This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Generation of Precursor Metabolites and Energy → Respiration → Anaerobic Respiration → Methanogenesis|
Expected Taxonomic Range: Euryarchaeota
Methanogenesis, the biological production of methane, is an anaerobic respiration process carried out by the methanogens, a group of microorganisms belonging to the Archaea domain. These organisms account for most of the biogenic methane production, which is estimated at 5x1014 g of methane per year.
Methanogenic pathways utilize a small group of fermentation products formed by other anaerobes as electron donors and acceptors. Typical electron donors are H2, formate, and one of several alcohols, such as ethanol, isopropanol, 2-butanol, or 3-methylbutanol. Typical electron acceptors are C-1 substrates (carbon-containing compounds that lack carbon-carbon bonds) such as CO2, methanol, and several different methylamines and methylsulfides. Acetate is a special methanogenic substrate, as it serves as both the electron donor and the electron acceptor after being split into two parts (see methanogenesis from acetate). Most of the methane in nature originates from acetate.
There are three main types of methanogenic pathways: the pathway from H2 and CO2 (see methanogenesis from H2 and CO2), the aceticlastic pathway from acetate (see methanogenesis from acetate), and methanogenesis from methylated C1 compounds. All pathways involve the transfer of a methyl group from the terminal acceptor to coenzyme M, forming methyl-CoM, which is then disproportionated into methane and CO2: One in four methyl-CoM molecules is oxidized to CO2 (see methyl-coenzyme M oxidation to CO2), providing the six electrons that are required for the reduction of three methyl-CoM molecules to methane (see methyl-coenzyme M reduction to methane) [Keltjens93, Pritchett05].
Methanogenic pathways do not afford substrate level phosphorylation. Moreover, in the aceticlastic pathway an ATP molecule is spent for acetyl-CoA formation. Instead, ATP is formed by a chemiosmotic mechanism. The CoB-CoM heterodisulfide, which is produced during methane formation (see methyl-coenzyme M reduction to methane), is restored into coenzyme B and coenzyme M by the action of two membrane-bound dehydrogenases ( F420H2 dehydrogenase and F420 non-reducing hydrogenase I), both of which translocate protons across the memberane (see coenzyme B/coenzyme M regeneration). The translocation results in a proton gradient that energizes ATP synthase enzymes.
About This Pathway
Methanosarcina barkeri is one of the few methanogenic species that are capable of methylotrohpic methanogenesis, and can grow on various methyl-containing substrates such as acetate, methanol, methylthiols and methylamines as a sole energy source.
The pathways for methanogenesis from methylated C1 compounds follow a similar route. Methyl transfer from the electron acceptor to coenzyme M is carried out by enzyme systems composed of three polypeptides: a protein that binds a corrinoid prosthetic group [Sauer98], and two methyltransferases, named MT1 and MT2. MT1 and the corrinoid protein form a tight heterodimeric complex. When the substrate binds to this complex, MT1 catalyzes the transfer of the methyl group from the substrate to the corrinoid group. MT2 then transfers the methyl group from the corrinoid to coenzyme M [vanderMeijden84]. Individual MT1 and corrinoid-binding proteins are highly specific for their respective substrate, and exhibit little or no activity with similar substrates [vanderMeijden83, Burke97, Ferguson97, Wassenaar98, Ferguson00].
Reconstitution of trimethylamine-dependent coenzyme M (CoM) methylation showed that three polypeptides were involved - trimethylamine--corrinoid protein Co-methyltransferase, trimethylamine-specific corrinoid protein and methylated [methylamine-specific corrinoid protein]:coenzyme M methyltransferase.
The first two proteins, of sizes 52 and 26 kDa, respectively, copurified as a single trimethylamine methyltransferase (TMA-MT). Gel permeation of the TMA-MT fraction demonstrated that the 52- kDa methyltransferase eluted with an apparent molecular mass of 280 kDa, suggesting a multimeric structure. The 26-kDa corrinoid protein eluted primarily as a monomer, although a small fraction of it eluted with the 280-kDa peak, indicating that the two proteins weakly associate [Ferguson97].
The third protein, methylated [methylamine-specific corrinoid protein]:coenzyme M methyltransferase (also known as MT2-A) was first discovered during studies of methanogenesis from acetate, but was later shown not to be involved in metabolism of acetate. Rather, it was found that MT2-A was the major isoform present in cells grown on trimethylamine [Yeliseev93], and subsequent studies showed that this form was functioning in methanogenesis of all three methylated amine substrates [Ferguson96].
Superpathways: superpathway of methanogenesis
Burke97: Burke SA, Krzycki JA (1997). "Reconstitution of Monomethylamine:Coenzyme M methyl transfer with a corrinoid protein and two methyltransferases purified from Methanosarcina barkeri." J Biol Chem 272(26);16570-7. PMID: 9195968
Ferguson00: Ferguson DJ, Gorlatova N, Grahame DA, Krzycki JA (2000). "Reconstitution of dimethylamine:coenzyme M methyl transfer with a discrete corrinoid protein and two methyltransferases purified from Methanosarcina barkeri." J Biol Chem 275(37);29053-60. PMID: 10852929
Ferguson96: Ferguson DJ, Krzycki JA, Grahame DA (1996). "Specific roles of methylcobamide:coenzyme M methyltransferase isozymes in metabolism of methanol and methylamines in Methanosarcina barkeri." J Biol Chem 271(9);5189-94. PMID: 8617801
Ferguson97: Ferguson DJ, Krzycki JA (1997). "Reconstitution of trimethylamine-dependent coenzyme M methylation with the trimethylamine corrinoid protein and the isozymes of methyltransferase II from Methanosarcina barkeri." J Bacteriol 179(3);846-52. PMID: 9006042
Pritchett05: Pritchett MA, Metcalf WW (2005). "Genetic, physiological and biochemical characterization of multiple methanol methyltransferase isozymes in Methanosarcina acetivorans C2A." Mol Microbiol 56(5);1183-94. PMID: 15882413
Sauer98: Sauer K, Thauer RK (1998). "Methanol:coenzyme M methyltransferase from Methanosarcina barkeri--identification of the active-site histidine in the corrinoid-harboring subunit MtaC by site-directed mutagenesis." Eur J Biochem 1998;253(3);698-705. PMID: 9654068
vanderMeijden83: van der Meijden P, Heythuysen HJ, Pouwels A, Houwen F, van der Drift C, Vogels GD (1983). "Methyltransferases involved in methanol conversion by Methanosarcina barkeri." Arch Microbiol 134(3);238-42. PMID: 6615129
vanderMeijden84: van der Meijden P, te Brommelstroet BW, Poirot CM, van der Drift C, Vogels GD (1984). "Purification and properties of methanol:5-hydroxybenzimidazolylcobamide methyltransferase from Methanosarcina barkeri." J Bacteriol 1984;160(2);629-35. PMID: 6438059
Wassenaar98: Wassenaar RW, Keltjens JT, van der Drift C, Vogels GD (1998). "Purification and characterization of dimethylamine:5-hydroxybenzimidazolyl-cobamide methyltransferase from Methanosarcina barkeri Fusaro." Eur J Biochem 253(3);692-7. PMID: 9654067
Yeliseev93: Yeliseev A, Gartner P, Harms U, Linder D, Thauer RK (1993). "Function of methylcobalamin: coenzyme M methyltransferase isoenzyme II in Methanosarcina barkeri." Arch Microbiol 159(6);530-6. PMID: 8352643
Burke95: Burke SA, Krzycki JA (1995). "Involvement of the "A" isozyme of methyltransferase II and the 29-kilodalton corrinoid protein in methanogenesis from monomethylamine." J Bacteriol 177(15);4410-6. PMID: 7635826
Grahame89: Grahame DA (1989). "Different isozymes of methylcobalamin:2-mercaptoethanesulfonate methyltransferase predominate in methanol- versus acetate-grown Methanosarcina barkeri." J Biol Chem 264(22);12890-4. PMID: 2753894
LeClerc96: LeClerc GM, Grahame DA (1996). "Methylcobamide:coenzyme M methyltransferase isozymes from Methanosarcina barkeri. Physicochemical characterization, cloning, sequence analysis, and heterologous gene expression." J Biol Chem 1996;271(31);18725-31. PMID: 8702528
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