Nitrogenase (EC 1.18.6.1) is the enzyme used by organisms to fix atmospheric nitrogen gas (N₂). Dinitrogen is relatively inert due to its triple bond. The enzyme therefore requires a lot of chemical energy in the form of ATP and reducing agents, such as NADH. The product of the reaction is ammonia, which is normally incorporated into glutamate to form glutamine. The enzyme is composed of a dimer of "MoFe" associated with a dimers of "Fe" each. Nitrogenase is supplied reducing power when it associates with the reduced, nucleotide-bound Fe homodimer. The heterocomplex undergoes cycles of association and disassociation to transfer one electron, which is the limiting step in the process. The process itself is very energy-intensive, requiring electron donors and ATP to provide reducing power. The exact mechanism of catalysis is unknown due to the difficulty in obtaining crystals of nitrogenase bound to nitrogen. This is because the resting state of MoFe 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 substrates and inhibitors, 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^[1]. The Enzyme Commission number (EC number) is a numerical classification scheme for enzymes, based on the chemical reactions they catalyze. ... Ribbon diagram of the enzyme TIM. TIM is catalytically perfect, meaning its conversion rate is limited, or nearly limited to its substrate diffusion rate. ... General Name, Symbol, Number nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15, 2, p Appearance colorless Atomic mass 14. ... Dinitrogen, N2, is the most common form of the element nitrogen in normal conditions. ... Adenosine 5-triphosphate (ATP) is a multifunctional nucleotide primarily known in biochemistry as the molecular currency of intracellular energy transfer. ... Nicotinamide adenine dinucleotide (NAD+) Nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) are two important coenzymes found in cells. ... Ammonia is a compound of nitrogen and hydrogen with the formula NH3. ... Glutamate is the anion of glutamic acid. ... Glutamine is one of the 20 amino acids encoded by the standard genetic code. ... Ribbon diagram of the enzyme TIM. TIM is catalytically perfect, meaning its conversion rate is limited, or nearly limited to its substrate diffusion rate. ... Sucrose, or common table sugar, is composed of glucose and fructose. ... Generic graph showing the effect of a catalyst in an hypotetical exothermic chemical reaction. ... In biochemistry, a substrate is a molecule which is acted upon by an enzyme. ... An inhibitor is a type of effector (biology) that decreases or prevents a chemical reaction. ...

All nitrogenases have an iron- and sulfur-containing cofactor that includes a metal atom assumed to be the active site. In most, this is molybdenum, though in some species it is replaced by vanadium or iron. General Name, Symbol, Number molybdenum, Mo, 42 Chemical series transition metals Group, Period, Block 6, 5, d Appearance gray metallic Atomic mass 95. ... General Name, Symbol, Number vanadium, V, 23 Chemical series transition metals Group, Period, Block 5, 4, d Appearance gray-white metal Atomic mass 50. ... General Name, Symbol, Number iron, Fe, 26 Chemical series transition metals Group, Period, Block 8, 4, d Appearance lustrous metallic with a grayish tinge Atomic mass 55. ...

Due to the similarity between oxygen and nitrogen gas, most nitrogenases are irreversibly inhibited by dioxygen in vitro. 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. 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 uM (ambient concentration is 230uM O2), as well as during additional nutrient limitations^[2]. General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless Atomic mass 15. ... Dioxygen, O2, is the most common form of the element oxygen in normal conditions. ... Wiktionary has a definition of: In vitro In vitro (Latin: within glass) means within a test tube, or, more generally, outside a living organism or cell. ... In vivo (Latin for (with)in the living). ... Streptomyces is a genus of Actinobacteria. ... Genera Azomonas Azotobacter The family Azotobacteraceae contains aerobic diazotrophs with two Genera, Azomonas and Azotobacter, distinguished by the ability to form cysts. ...

The reaction that this enzyme performs is:

N₂ + 8H⁺ + 8e^- + 16 ATP → 2NH₃ + H₂ + 16ADP + 16 Pi

Organisms that synthesise nitrogenase

• Diazotrophs
• Heterocysts
• Azotobacteraceae
• Rhizobia

Diazotrophs are microorganisms that fix atmospheric nitrogen gas in to a more usable form such as ammonia. ... Heterocysts are specialized nitrogen-fixing cells formed by some filamentous cyanobacteria, such as Nostoc punctiforme and Anabaena sperica, during nitrogen starvation. ... Genera Azomonas Azotobacter The family Azotobacteraceae contains aerobic diazotrophs with two Genera, Azomonas and Azotobacter, distinguished by the ability to form cysts. ... Rhizobia (from the Greek words Riza = Root and Bios = Life) are soil bacteria that fix nitrogen (diazotrophy) after becoming established inside root nodules of legumes (Fabaceae). ...

See also

• Nitrogen fixation

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

References

1. ^ Seefeldt LC, Dance IG, Dean DR. 2004. Substrate interactions with nitrogenase: Fe versus Mo. Biochemistry. 43(6):1401-9.
2. ^ 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.