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Mechanism of Nitrogen Fixation

	Mechanism of Nitrogen Fixation -The mechanism of nitrogen fixation is not fully understood but the first product detectable by isolopic studies is ammonia. Reduction of nitrogen to ammonia requires a valence change (0 to -3). Six electrons arc therefore required to reduce one mole of nitrogen to two moles of ammonia. This requires the breakup of nitrogen bonds (N =N).
It is agreed that atoms of oxygen are separated in biological oxidation through change in valency of the metal ion (Fe) in the enzyme cytochrome oxidase. It is postulated that .atoms of nitrogen are separated trough change in the valency of the metal ion (molybdenum) bound to the enzyme involved in the reduction of nitrogen.
Non symbiotic nitrogen fixation is better understood because a cell free nitrogen fixing system is available. A cell free preparation, made of two fractions, has been isolated from Cl. pasteurianum. Fraction one is known as electron donating and A TP generating system. It is involved in the metabolism of pyruvic acid.
The second fraction involved in actual nitrogen fixation includes the enzyme nitrogenase. It is repressed by ammonium ions. It requires ferredoxin (non-haem Fe -compound) or flavodoxin (FMN containing protein) as a carrier of reducing power. Ferredoxin or flavodoxin (Fd) is the first reducer, which in turn reduces nitrogen bound to the enzyme nitrogenase in presence of ATP.
	Inorganic intermediates between N2 and NH3 have not been detected. It is therefore presumed that reduction takes place directly on the enzyme surface. The enzyme nitrogenase has been studied and following characteristics have been noted: 1) The enzyme is made up of at least two components; one contains both Fe and Mo, and the other contains only Fe. 2) The enzyme can reduce other substrates besides nitrogen, such as nitrous oxide, azide, cyanide, acetylene etc. 3) If no suitable electron acceptor is available, the enzyme forms molecular hydrogen.

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Sulphur Cycle Ammonification in Soil Nitrification in Soil Nitrate Reduction in Soil Denitrification in Soil Nitrogen Fixation Symbiotic Nitrogen Fixation Rhizobium Specificity of Bacteria and Legumes Non-Symbiotic Nitrogen Fixation Non-symbiotic Nitrogen Fixing Microorganisms Mechanism of Nitrogen Fixation Oxidation of Sulphur and Sulphur Compounds by Bacteria

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