CO2 methanogenesis

Fig. 1. The pathways of methanogenesis. Intermediates are abbreviated as in the text. The thick lines indicate the pathway for H2-CO2 methanogenesis, which is also in common to some extent with methanogenesis from one or more other substrates. The thin lines indicate specialized portions of pathways of methanogenesis from methanol, formate, and acetate, (a) Two different dehydrogenases have been reported, one dependent on H2F420, and one dependent on H2. (b) The source of these electrons may be H2F420 in some cases, but in other cases it is unknown see the text, (c) This is a possible alternative for methanol oxidation to the methylene-RjMPT level see the text for details.

Methane formation from CH3-C0M in acetate methanogenesis follows the same course described for H2-CO2 methanogenesis methyl-coenzyme M reductases have been purified from acetate-grown Methanosarcina and Methanothrix [242,26 i]. 

Methenyl-H4MPT cyclohydrolase is essential for H2—CO2 methanogenesis. Pathway analysis and enzyme level studies suggest that this enzyme is also involved in methanol oxidation [224,346]. In acetate catabolism it does not have an obvious role, and in Methanosarcina growing on acetate this enzyme is down-modulated [224,346]. 

Several portions of the pathway are still unclear. How the methyl of methanol enters the reversed methanogenic pathway is unknown, since some suggest it may not proceed via methyl-CoM nonetheless, evidence is clear that most of the reversed H2-CO2 path is used. Also, our understanding of methyl-transfer reactions at several portions of the pathways remains incomplete, e.g. the way in which methanol is reduced to methane, the enzymes involved in methyl transfer in CO2 methanogenesis, and routes of nonmethanol methyl-substrate entry into the path. In several cases, the source of electrons for a reductive step is unknown, e.g. the heterodisulfide reductase and formyl-methanofuran dehydrogenase steps. 

Methanogenesis from CO2 via methenyl H MPT, methylene H MPT, and methyl H MPT using coenzyme F420H2 (5,6,7,8-tetrahydrofolate). 

Methanogenesis Co2+ 40 pM Methanosarcina sp. Mineral salts medium containing bicarbonate buffer and 1 mM sulfur 66... 

Fermentation of the wastes - conversion to acetates Acetogenesis - conversion to acids, formaldehyde and Hydrogen, and Methanogenesis - conversion of formaldehyde, acetates, and acids to CO2 and Methane... 

Methanococcus mazeii 2[NiFe] hydrogenases Membrane-bound cytochrome b reducing (Vht) Membrane-bound cytochrome b reducing (Vho) Methanogenesis Methanogenesis H2/CO2, methanol, acetate Constitutive 10... 

Methanogenesis. The methanogenic population (X2) uses as substrate the volatile fatty acids S2) to produce CO2 and CH4... 

The standard free energy change for methanogenesis from hydrogen and CO2 is more exergonic than that of acetogenesis (AG o = -135.6kJ per mole methane and AG o = -104.6kJ per mole acetate, respectively). However, certain conditions compromise or inhibit methanogenesis (Fig. 13.5). In a complex ecosystem, the metabolic interactions of the anaerobic populations... 

Methanogenesis H2 C02 OO2, possibly formate, acetate 4H2+ CO2 CH4 + 2H2O (CO2 reduction) CH3COOH CH4 + CO2 (acetate fermentation) Mesophilic to hyperthermophilic archaea at vents and seeps... 

The total flux of CH4 to the atmosphere today is about 34 x 1012 moles y1 about 20% of this flux results from enteric fermentation processes in animals, termites, and humans. Of the remaining flux, about 60% comes from the process of methanogenesis in sediments of various types-primarily muds of rice paddy fields, swamps, and marshes. The overall bacterial reactions in sediments are the disproportionation of organic matter to CH4 and CO2, that is ... 

Cobamides or corrinoids are involved in methyl-transfer reactions in the methanogenic pathways, especially from methyl substrates [143-156]. At least one cobamide is involved in methanogenesis from H2-CO2 by M. thermoautotrophicum, where it is found in the methyl-H4MPT CoM methyltransferase [157]. The majority are either associated with membranes or bound to soluble proteins [143,158]. Methanogens are inhibited by corrinoid antagonists, suggesting an important metabolic role [159]. 

The pathways for methanogenesis from CO2, HCOO, CH3OH, and CH3COO are shown in Fig. 1. Generally, the H4MPT-mediated steps in these pathways show close homology to the H4folate system [182]. 

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