MetaCyc Polypeptide: dinitrogenase β subunit

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.

Unification Links: Protein Model Portal:Q44486 , SMR:Q44486 , UniProt:Q44486

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

Gene-Reaction Schematic: ?

Gene-Reaction Schematic

Created 01-Apr-2011 by Weerasinghe D , SRI International

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 the dinitrogen molecule to ammonia, a process known as nitrogen fixation, is catalyzed by the nitrogenase enzyme system in diazotrophs. The enzyme complex consists of two oxygen-sensitive metalloprotein components [Richards94]. The dinitrogenase reductase component is a Fe protein containing a single [4Fe4S] center that transfers electrons from an external electron donor (a ferredoxin or a flavodoxin, depending on the species) to the other component, dinitrogenase [OrmeJohnson92].

Three such systems have been described, and they differ in their protein structures and metal content in the active site. Type I contains molybedenum and iron (MoFe), type II contains vanadium and iron (VFe) and type III contains only iron (FeFe). Different genes and gene products make up the different types [Eady96]. All dinitrogenases contain two unique cofactors, an Fe8S7 complex known as the P-cluster, and the metal (Mo, V or Fe) containing cofactor. All diazotrophic organisms sequenced to date encode a molybdenum-dependent nitrogenase, but some also have one or more alternative nitrogenases.

About this Enzyme

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.

Citations: [Tamagnini02]

Created 01-Apr-2011 by Weerasinghe D , SRI International

Enzymatic reaction of: nitrogenase

EC Number:

8 reduced ferredoxins + N2 + 16 ATP + 16 H2O <=> 8 oxidized ferredoxins + 2 ammonium + 16 ADP + 16 phosphate + H2 + 6 H+

The reaction direction shown, that is, A + B ↔ C + D versus C + D ↔ A + B, is in accordance with the direction in which it was curated.

The reaction is physiologically favored in the direction shown.

In Pathways: nitrogen fixation I (ferredoxin)

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: iron-molybdenum cofactor , an [8Fe-7S] cluster , Mg2+


Eady72: Eady RR, Smith BE, Cook KA, Postgate JR (1972). "Nitrogenase of Klebsiella pneumoniae. Purification and properties of the component proteins." Biochem J 1972;128(3);655-75. PMID: 4344006

Eady96: Eady RR (1996). "Structureminus signFunction Relationships of Alternative Nitrogenases." Chem Rev 96(7);3013-3030. PMID: 11848850

OrmeJohnson92: Orme-Johnson WH (1992). "Nitrogenase structure: where to now?." Science 257(5077);1639-40. PMID: 1529351

Rao02: Rao K. K, Cammack R (2002). "Hydrogen as a Fuel Producing hydrogen as a fuel." Learning from Nature.

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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Please cite the following article in publications resulting from the use of MetaCyc: Caspi et al, Nucleic Acids Research 42:D459-D471 2014
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