Nitrogenase

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Nitrogenase
Identifier
Gene name (s) nif, anf, vnf
Enzyme classification
EC, category 1.18.6.1 ,  oxidoreductase
Substrate 8 Ferredoxin red. + 8 H + + N 2 + 16 ATP + 16 H 2 O
Products 8 ferredoxin ox. + H 2 + 2 NH 3 + 16 ADP + 16 phosphate
Occurrence
Parent taxon Bacteria, archaea

Nitrogenase is an enzyme complex that is able to reduce elemental, molecular nitrogen (N 2 ) and thus convert it into a biologically available form. This process is known as nitrogen fixation . Nitrogenases are present in various bacteria and some archaea . Nitrogen-fixing actinomycetes are known as well as cyanobacteria (for example Anabaena ) and proteobacteria (for example Azotobacter ).

Structure and properties

FeMoCo: iron-molybdenum cofactor in dinitrogenase.

The enzyme complex consists of two enzymatically active proteins, the dinitrogenase (hetero tetramer α 2 β 2 ) and the dinitrogenase reductase (homo dimer ). The reaction takes place in the dinitrogenase, the reductase transfers the electrons it receives from ferredoxin by means of a [4Fe-4S] iron-sulfur cluster . The active center of dinitrogenase consists of another [8Fe-7S] iron-sulfur cluster and the iron-sulfur-molybdenum cofactor (FeMoCo). For a long time the identity of the central atom of the FeMoCo factor was unclear. Both a carbon, an oxygen, and a nitrogen atom were plausible candidates. Two independent X-ray emission spectroscopy studies in 2011 showed that the interstitial atom in the center of the cluster is a carbon atom. The strength of the central carbon was also determined in 2017 using more recent computer simulations. In the case of a molybdenum deficiency, some bacteria are able to produce alternative cofactors that contain vanadium or only iron instead of molybdenum . The nitrogenase of most bacteria, like all enzymes with iron-sulfur clusters, is extremely sensitive to oxygen. To protect the enzyme against oxygen, bacteria have developed various adaptations, such as thick mucous capsules or particularly thick-walled cells. Bacteria that carry out oxygenic photosynthesis separate nitrogen-fixing cells ( heterocysts ) spatially from oxygen-releasing cells or they only assimilate nitrogen at night when the light reaction of photosynthesis is idle. Some bacteria can only fix nitrogen in symbiosis, for example rhizobia (nodule bacteria) that live with plants (often legumes ). Since the plants themselves are not able to fix elementary, molecular nitrogen, they are dependent on the product of the bacterial nitrogenase. The ammonia formed is the starting material for the formation of glutamic acid and glutamine .

Catalyzed reaction

Since N 2 is a very stable and inert molecule (the binding energy is 945 kJ / mol), a large amount of energy is required to break the triple bond between the two nitrogen atoms . The energy required for this is provided by the energy carrier ATP . Ammonia is the product of this reaction .

The sum equation of the reaction catalyzed by nitrogenase is:

The activity of nitrogenase is not limited to nitrogen; the enzyme also reduces other triple bonds, for example those of ethyne , cyanide , azides or nitrogen oxides . Under natural conditions, this property is probably of no importance. However, the reduction of ethyne to ethene is used to experimentally detect nitrogenase.

regulation

The entire process of biological nitrogen fixation is relatively complex and requires the interaction of several enzymes, of which nitrogenase is the most important. The genes of these enzymes are subject to strict regulation. Your transcription is switched off , for example, by oxygen and nitrogen compounds such as nitrate and some amino acids . Switching off with nitrogen compounds is advantageous because the use of nitrogen from these sources consumes much less energy.

In Klebsiella pneumoniae, nitrogenase activity is regulated on the transcriptional level via the nifLA operon. The protein NifL is a sensor for O 2 , NifA is, among other things, a transcriptional activator for the nifHDKY operon, which encodes the structural elements of nitrogenase. If the bacterium is exposed to oxygen, NifL and NifA form a heterodimer. As a result, NifA cannot perform the activator function and nitrogenase is not expressed. This effect is meaningful, since nitrogenase would be inhibited by O 2 and expression in the presence of O 2 would be a pure waste of energy. The expression of the genes nifL and nifA itself is started by NtrC. This factor signals the need to fix nitrogen.

See also

Individual evidence

  1. ↑ T. Spatzal, M. Aksoyoglu, L. Zhang, SLA Andrade, E. Schleicher, S. Weber, DC Rees, O. Einsle: Evidence for Interstitial Carbon in Nitrogenase FeMo Cofactor. In: Science. 334, 6058, p. 940, doi: 10.1126 / science.1206445 .
  2. ↑ KM Lancaster, M. Roemelt, P. Ettenhuber, Y. Hu, MW Ribbe, F. Neese, U. Bergmann, S. DeBeer: X-ray Emission Spectroscopy Evidences a Central Carbon in the Nitrogenase Iron-Molybdenum Cofactor. In: Science. 334, 2011, pp. 974-977, doi: 10.1126 / science.1206445 .
  3. ↑ J. Grunenberg: The interstitially bound carbon of nitrogenase is significantly more stable than previously assumed. In: Angewandte Chemie. 2017, 129, pp. 7394-7397, doi: 10.1002 / anie.201701790 .
  4. ↑ Swiss Institute of Bioinformatics (SIB): PROSITE documentation PDOC00085. Retrieved September 20, 2011 .
  5. ↑ Swiss Institute of Bioinformatics (SIB): PROSITE documentation PDOC00580. Retrieved September 20, 2011 .
  6. ↑ RN Pau, ME Eldridge, DJ Lowe, LA Mitchenall, RR Eady: Molybdenum-independent nitrogenases of Azotobacter vinelandii: a functional species of alternative nitrogenase-3 isolated from a molybdenum-tolerant strain contains an iron-molybdenum cofactor . In: Biochem. J. . 293 (Pt 1), July 1993, pp. 101-7. PMID 8392330 . PMC 1134325 (free full text).
  7. ↑ AN Glazer, KJ Kechris: Conserved amino acid sequence features in the alpha subunits of MoFe, VFe, and FeFe nitrogenases . In: PLoS ONE . 4, No. 7, 2009, p. E6136. doi : 10.1371 / journal.pone.0006136 . PMID 19578539 . PMC 2700964 (free full text).
  8. ↑ The use of cloned nif (nitrogen fixation) DNA to investigate transcriptional regulation of nif expression in Klebsiella pneumoniae
  9. ↑ Positive control and autogenous regulation of the promoter nifLA in Klebsiella pneumoniae