Editing Nitrogen fixation (section)

=== Nitrogenase ===
{{Main|Nitrogenase}}
The protein complex nitrogenase is responsible for [[Catalysis|catalyzing]] the reduction of nitrogen gas (N<sub>2</sub>) to ammonia (NH<sub>3</sub>).<ref>{{cite journal |doi=10.1021/acs.chemrev.9b00556 |title=Reduction of Substrates by Nitrogenases |date=2020 |last1=Seefeldt |first1=Lance C. |last2=Yang |first2=Zhi-Yong |last3=Lukoyanov |first3=Dmitriy A. |last4=Harris |first4=Derek F. |last5=Dean |first5=Dennis R. |last6=Raugei |first6=Simone |last7=Hoffman |first7=Brian M. |journal=Chemical Reviews |volume=120 |issue=12 |pages=5082–5106 |pmid=32176472 |pmc=7703680 }}</ref><ref>{{cite journal |doi=10.1002/1873-3468.14534 |title=Biological nitrogen fixation in theory, practice, and reality: A perspective on the molybdenum nitrogenase system |date=2023 |last1=Threatt |first1=Stephanie D. |last2=Rees |first2=Douglas C. |journal=FEBS Letters |volume=597 |issue=1 |pages=45–58 |pmid=36344435 |pmc=10100503 }}</ref> In [[cyanobacteria]], this [[enzyme]] system is housed in a specialized cell called the [[heterocyst]].<ref>{{cite journal | vauthors = Peterson RB, Wolk CP | title = High recovery of nitrogenase activity and of Fe-labeled nitrogenase in heterocysts isolated from Anabaena variabilis | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 75 | issue = 12 | pages = 6271–6275 | date = December 1978 | pmid = 16592599 | pmc = 393163 | doi = 10.1073/pnas.75.12.6271 | doi-access = free | bibcode = 1978PNAS...75.6271P }}</ref> The production of the [[nitrogenase]] complex is genetically regulated, and the activity of the protein complex is dependent on ambient oxygen concentrations, and intra- and extracellular concentrations of ammonia and oxidized nitrogen species (nitrate and nitrite).<ref>{{cite journal | vauthors = Beversdorf LJ, Miller TR, McMahon KD | title = The role of nitrogen fixation in cyanobacterial bloom toxicity in a temperate, eutrophic lake | journal = PLOS ONE | volume = 8 | issue = 2 | pages = e56103 | date = 2013-02-06 | pmid = 23405255 | pmc = 3566065 | doi = 10.1371/journal.pone.0056103 | doi-access = free | bibcode = 2013PLoSO...856103B }}</ref><ref>{{Cite journal| vauthors = Gallon JR |date=2001-03-01|title=N2 fixation in phototrophs: adaptation to a specialized way of life |journal=Plant and Soil|language=en|volume=230|issue=1|pages=39–48 |doi=10.1023/A:1004640219659|bibcode=2001PlSoi.230...39G |s2cid=22893775|issn=1573-5036}}</ref><ref>{{cite journal | vauthors = Paerl H | title = The cyanobacterial nitrogen fixation paradox in natural waters | journal = F1000Research | volume = 6 | pages = 244 | date = 2017-03-09 | pmid = 28357051 | pmc = 5345769 | doi = 10.12688/f1000research.10603.1 | doi-access = free }}</ref> Additionally, the combined concentrations of both ammonium and nitrate are thought to inhibit N<sub>Fix</sub>, specifically when intracellular concentrations of 2-[[Ketoglutaric acid|oxoglutarate]] (2-OG) exceed a critical threshold.<ref>{{cite journal | vauthors = Li JH, Laurent S, Konde V, Bédu S, Zhang CC | title = An increase in the level of 2-oxoglutarate promotes heterocyst development in the cyanobacterium Anabaena sp. strain PCC 7120 | journal = Microbiology | volume = 149 | issue = Pt 11 | pages = 3257–3263 | date = November 2003 | pmid = 14600238 | doi = 10.1099/mic.0.26462-0 | doi-access = free }}</ref> The specialized heterocyst cell is necessary for the performance of nitrogenase as a result of its sensitivity to ambient oxygen.<ref>{{cite book | vauthors = Wolk CP, Ernst A, Elhai J | chapter = Heterocyst Metabolism and Development|date=1994 | title = The Molecular Biology of Cyanobacteria |pages=769–823| veditors = Bryant DA |series=Advances in Photosynthesis|place=Dordrecht|publisher=Springer Netherlands|language=en|doi=10.1007/978-94-011-0227-8_27|isbn=978-94-011-0227-8 }}</ref>

Nitrogenase consist of two proteins, a catalytic iron-dependent protein, commonly referred to as MoFe protein and a reducing iron-only protein (Fe protein). Three iron-dependent proteins are known: [[molybdenum]]-dependent, [[vanadium]]-dependent, and [[iron]]-only, with all three nitrogenase protein variations containing an iron protein component. Molybdenum-dependent nitrogenase is most common.<ref name=Rees/> The different types of nitrogenase can be determined by the specific iron protein component.<ref>{{cite book | vauthors = Schneider K, Müller A | title = Catalysts for Nitrogen Fixation| chapter = Iron-Only Nitrogenase: Exceptional Catalytic, Structural and Spectroscopic Features|date=2004 |pages=281–307| veditors = Smith BE, Richards RL, Newton WE |series=Nitrogen Fixation: Origins, Applications, and Research Progress|place=Dordrecht|publisher=Springer Netherlands|language=en|doi=10.1007/978-1-4020-3611-8_11|isbn=978-1-4020-3611-8 }}</ref> Nitrogenase is highly conserved. [[Gene expression]] through [[DNA sequencing]] can distinguish which protein complex is present in the microorganism and potentially being expressed. Most frequently, the [[Nif gene|''nif''H gene]] is used to identify the presence of molybdenum-dependent nitrogenase, followed by closely related nitrogenase reductases (component II) ''vnf''H and ''anf''H representing vanadium-dependent and iron-only nitrogenase, respectively.<ref>{{Cite journal| vauthors = Knoche KL, Aoyama E, Hasan K, Minteer SD |date=2017|title=Role of Nitrogenase and Ferredoxin in the Mechanism of Bioelectrocatalytic Nitrogen Fixation by the Cyanobacteria Anabaena variabilis SA-1 Mutant Immobilized on Indium Tin Oxide (ITO) Electrodes|url=https://www.cheric.org/research/tech/periodicals/view.php?seq=1531452|journal=Electrochimica Acta|language=ko|volume=232|pages=396–403|doi=10.1016/j.electacta.2017.02.148}}</ref> In studying the ecology and evolution of [[Diazotroph|nitrogen-fixing bacteria]], the ''nifH'' gene is the [[biomarker]] most widely used.<ref>{{cite journal | vauthors = Raymond J, Siefert JL, Staples CR, Blankenship RE | title = The natural history of nitrogen fixation | journal = Molecular Biology and Evolution | volume = 21 | issue = 3 | pages = 541–554 | date = March 2004 | pmid = 14694078 | doi = 10.1093/molbev/msh047 | doi-access = free | author4-link = Robert E. Blankenship }}</ref> ''nif''H has two similar genes ''anf''H and vnfH that also encode for the nitrogenase reductase component of the nitrogenase complex.<ref>{{cite journal | vauthors = Schüddekopf K, Hennecke S, Liese U, Kutsche M, Klipp W | title = Characterization of anf genes specific for the alternative nitrogenase and identification of nif genes required for both nitrogenases in Rhodobacter capsulatus | journal = Molecular Microbiology | volume = 8 | issue = 4 | pages = 673–684 | date = May 1993 | pmid = 8332060 | doi = 10.1111/j.1365-2958.1993.tb01611.x | s2cid = 42057860 }}</ref>