|PDB structures||RCSB PDB PDBe PDBsum|
Nitrogenases (EC 126.96.36.199EC 188.8.131.52) are enzymes used by some organisms to fix atmospheric nitrogen gas (N2). There is only one known family of enzymes that accomplishes this process. Dinitrogen is quite inert because of the strength of its N≡N triple bond.
Whilst the equilibrium formation of ammonia from molecular hydrogen and nitrogen has an overall negative enthalpy of reaction (), the energy barrier to activation is very high () without the assistance of catalysis.
In addition to reducing agents, such as dithionite in vitro, or ferredoxin or flavodoxin in vivo, the enzymatic reduction of dinitrogen to ammonia therefore also requires an input of chemical energy, released from the hydrolysis of ATP, to overcome the activation energy barrier. The enzyme is composed of the heterotetrameric MoFe protein that is transiently associated with the homodimeric Fe protein. Electrons for the reduction of nitrogen are supplied to nitrogenase when it associates with the reduced, nucleotide-bound homodimeric Fe protein. The heterocomplex undergoes cycles of association and disassociation to transfer one electron, which is the rate-limiting step in nitrogen reduction. ATP supplies the energy to drive the transfer of electrons from the Fe protein to the MoFe protein. The reduction potential of each electron transferred to the MoFe protein is sufficient to break one of dinitrogen's chemical bonds, though it has not yet been shown that exactly three cycles are sufficient to convert one molecule of N2 to ammonia. Nitrogenase ultimately bonds each atom of nitrogen to three hydrogen atoms to form ammonia (NH3), which is in turn bonded to glutamate to form glutamine. The nitrogenase reaction additionally produces molecular hydrogen as a side product.
The exact mechanism of catalysis is unknown due to the difficulty in obtaining crystals of nitrogen bound to nitrogenase. This is because the resting state of the MoFe protein does not bind nitrogen and also requires at least three electron transfers to perform catalysis. Nitrogenase is able to reduce acetylene, but is inhibited by carbon monoxide, which binds to the enzyme and thereby prevents binding of dinitrogen. Dinitrogen will prevent acetylene binding, but acetylene does not inhibit binding of dinitrogen and requires only one electron for reduction to ethylene.
All nitrogenases have an iron- and sulfur-containing cofactor that includes a heterometal complex in the active site (e.g., FeMoCo). In most, this heterometal complex has a central molybdenum atom, though in some species it is replaced by a vanadium  or iron atom.
Due to the oxidative properties of oxygen, most nitrogenases are irreversibly inhibited by dioxygen, which degradatively oxidizes the Fe-S cofactors. This requires mechanisms for nitrogen fixers to protect nitrogenase from oxygen in vivo. Despite this problem, many use oxygen as a terminal electron acceptor for respiration. One known exception is the nitrogenase of Streptomyces thermoautotrophicus, which is unaffected by the presence of oxygen. Although the ability of some nitrogen fixers such as Azotobacteraceae to employ an oxygen-labile nitrogenase under aerobic conditions has been attributed to a high metabolic rate, allowing oxygen reduction at the cell membrane, the effectiveness of such a mechanism has been questioned at oxygen concentrations above 70 µM (ambient concentration is 230 µM O2), as well as during additional nutrient limitations.
The reaction that this enzyme performs is:
- HC≡CH → H2C=CH2
- N–=N+=O → N2 + H2O
- N=N=N– → N2 + NH3
→ CH4, NH3, H3C–CH3, H2C=CH2 (CH3NH2)
- N≡C–R → RCH3 + NH3
- C≡N–R → CH4, H3C–CH3, H2C=CH2, C3H8, C3H6, RNH2
- O=C=S → CO + H2S 
- O=C=O → CO + H2O 
- S=C=N– → H2S + HCN 
- O=C=N– → H2O + HCN, CO + NH3 
Furthermore, dihydrogen functions as a competitive inhibitor, carbon monoxide functions as a non-competitive inhibitor, and carbon disulfide functions as a rapid-equilibrium inhibitor of nitrogenase.
Organisms that synthesize nitrogenase
- Free-living diazotrophs, e.g.
- Symbiotic diazotrophs, e.g.
Typically nitrogenases are coded by Nif genes.
Similarity to other proteins
The three subunits of nitrogenase exhibit significant sequence similarity to three subunits of the light-independent version of protochlorophyllide reductase that performs the conversion of protochlorophyllide to chlorophyll. This protein is present in gymnosperms, algae, and photosynthetic bacteria but has been lost by angiosperms during evolution.
Separately, two of the nitrogenase subunits (NifD and NifD) have homologues in methanogens that do not fix nitrogen e.g. Methanocaldococcus jannaschii. Little is understood about the function of these "class IV" nif genes, though they occur in many methanogens. In M. jannaschii they are known to interact with each other and are constitutively expressed.
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