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Review
. 2011 Jun;22(3):351-7.
doi: 10.1016/j.copbio.2011.04.011. Epub 2011 May 17.

Fundamentals of methanogenic pathways that are key to the biomethanation of complex biomass

James G Ferry  1
Affiliations
Review

Fundamentals of methanogenic pathways that are key to the biomethanation of complex biomass

James G Ferry. Curr Opin Biotechnol. 2011 Jun.

Abstract

The conversion of biomass to CH4 (biomethanation) involves an anaerobic microbial food chain composed of at least three metabolic groups of which the first two decompose the complex biomass primarily to acetate, formate, and H2. The thermodynamics of these conversions are unfavorable requiring a symbiosis with the CH4-producing group (methanogens) that metabolize the decomposition products to favorable concentrations. The methanogens produce CH4 by two major pathways, conversion of the methyl group of acetate and reduction of CO2 coupled to the oxidation of formate or H2. This review covers recent advances in the fundamental understanding of both methanogenic pathways with the view of stimulating research towards improving the rate and reliability of the overall biomethanation process.

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Figures

Figure 1
Figure 1
The global carbon cycle. Aerobic O2-requiring conversions are shown in solid red arrows and anaerobic conversions in solid blue arrows. The brackets denote aerobic (+O2) and anaerobic (−O2) habitats. Black dotted arrows symbolize diffusion of substrates and products across the interface of zones.
Figure 2
Figure 2
Composite of CO2-reduction and aceticlastic methane-producing pathways. The left arm leading to CH3-H4M(S)PT shows reactions (1–4) unique to the aceticlastic pathway, and the right arm leading to CH3-H4M(S)PT shows reactions (5–9) unique to the CO2-reduction pathway. Both pathways have in common reactions (10–12) leading to the formation of CH4 from the methyl groups of CH3-H4M(S)PT. Abbreviations: ATP, adenosine triphosphate; H4SPT, tetrahydrosarcinapterin; H4MPT, tetrahydromethanopterin, Fd, ferredoxin; CoA, coenzyme A; CoM, coenzyme M; CoB, coenzyme B; MF, methanofuran; F420, coenzyme F420.
Figure 3
Figure 3
Proposed model of ferredoxin-dependent electron transport chain in the freshwater isolate Methanosarcina mazei. H2ase, F420-nonreducing hydrogenase (Vho); HDR, heterodisulfide reductase; MP, methanophenazine. By permission from Ref. [16].
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