Nitrogenase

nitrogenase
Identifiers
EC number	1.18.6.1
CAS number	9013-04-1
Databases
IntEnz	IntEnz view
BRENDA	BRENDA entry
ExPASy	NiceZyme view
KEGG	KEGG entry
MetaCyc	metabolic pathway
PRIAM	profile
PDB structures	RCSB PDB PDBe PDBsum
Search PMC articles PubMed articles

Nitrogenases (EC 1.18.6.1EC 1.19.6.1) are enzymes used by some organisms to fix atmospheric nitrogen gas (N₂). It is the only known family of enzymes that accomplish this process. Dinitrogen is quite inert because of the strength of its N-N triple bond. To break one nitrogen atom away from another requires breaking all three of these chemical bonds.

Whilst the equilibrium formation of ammonia from molecular hydrogen and nitrogen has an overall negative enthalpy of reaction (ΔH⁰ = -45.2 kJ mol⁻¹ NH₃), the energy barrier to activation is very high (E_A = 420 kJ mol⁻¹) without the assistance of catalysis.^[1]

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 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.^[2]

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 ^[3] 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 [1]. 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.^[4]

The reaction that this enzyme performs is:

N₂ + 8 H⁺ + 8 e⁻ + 16 ATP → 2 NH₃ + H₂ + 16 ADP + 16 P_i

Nonspecific Reactions

In addition to performing the reaction N≡N → 2 NH₃, nitrogenase is also capable of catalyzing the following reactions:^[5][6]

HC≡CH → H₂C=CH₂

N≡N–O → N₂ + H₂O

N=N=N^– → N₂ + NH₃

C≡N⁻ → 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 ^[7]

O=C=O → CO + H₂O ^[7]

S=C=N^– → H₂S + HCN ^[8]

S=C=O → H₂S + CO ^[8]

O=C=N^– → H₂O + HCN, CO + NH₃ ^[8]

Furthermore, dihydrogen functions as a competitive inhibitor,^[9] carbon monoxide functions as a non-competitive inhibitor,^[5][6] and carbon disulfide functions as a rapid-equilibrium inhibitor^[7] of nitrogenase.

Vanadium nitrogenases have also been shown to catalzye the conversion of CO into alkanes through a reaction comparable to Fischer-Tropsch synthesis.

Organisms that synthesize nitrogenase

• Free-living diazotrophs, e.g.
  • Cyanobacteria (by means of differentiated heterocysts)
  • Azotobacteraceae
• Symbiotic diazotrophs, e.g.
  • Rhizobia
  • Frankia

Similarity to protochlorophyllide reductase

The light-independent version of protochlorophyllide reductase that performs the conversion of protochlorophyllide to chlorophyll also consists of three subunits that exhibit significant sequence similarity to the three subunits of nitrogenase. This protein is present in gymnosperms, algae, and photosynthetic bacteria but has been lost by angiosperms during evolution [2].

See also

• Nitrogen fixation

References

• Zumft WG, Mortenson LE (1975). "The nitrogen-fixing complex of bacteria". Biochim. Biophys. Acta. 416 (1): 1–52. PMID 164247. 

1. ^ Modak, J. M., 2002, Haber Process for Ammonia Synthesis, Resonance. 7, 69-77.
2. ^ Seefeldt LC, Dance IG, Dean DR. 2004. Substrate interactions with nitrogenase: Fe versus Mo. Biochemistry. 43(6):1401-9.
3. ^ Robson, R. L.; Eady, R. R.; Richardson, T. H.; Miller, R. W.; Hawkins, M.; Postgate, J. R., 1986, The alternative nitrogenase of Azotobacter chroococcum is a vanadium enzyme, Nature (London). 322, 388-390.
4. ^ 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.
5. ^ ^{a b} Rivera-Ortiz, José M., and Burris, Robert H. (1975). "Interactions among substrates and inhibitors of nitrogenase". J Bacteriol 123 (2): 537–545. PMC 235759. PMID 1150625. http://jb.asm.org/cgi/content/abstract/123/2/537. 
6. ^ ^{a b} G. N. Schrauzer (2003). "Nonenzymatic Simulation of Nitrogenase Reactions and the Mechanism of Biological Nitrogen Fixation". Angewandte Chemie International Edition in English 14 (8): 514–522. doi:10.1002/anie.197505141. PMID 810048. http://www3.interscience.wiley.com/journal/106580477/abstract. 
7. ^ ^{a b c} Lance C. Seefeldt, Madeline E. Rasche, Scott A. Ensign (1995). "Carbonyl sulfide and carbon dioxide as new substrates, and carbon disulfide as a new inhibitor, of nitrogenase". Biochemistry 34 (16): 5382–5389. doi:10.1021/bi00016a009. PMID 7727396. http://pubs.acs.org/doi/abs/10.1021/bi00016a009. 
8. ^ ^{a b c} Madeline E. Rasche and Lance C. Seefeldt (1997). "Reduction of Thiocyanate, Cyanate, and Carbon Disulfide by Nitrogenase: Kinetic Characterization and EPR Spectroscopic Analysis". Biochemistry 36 (28): 8574–8585. doi:10.1021/bi970217e. PMID 9214303. http://pubs.acs.org/doi/abs/10.1021/bi970217e. 
9. ^ Joseph H. Guth, Robert H. Burris (1983). "Inhibition of nitrogenase-catalyzed ammonia formation by hydrogen". Biochemistry 22 (22): 5111–5122. doi:10.1021/bi00291a010. PMID 6360203. http://pubs.acs.org/doi/abs/10.1021/bi00291a010.