Nitrogen fixation

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Nitrogen fixation is the process by which nitrogen is taken from its natural, relatively inert molecular form (N2) in the atmosphere and converted into nitrogen compounds (such as, notably, ammonia, nitrate and nitrogen dioxide).

Nitrogen fixation can occur as a result of non-biological processes. These include lightning, industrially through the Haber-Bosch Process, and combustion.

But Nitrogen fixation can also be performed naturally by a number of different [prokaryotes]including some cyanobacteria, actinobacteria, heterotrophic bacteria and certain types of anaerobic bacteria, and this is called Biological Nitrogen fixation. Microorganisms that fix nitrogen are called diazotrophs. Biological nitrogen fixation can occur in terrestrial and aquatic environments.

Biological Nitrogen fixation occurs when atmospheric nitrogen is converted to ammonia by a pair of bacterial enzymes called nitrogenase. In the biological nitrogen fixation process, the triple bond of N2 is broken down in the presence of energy and nitrogenase enzyme to ammonia with concurrent evolution of H2 according to the following reaction: N2 + 8H+ + 8eāˆ’ + 16 ATP ā†’ 2NH3 + H2 + 16ADP + 16 Pi

Although ammonia (NH3) is the direct product of this reaction, it is quickly protonated into ammonium (NH4+). In free-living diazotrophs, the nitrogenase-generated ammonium is assimilated into glutamate through the glutamine synthetase/glutamate synthase pathway.

All diazotrophs are prokaryotes and so far there is no eukaryotic organism known to fix nitrogen. These diazotrophs are very important in the global nitrogen cycle as they can utilise the abundantly available gaseous nitrogen (78% by weight of air) and grow successfully in the absence of combined nitrogen such as NH3, nitrate or amino-acids.

In most bacteria, the nitrogenase enzymes are very susceptible to destruction by oxygen (and many bacteria cease production of the enzyme in the presence of oxygen)[1]. Low oxygen tension is achieved by different bacteria by: living in anaerobic conditions, respiring to draw down oxygen levels, or binding the oxygen with a protein (e.g. leghaemoglobin). The great majority of legumes have this association, but a few genera (e.g., Styphnolobium) do not.

The enzyme nitrogenase is not specific to N2 but it can also reduce a number of triple bonded molecules such as azide, cyanide, and acetylene. Acetylene has been used extensively as a substrate for the biochemical assay of nitrogenase, the reaction for reduction of acetylene is: C2H2 + 2H+ + 2e- ---- C2H4

The reduction of acetylene consumes a small amount of energy and serves no useful purpose to cells but does provide the experiment with a simple way of measuring the activity of nitrogenase. This method is mostly used for analysis of nitrogen fixation and it is known as Acetylene Reduction Assay, first described by Stewart et al. (1967).

The enzyme nitrogenase is only found in diazotrophs, a phylogenetically diverse group of prokaryotes (eubacteria and archaebacteria). In these organisms the enzyme has been highly conserved and extremely O2 sensitive even in strict aerobes as the enzyme is readily inactivated by oxygen.

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