Nitrate Reduction

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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

Nitrate Reduction

Nitrate Reduction -
The nitrification process is reversed by many microorganisms, which are capable of reducing nitrite to nitrate and then to ammonia. The nitrate serves as a terminal electron acceptor for anaerobic respiration.

This process is known as assimilatory nitrate reduction, because the organisms are able to obtain cellular nitrogen through the assimilation of the ammonia formed. The process is mediated through two enzyme complexes called nitrate reductase.

E.coli can live anaerobically by reducing nitrate to nitrite.

NO; + 2H+ + 2e--+ NO; + HP

The nitrate reductase of E.coli has a MW of 720,000. It consists of four large and four small polypeptide chains, and four atoms of molybdenum. (Mo). It is believed that the nitrate interacts directly with molybdenum. As similatory nitrate reductases from bacteria and plants require NADH as the electron source, while those from fungi preferentially utilize NADPH.

The nitrate reductases from all organisms are essentially similar. They are high MW complexes consisting of two proteins, an FAD dependent protein and a molybdenum containing protein. The FAD dependent protein accepts electron from pyridine nucleotides. The molybdenum containing protein brings about the reduction of nitrate to nitrite.

The assimilatory nitrate reductase of Neurospora crassa contains not only Mo but also cytochrome b557. The following electron transport path way has been proposed.

Assimilatory nitrate reductase of bacteria utilizes NADH as the electron donor. It requires FAD for activity. In algae and higher plants, ferredoxin in used instead of FAD. The nitrite to ammonia reduction involves the addition of six electrons. Assuming that the reduction takes place in two electron states, it would pass through + 1 and -1 oxidation states.

Hyponitrous acid and hydroxylamine are suggested to be the two inter mediates. However, there is also evidence for a direct reduction from nitrate to ammonia.

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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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