Biochemistry of Acetotrophic Methanogenesis
Two-thirds of the estimated one billion metric tones of methane produced each year in the Earth’s biosphere derives from the methyl group of acetate via the acetotrophic pathway. The pathway is best understood in species from the genus Methanosarcina, one
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K. N. Timmis (ed.), Handbook of Hydrocarbon and Lipid Microbiology, DOI 10.1007/978-3-540-77587-4_23, # Springer-Verlag Berlin Heidelberg, 2010
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Biochemistry of Acetotrophic Methanogenesis
Abstract: Two-thirds of the estimated one billion metric tones of methane produced each year in the Earth’s biosphere derives from the methyl group of acetate via the acetotrophic pathway. The pathway is best understood in species from the genus Methanosarcina, one of only two genera known to obtain energy for growth by converting acetate to methane and carbon dioxide. The pathway commences by activation of acetate to acetyl-CoA of which the C–C and C–S bonds are cleaved yielding methyl and carbonyl groups. The methyl group is ultimately transferred to coenzyme M that is reductively demethylated to methane with electrons derived from oxidation of the carbonyl group of acetate. Cells obtain energy for growth by the transfer of electrons through a membrane-bound electron transport chain coupled to generation of an ion gradient that drives ATP synthesis. This review summarizes the current biochemical understanding of the pathway contrasting marine and freshwater Methanosarcina species with a focus on the mechanism of key enzymes.
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Introduction
Methane is the end product of the decomposition of complex organic matter in diverse O2-free (anaerobic) environments, producing nearly 1 billion metric tons of methane each year. The process is an essential link in Earth’s carbon cycle (> Fig. 1). In the cycle, CO2 is fixed into complex organic matter by photosynthesis (step 1) and in aerobic zones is oxidized back to CO2 by O2-requiring microbes (step 2). A portion of the organic matter is deposited in a variety of anaerobic environments where diverse anaerobes decompose the organic matter (step 3) to products that are substrates for methane-producing species (steps 4 and 5). Some of the methane is utilized by sulfate- or nitrate-reducing anaerobic methane oxidizers (step 6) (Thauer and Seigo, 2007) and the remainder escapes into aerobic habitats where it is oxidized to CO2 by O2-requiring methylotrophic microbes (step 7). Approximately one-third of the methane produced in Earth’s biosphere is generated by the reduction of CO2 with electrons derived from the oxidation of H2 (> Fig. 1, step 4): CO2 þ 4H2 ! CH4 þ 2H2
ð1Þ
The remaining two-thirds originates from the methyl group of acetate (> Fig. 1, step 5) by the ‘‘aceticlastic’’ pathway: CH3 COO þ Hþ ! CH4 þ CO2
. Figure 1 The global carbon cycle.
ð2Þ
Biochemistry of Acetotrophic Methanogenesis
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Only two genera of methanogens (> Fig. 2), Methanosarcina (Msr.) and Methanosaeta (Mse.), contain acetate-utilizing species (Ferry and Kastead, 2007). Methanosaeta (previously Methanothrix) species have a higher affinity for acetate than Methanosarcina and dominate in methanogenic habitats with low concentrations of acetate. The biochemistry of the aceticlastic pathway has been investigated primarily in the freshwater Methanosarcina species Msr. barkeri, Msr. mazei and Msr.
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