction of nitrogen to ammonia takes place in different site of the cell. Respiration occurs in different sites. These two sites are separated by compartment. Thus nitrogen reduction occurs in the bacterioids under anaerobic conditions.Higher plants lack enzyme nitrogenase. This enzyme is provided by bacteria. Therefore, higher plants cannot fix molecular nitrogen without bacteria. However, genes of this enzyme are injected into higher plant by genetic engineering. Therefore, some transgenic plants can fix atmospheric nitrogen without bacteria,

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