|Gene:||nifK||Accession Number: G-12734 (MetaCyc)|
Synonyms: nitrogenase component I
Species: Anabaena variabilis
Component of: Mo-dependent nitrogenase complex (extended summary available)
This subunit is part of the nitrogenase complex that catalyzes the key enzymatic reactions in nitrogen fixation.
Relationship Links: Entrez-Nucleotide:Homolog:AAA64711.1 , InterPro:IN-FAMILY:IPR000318 , InterPro:IN-FAMILY:IPR000510 , InterPro:IN-FAMILY:IPR005976 , InterPro:IN-FAMILY:IPR024564 , Pfam:IN-FAMILY:PF00148 , Pfam:IN-FAMILY:PF11844 , Prosite:IN-FAMILY:PS00090 , Prosite:IN-FAMILY:PS00699
Subunit of: Mo-dependent nitrogenase complex
Species: Anabaena variabilis
Subunit composition of
Mo-dependent nitrogenase complex = [NifH]2[NifD]2[NifK]2
dinitrogenase reductase subunit = NifH (summary available)
dinitrogenase α subunit = NifD (summary available)
dinitrogenase β subunit = NifK (summary available)
The reduction of nitrogen to ammonia and the production of H2 is catalyzed by more than one nitrogenase enzyme system in diazotrophs. The enzyme complex consists of two oxygen-sensitive metalloproteins [Richards94]. The dinitrogenase reductase is a Fe protein containing a single [4Fe4S] center which acts as a specific ATP-dependent electron donor, transferring electrons from the external electron donor (a ferredoxin or a flavoprotein) to the second protein, dinitrogenase [OrmeJohnson92]. Dinitrogenase can be a [FeFe], [MoFe] or [VFe] protein, with two types of redox centers, two P clusters and two metal containing cofactor centers [Davis96].
About this Enzyme
The Mo-dependent nitrogenase complex is composed of two separate protein components, encoded by the nifDHK gene cluster [Zumft97]. Component I is known as dinitrogenase. It binds and reduces N2 and other substrates [Homer95]. The dinitrogenase is an α2-β2 heterotetramer of about 220 to 240 kDa, containing the Mo-Fe cofactor. It is encoded by the genes nifD and nifK.
Component II is the dinitrogenase reductase, a homodimer of about 60 to 70 kDa, encoded by nifH [Homer95]. It plays the specific role of transferring electrons from the external electron donor (a ferredoxin or a flavin) one at a time to dinitrogenase. Both components are necessary for nitrogenase activity [Burris91, Zumft75].
Anabaena variabilis expresses two Mo-dependent nitrogenases, the nif1 gene cluster encoded enzyme functions only in heterocysts, while the nif2 gene cluster encoded enzyme functions in both vegetative cells and heterocysts [Rao02]. The nif1 enzyme functions under both aerobic and anaerobic conditions while the nif2 enzyme is strictly anaerobic.
Enzymatic reaction of: nitrogenase
EC Number: 188.8.131.52
The reaction direction shown, that is, A + B ↔ C + D versus C + D ↔ A + B, is in accordance with the Enzyme Commission system.
The reaction is physiologically favored in the direction shown.
In Pathways: nitrogen fixation
It is composed of two proteins that can be separated but are both required for nitrogenase activity.
Dinitrogen reductase is a [4Fe-4S] protein, which for every two molecules of ATP, transfers an electron from the external electron donor ferredoxin, to dinitrogenase.
Dinitrogenase is a molybdenum-iron protein that breaks apart the atoms of nitrogen. It reduces dinitrogen in three successive two-electron reductions from nitrogen to diimine to hydrazine to two molecules of ammonia. The reduction is initiated by formation of hydrogen in stoichiometric amounts. The complex can also reduce acetylene to ethylene and very slowly to ethane, azide to nitrogen and ammonia, and cyanide to methane and ammonia. Ferredoxin may be replaced by flavodoxin.
The molybdenum in the enzyme can be replaced by vanadium or iron.
Cofactors or Prosthetic Groups: Mg2+
Davis96: Davis R, Lehman L, Petrovich R, Shah VK, Roberts GP, Ludden PW (1996). "Purification and characterization of the alternative nitrogenase from the photosynthetic bacterium Rhodospirillum rubrum." J Bacteriol 178(5);1445-50. PMID: 8631723
Homer95: Homer MJ, Dean DR, Roberts GP (1995). "Characterization of the gamma protein and its involvement in the metallocluster assembly and maturation of dinitrogenase from Azotobacter vinelandii." J Biol Chem 270(42);24745-52. PMID: 7559591
Richards94: Richards AJ, Lowe DJ, Richards RL, Thomson AJ, Smith BE (1994). "Electron-paramagnetic-resonance and magnetic-circular-dichroism studies of the binding of cyanide and thiols to the thiols to the iron-molybdenum cofactor from Klebsiella pneumoniae nitrogenase." Biochem J 297 ( Pt 2);373-8. PMID: 8297344
Tamagnini02: Tamagnini P, Axelsson R, Lindberg P, Oxelfelt F, Wunschiers R, Lindblad P (2002). "Hydrogenases and hydrogen metabolism of cyanobacteria." Microbiol Mol Biol Rev 66(1);1-20, table of contents. PMID: 11875125
Vandecasteele70: Vandecasteele JP, Burris RH (1970). "Purification and properties of the constituents of the nitrogenase complex from Clostridium pasteurianum." J Bacteriol 1970;101(3);794-801. PMID: 5438048
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