Nitrogenase
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224748535
2008-07-10T06:58:09Z
Inwind
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merged
[[Image:Nitrogenase.PNG|thumb|Nitrogenase (PDB code=1n2c)
([http://www.pdb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb26_1.html more details...])]]
'''Nitrogenase''' ({{EC number|1.18.6.1}}) is the [[enzyme]] used by some organisms to fix atmospheric [[nitrogen]] gas (N<sub>2</sub>). It is the only known family of enzymes which accomplishes this process. [[Dinitrogen]] is relatively inert because each atom of nitrogen has three open [[orbitals]] in its outer [[electron shell]] to bond with another atom, so that if two nitrogen atoms bond to each other, they do so in all three of these orbitals. To break one nitrogen atom away from another requires breaking all three of these chemical bonds. This is referred to as having a [[triple bond]].
Nitrogenase is a [[catalyst]] for the reaction:
:N<sub>2</sub> + 6H + energy → 2NH<sub>3</sub>
Whilst the equilibrium formation of ammonia from molecular hydrogen and nitrogen has an overall negative enthalpy of reaction (ΔH<sup>0</sup> = -45.2 kJ mol<sup>-1</sup> NH<sub>3</sub>), the energy barrier to activation is generally insurmountable (E<sub>A</sub> = 420 kJ mol<sup>-1</sup>) without the assistance of catalysis<ref>Modak, J. M., 2002, Haber Process for Ammonia Synthesis, ''Resonance''. 7, 69-77.</ref>.
Nitrogenase thus breaks the triple bond by getting [[electron donor]]s for each of the three bonds, and then bonds the nitrogen to hydrogen atoms. The process is complex because each bond is broken individually, and is not completely understood. Nitrogenase requires both the MoFe protein and [[adenosine triphosphate|ATP]], which supplies the energy. Nitrogenase bonds each atom of nitrogen to three atoms of hydrogen to form [[ammonia]] or NH<sub>3</sub>, and then ammonia is bonded to [[glutamate]] and becomes [[glutamine]]. Nitrogenase associates with a second protein, and each cycle transfers one electron from an [[electron donor]] which is enough to break one of the nitrogen chemical bonds. However, it has not been proven that exactly three cycles are sufficient to fix an atom of nitrogen.
The enzyme therefore requires a great deal of chemical energy, released from the [[hydrolysis]] of ATP, and [[reducing agents]], such as [[dithionite]] [[in vitro|''in vitro'']] or [[ferredoxin]] [[in vivo|''in vivo'']]. The [[enzyme]] is composed of the [[heterotetrameric]] MoFe protein that is transiently associated with the [[homodimeric]] Fe [[protein]]. Nitrogenase is supplied [[reducing power]] when it associates with the reduced, [[nucleotide-bound]] homodimeric Fe protein. The [[heterocomplex]] undergoes cycles of [[cycle of association|association]] and disassociation to transfer one [[electron]], which is the [[limiting step]] in the process. ATP supplies the reducing power.
The exact mechanism of [[catalysis]] is unknown due to the difficulty in obtaining [[crystal]]s of nitrogen bound to nitrogenase. This is because the [[resting state]] of MoFe protein does not bind nitrogen and also requires at least three electron transfers to perform [[catalysis]]. Nitrogenase is able to bind acetylene and carbon monoxide, which are noncompetitive [[substrate (biochemistry)|substrate]]s and [[Enzyme inhibitor|inhibitor]]s, respectively. Dinitrogen, however, is a competitive substrate for acetylene. This is because binding of dinitrogen prevents acetylene binding, and acetylene requires only one electron to be reduced, and it does not inhibit.<ref>Seefeldt LC, Dance IG, Dean DR. 2004. Substrate interactions with nitrogenase: Fe versus Mo. ''Biochemistry''. 43(6):1401-9.</ref>
All nitrogenases have an iron- and sulfur-containing cofactor that includes [[heterometal]] atom in the active site (e.g. [[FeMoCo]]). In most, this heterometal is [[molybdenum]], though in some species it is replaced by [[vanadium]] or [[iron]].
Due to the oxidiative properties of [[oxygen]], most nitrogenases are irreversibly inhibited by [[dioxygen]], which degradatively oxidizes the Fe-S cofactors. This requires mechanisms for nitrogen fixers to avoid oxygen ''[[in vivo]]''. Despite this problem, many use oxygen as a terminal electron acceptor for respiration. One known exception, a recently-discovered nitrogenase of ''[[Streptomyces thermoautotrophicus]]'', is unaffected by the presence of oxygen [http://www.jbc.org/cgi/reprint/272/42/26627.pdf]. The [[Azotobacteraceae]] are unique in their ability to employ an [[oxygen-labile]] nitrogenase under [[aerobic]] conditions. This ability has been attributed to a high [[metabolic rate]] allowing oxygen reduction at the [[membrane]], but this idea has been shown to be unfounded and impossible at oxygen concentrations above 70 µM (ambient concentration is 230 µM O2), as well as during additional nutrient limitations.<ref>Oelze J. 2000. Respiratory protection of nitrogenase in [[Azotobacter]] species: Is a widely-held hypothesis unequivocally supported by experimental evidence? ''FEMS Microbiol Rev''. 24(4):321-33.</ref>
The reaction that this enzyme performs is:
: N<sub>2</sub> + 8H<sup>+</sup> + 8e<sup>-</sup> + 16 ATP → 2NH<sub>3</sub> + H<sub>2</sub> + 16ADP + 16 Pi
==Organisms that synthesize nitrogenase==
* [[Diazotroph]]s
* [[Heterocyst]]s
* [[Azotobacteraceae]]
* [[Rhizobia]]
* [[Frankia]]
==See also==
* [[Nitrogen fixation]]
* [[Dinitrogenase]]
== References ==
<references/>
{{Other oxidoreductases}}
[[Category:EC 1.18.6]]
[[Category:Iron-sulfur proteins]]
[[Category:Nitrogen metabolism]]
[[cs:Nitrogenáza]]
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