Editing Methanogen (section)

=== Methane production ===
Methanogens are known to produce methane from substrates such as H<sub>2</sub>/CO<sub>2</sub>, acetate, [[Formatotrophs|formate]], methanol and methylamines in a process called [[methanogenesis]].<ref name=":6">{{Cite journal|last=Blaut|first=M.|date=1994|title=Metabolism of methanogens|journal=Antonie van Leeuwenhoek|volume=66|issue=1–3|pages=187–208|issn=0003-6072|pmid=7747931|doi=10.1007/bf00871639|s2cid=23706408}}</ref> Different methanogenic reactions are catalyzed by unique sets of [[enzyme]]s and [[coenzymes]]. While reaction mechanism and energetics vary between one reaction and another, all of these reactions contribute to net positive energy production by creating ion [[concentration gradient]]s that are used to drive ATP synthesis.<ref>{{Cite journal|last1=Dybas|first1=M|last2=Konisky|first2=J|date=1992|title=Energy transduction in the methanogen Methanococcus voltae is based on a sodium current.|journal=J Bacteriol|volume=174 | issue = 17 |pages=5575–5583|doi=10.1128/jb.174.17.5575-5583.1992|pmid=1324904|pmc=206501}}</ref> The overall reaction for H<sub>2</sub>/CO<sub>2</sub> methanogenesis is:

:<chem>CO2 + 4 H2 -> CH4 + 2 H2O</chem> (∆G˚' = -134 kJ/mol CH<sub>4</sub>)

Well-studied organisms that produce methane via H<sub>2</sub>/CO<sub>2</sub> methanogenesis include ''Methanosarcina barkeri'', ''Methanobacterium thermoautotrophicum'', and ''Methanobacterium wolfei''.<ref>{{Cite journal|last1=Karrasch|first1=M.|last2=Börner|first2=G.|last3=Enssle|first3=M.|last4=Thauer|first4=R. K.|date=1990-12-12|title=The molybdoenzyme formylmethanofuran dehydrogenase from Methanosarcina barkeri contains a pterin cofactor|journal=European Journal of Biochemistry|volume=194|issue=2|pages=367–372|issn=0014-2956|pmid=2125267|doi=10.1111/j.1432-1033.1990.tb15627.x}}</ref><ref>{{Cite journal|last1=Börner|first1=G.|last2=Karrasch|first2=M.|last3=Thauer|first3=R. K.|date=1991-09-23|title=Molybdopterin adenine dinucleotide and molybdopterin hypoxanthine dinucleotide in formylmethanofuran dehydrogenase from Methanobacterium thermoautotrophicum (Marburg)|journal=FEBS Letters|volume=290|issue=1–2|pages=31–34|issn=0014-5793|pmid=1915887|doi=10.1016/0014-5793(91)81218-w|s2cid=24174561|doi-access=free|bibcode=1991FEBSL.290...31B }}</ref><ref>{{Cite journal|last1=Schmitz|first1=Ruth A.|last2=Albracht|first2=Simon P. J.|last3=Thauer|first3=Rudolf K.|date=1992-11-01|title=A molybdenum and a tungsten isoenzyme of formylmethanofuran dehydrogenase in the thermophilic archaeon Methanobacterium wolfei|journal=European Journal of Biochemistry|language=en|volume=209|issue=3|pages=1013–1018|doi=10.1111/j.1432-1033.1992.tb17376.x|pmid=1330558|issn=1432-1033|doi-access=free}}</ref> These organisms are typically found in anaerobic environments.<ref name=":6" />

In the earliest stage of H<sub>2</sub>/CO<sub>2</sub> methanogenesis, CO<sub>2</sub> binds to [[methanofuran]] (MF) and is reduced to formyl-MF. This [[endergonic]] reductive process (∆G˚'= +16 kJ/mol) is dependent on the availability of H<sub>2</sub> and is catalyzed by the enzyme formyl-MF dehydrogenase.<ref name=":6" />

:<chem>CO2 + H2 + MF -> HCO-MF + H2O</chem>

The formyl constituent of formyl-MF is then transferred to the coenzyme [[tetrahydromethanopterin]] (H4MPT) and is catalyzed by a soluble enzyme known as [[formyltransferase]]. This results in the formation of formyl-H4MPT.<ref name=":6" />

:<chem>HCO-MF + H4MPT -> HCO-H4MPT + MF</chem>

Formyl-H4MPT is subsequently reduced to methenyl-H4MPT. Methenyl-H4MPT then undergoes a one-step hydrolysis followed by a two-step reduction to methyl-H4MPT. The two-step reversible reduction is assisted by [[Coenzyme F420|coenzyme F<sub>420</sub>]] whose hydride acceptor spontaneously oxidizes.<ref name=":6" /> Once oxidized, F<sub>420</sub>'s electron supply is replenished by accepting electrons from H<sub>2</sub>. This step is catalyzed by methylene H4MPT dehydrogenase.<ref>{{Cite journal|last=Zirngibl|first=C|date=February 1990|title=N5,N10-Methylenetetrahydromethanopterin dehydrogenase from Methanobacterium thermoautotrophicum has hydrogenase activity|journal=Laboratorium Fir Mikrobiologie|volume=261 | issue = 1 |pages=112–116|doi=10.1016/0014-5793(90)80649-4|doi-access=free|bibcode=1990FEBSL.261..112Z}}</ref>

:<chem>HCO-H4MPT + H+ -> CH-H4MPT+ + H2O</chem> (Formyl-H4MPT reduction)

:<chem>CH-H4MPT+ + F420H2 -> CH2=H4MPT + F420 + H+</chem>(Methenyl-H4MPT hydrolysis)

:<chem>CH2=H4MPT + H2 -> CH3-H4MPT + H+</chem>(H4MPT reduction)

Next, the methyl group of methyl-M4MPT is transferred to coenzyme M via a methyltransferase-catalyzed reaction.<ref>{{Cite journal|last1=te Brömmelstroet|first1=B. W.|last2=Geerts|first2=W. J.|last3=Keltjens|first3=J. T.|last4=van der Drift|first4=C.|last5=Vogels|first5=G. D.|date=1991-09-20|title=Purification and properties of 5,10-methylenetetrahydromethanopterin dehydrogenase and 5,10-methylenetetrahydromethanopterin reductase, two coenzyme F420-dependent enzymes, from Methanosarcina barkeri|journal=Biochimica et Biophysica Acta (BBA) - Protein Structure and Molecular Enzymology|volume=1079|issue=3|pages=293–302|issn=0006-3002|pmid=1911853|doi=10.1016/0167-4838(91)90072-8}}</ref><ref>{{Cite journal|last1=Kengen|first1=Servé W. M.|last2=Mosterd|first2=Judith J.|last3=Nelissen|first3=Rob L. H.|last4=Keltjens|first4=Jan T.|last5=Drift|first5=Chris van der|last6=Vogels|first6=Godfried D.|date=1988-08-01|title=Reductive activation of the methyl-tetrahydromethanopterin: coenzyme M methyltransferase from Methanobacterium thermoautotrophicum strain ΔH|journal=Archives of Microbiology|language=en|volume=150|issue=4|pages=405–412|doi=10.1007/BF00408315|bibcode=1988ArMic.150..405K |s2cid=36366503|issn=0302-8933}}</ref>

:<chem>CH3-H4MPT + HS-CoM -> CH3-S-CoM + H4MPT</chem>

The final step of H<sub>2</sub>/CO<sub>2</sub> methanogenesis involves [[methyl-coenzyme M reductase]] and two coenzymes: N-7 mercaptoheptanoylthreonine phosphate (HS-HTP) and coenzyme [[Cofactor F430|F<sub>430</sub>]]. HS-HTP donates electrons to methyl-coenzyme M allowing the formation of methane and mixed disulfide of HS-CoM.<ref>{{Cite journal|last1=Bobik|first1=T. A.|last2=Olson|first2=K. D.|last3=Noll|first3=K. M.|last4=Wolfe|first4=R. S.|date=1987-12-16|title=Evidence that the heterodisulfide of coenzyme M and 7-mercaptoheptanoylthreonine phosphate is a product of the methylreductase reaction in Methanobacterium|journal=Biochemical and Biophysical Research Communications|volume=149|issue=2|pages=455–460|issn=0006-291X|pmid=3122735|doi=10.1016/0006-291x(87)90389-5}}</ref> F<sub>430</sub>, on the other hand, serves as a prosthetic group to the reductase. H<sub>2</sub> donates electrons to the mixed disulfide of HS-CoM and regenerates coenzyme M.<ref>{{Cite journal|last1=Ellermann|first1=J.|last2=Hedderich|first2=R.|last3=Böcher|first3=R.|last4=Thauer|first4=R. K.|date=1988-03-15|title=The final step in methane formation. Investigations with highly purified methyl-CoM reductase (component C) from Methanobacterium thermoautotrophicum (strain Marburg)|journal=European Journal of Biochemistry|volume=172|issue=3|pages=669–677|issn=0014-2956|pmid=3350018|doi=10.1111/j.1432-1033.1988.tb13941.x|doi-access=free}}</ref>

:<chem>CH3-S-CoM + HS-HTP -> CH4 + CoM-S-S-HTP</chem> (Formation of methane)

:<chem>CoM-S-S-HTP + H2 -> HS-CoM + HS-HTP</chem> (Regeneration of coenzyme M)
: