Methanogenesis from acetate

Methanogenesis from Acetate, Residence Time and Size of Ruminants... 

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... 

The excreted end-products of fermentations, including hydrogen gas, can be used by other anaerobic bacteria, and what is known as an anaerobic food chain develops. At the bottom of the food chain are the archaeal methanogens, different species of which convert hydrogen gas, carbon dioxide, or acetate to CH4. The process of methanogenesis from H2 and C02 occurs as follows ... 

H2 + HCOj- + H+ -> CH4 + 3 H20 Methanogenesis from acetate occurs as follows ... 

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]. 

Methanogenesis from acetate in extracts of Methanosarcina does not require membrane addition [260]. However, this does not exclude a function for cytochromes in acetoclastic methanogenesis by whole cells. Rather, the role of H2 in cell extracts, the ability of cytochrome b from Methanosarcina species to react with CO, and the observation that membrane-bound cytochromeb of M. barkeri is reduced by H2, and is oxidized by CH3-CoM -I- ATP or CH3-C0M + acetyl-phosphate, all point to the participation of cytochromes in Methanosarcina. A role of cytochromes in transport of electrons generated from carboxyl-group oxidation to the heterodisulfide reductase is a logical hypothesis. 

Fig. 10. Proposed pathway of methanogenesis from acetate (Methanosarcina, Methanothrix) and from pyruvate (Methanosarcina) Intermediates, enzymes and a site for Na dependence. CoA, coenzyme A PP, pyrophosphate CH3-H4MPT, methyl-tetrahydromethanopterin, CH3-S-C0M, methyl-coenzyme M [CO], CO bound to carbon monoxide dehydrogenase. Numbers in circles refer to enzymes involved (1) acetyl-CoA synthetase (2) pyrophosphatase (3) acetate kinase (4) phosphate acetyltransferase (5) pyruvate ferredoxin oxidoreductase (6) carbon monoxide dehydrogenase (7) CH3-H4MPT FI-S-CoM methyltransferase (8) methyl-coenzyme M reductase (9) heterodisulfide...

Fig. 12. Proposed role of methyl-tetrahydromethanopterin (CH3-H4MPT) coenzymeM (H-S-CoM) methyltransferase in methanogenesis from CO2, acetate and methanol methyltransferase as a reversible Na -translocating membrane protein. During CH4 formation from CO2 and from acetate, methyltransferase is involved in primary Na" extrusion generating A/INa during methanol disproportionation to CH4 and CO2 the enzyme is involved in A/iNa -driven methanol oxidation.

Acetate formation from pyruvate in the absence of methanogenesis... 

Allen GWJ, Zinder SH (1996) Methanogenesis from acetate by cell-free extracts of the thermophilic acetotrophic methanogen Methanothrix thermophila (ALS-1). Arch Microbiol 166 275-281... 

In marine sediments below the sulfate zone, methanogenesis is the predominant terminal pathway of organic carbon degradation. Methane may also be formed from acetate or from organic Cj-compounds such as methanol or methylamines ... 

Fig. 5.15 Inhibitor experiment for the demonstration of substrates used by sulfate reducing bacteria in a coastal marine sediment. The concentrations of volatile fatty acids, hydrogen and methane are followed during a time-course experiment over 8 hours. At 3.5 hours (arrow) molybdate was added and the substrates accumulate at a rate corresponding to their rate of consumption before inhibition. The formation of methane shows the release of competition for the common substrates for methanogenesis and sulfate reduction (H and acetate). Data from Sorensen et al. (1981).

---