Nitrogen fixation

Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmospheric nitrogen is molecular dinitrogen, a relatively nonreactive molecule that is metabolically useless to all but a few microorganisms. Biological nitrogen fixation or ''diazotrophy'' is an important microbials mediated process that converts dinitrogen (N₂) gas to ammonia (NH₃) using the nitrogenase protein complex (Nif).

Nitrogen fixation is essential to life because fixed inorganic nitrogen compounds are required for the biosynthesis of all nitrogen-containing organic compounds, such as amino acids and proteins, nucleoside triphosphates and nucleic acids. As part of the nitrogen cycle, it is essential for agriculture and the manufacture of fertilizer. It is also, indirectly, relevant to the manufacture of all nitrogen chemical compounds, which includes some explosives, pharmaceuticals, and dyes.

Nitrogen fixation is carried out naturally in soil by microorganisms termed diazotrophs that include bacteria, such as '' Azotobacter,'' and archaea. Some nitrogen-fixing bacteria have symbiotic relationships with plant groups, especially legumes. Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on rice roots. Nitrogen fixation occurs between some termites and fungi. It occurs naturally in the air by means of NO_x production by lightning.

All biological reactions involving the process of nitrogen fixation are catalyzed by enzymes called nitrogenases. These enzymes contain iron, often with a second metal, usually molybdenum but sometimes vanadium.

History

Biological nitrogen fixation was discovered by Jean-Baptiste Boussingault in 1838. Later, in 1880, the process by which it happens was discovered by German agronomist Hermann Hellriegel and and was fully described by Dutch microbiologist Martinus Beijerinck.

"The protracted investigations of the relation of plants to the acquisition of nitrogen begun by Saussure, Ville, Lawes and Gilbert and others culminated in the discovery of symbiotic fixation by Hellriegel and Wilfarth in 1887."

"Experiments by Bossingault in 1855 and Pugh, Gilbert & Lawes in 1887 had shown that nitrogen did not enter the plant directly. The discovery of the role of nitrogen fixing bacteria by Herman Hellriegel and Herman Wilfarth in 1886-8 would open a new era of soil science."

In 1901 Beijerinck showed that '' Azotobacter chroococcum'' was able to fix atmospheric nitrogen. This was the first species of the azotobacter genus, so-named by him. It is also the first known diazotroph, species that use diatomic nitrogen as a step in the complete nitrogen cycle.

Biological

Biological nitrogen fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by a nitrogenase enzyme. The overall reaction for BNF is:

:N2 + 16ATP + 16H2O + 8e- + 8H+ -> 2NH3 +H2 + 16ADP + 16$\text_i$

The process is coupled to the hydrolysis of 16 equivalents of ATP and is accompanied by the co-formation of one equivalent of . The conversion of into ammonia occurs at a metal cluster called FeMoco, an abbreviation for the iron-molybdenum cofactor. The mechanism proceeds via a series of protonation and reduction steps wherein the FeMoco active site hydrogenates the substrate. In free-living diazotrophs, nitrogenase-generated ammonia is assimilated into glutamate through the glutamine synthetase/glutamate synthase pathway. The microbial nif genes required for nitrogen fixation are widely distributed in diverse environments.

For example, decomposing wood, which generally has a low nitrogen content, has been shown to host a diazotrophic community. The bacteria enrich the wood substrate with nitrogen through fixation, thus enabling deadwood decomposition by fungi.

Nitrogenases are rapidly degraded by oxygen. For this reason, many bacteria cease production of the enzyme in the presence of oxygen. Many nitrogen-fixing organisms exist only in anaerobic conditions, respiring to draw down oxygen levels, or binding the oxygen with a protein such as leghemoglobin.

Importance of nitrogen

Atmospheric nitrogen is inaccessible to most organisms, because its triple covalent bond is very strong. Life takes up fixed nitrogen in various ways. Considering atom acquisition, for every 100 atoms of carbon, roughly 2 to 20 atoms of nitrogen are assimilated. The atomic ratio of carbon (C) : nitrogen (N) : phosphorus (P) observed on average in planktonic biomass was originally described by Alfred Redfield. The Redfield Ratio, the stoichiometric relationship between C:N:P atoms, is 106:16:1.

Nitrogenase

The protein complex nitrogenase is responsible for catalyzing the reduction of nitrogen gas (N₂) to ammonia (NH₃). In Cyanobacteria, this enzyme system is housed in a specialized cell called the heterocyst. The production of the nitrogenase complex is genetically regulated, and the activity of the protein complex is dependent on ambient oxygen concentrations, and intra- and extracellular concentrations of ammonia and oxidized nitrogen species (nitrate and nitrite). Additionally, the combined concentrations of both ammonium and nitrate are thought to inhibit N_Fix, specifically when intracellular concentrations of 2-oxoglutarate (2-OG) exceed a critical threshold. The specialized heterocyst cell is necessary for the performance of nitrogenase as a result of its sensitivity to ambient oxygen.

Nitrogenase consist of two proteins, a catalytic iron-dependent protein, commonly referred to as MoFe protein and a reducing iron-only protein (Fe protein). There are three different iron dependent p