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 → Methanogenesis|
Expected Taxonomic Range: Euryarchaeota
Methanogenesis, the biological production of methane, is 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.
A limited number of substrates can be converted to methane, and most species are capable of utilizing only one or two growth substrates. Most of the methane in nature originates from acetate, but some C-1 substrates (carbon-containing compounds that lack carbon-carbon bonds) such as methanol and several different methylamines and methylsulfides can also be used by some organisms.
The pathways for the conversion of all of these compounds to methane follows a similar path. The pathway always involves the transfer of a methyl group from the substrate 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].
The methyl transfer to coenzyme M is carried out by enzyme systems composed of three polypeptides: methyltransferase 1, or MT1, a protein that binds a corrinoid prosthetic group [Sauer98], and methyltransferase 2, or 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].
Several such MT1/MT2 systems were identified in Methanosarcina barkeri and related organisms. 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]. Specific MT1/corrinoid pairs have been identified for methanol (MtaC, MtaB), trimethylamine (MttC, MttB), dimethylamine (MtbC, MtbB), methylamine (MtmC, MtmB) and methanethiol (MtsA, MtsB). MT2 proteins are somewhat less specific - MtaA is used mostly for methanol, while MtbA is used for all three methylamines [Ferguson96]. An exception is the MtsA protein, which catalyses both MT1 and MT2 reactions in the activation of dimethyl sulfide [Tallant01].
The genes encoding the various MT1/2 enzymes from Methanosarcina barkeri and related organisms have been identified [Burke98, Paul00]. Surprisingly, multiple copies of each MT1 and corrinoid protein pair were identified. For example, the Methanosarcina acetivorans genome encodes 23 putative MT1 and MT2 methyltransferases, with three copies each of mtaCB (encoding putative methanol-specific corrinoid and MT1 proteins) and of mtmCB (monomethylamine-specific), and two copies each of mtbCB (dimethylamine-specific) and of mttCB (trimethylamine-specific). In addition, other apparent corrinoid proteins and MT2 enzymes of as yet unknown specificity were also identified [Pritchett05].
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.
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
Paul00: Paul L, Ferguson DJ, Krzycki JA (2000). "The trimethylamine methyltransferase gene and multiple dimethylamine methyltransferase genes of Methanosarcina barkeri contain in-frame and read-through amber codons." J Bacteriol 182(9);2520-9. PMID: 10762254
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
Tallant01: Tallant TC, Paul L, Krzycki JA (2001). "The MtsA subunit of the methylthiol:coenzyme M methyltransferase of Methanosarcina barkeri catalyses both half-reactions of corrinoid-dependent dimethylsulfide: coenzyme M methyl transfer." J Biol Chem 276(6);4485-93. PMID: 11073950
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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