Energetics of CH4 formation from CO2 reduction by alcohols

Several methanogenic species have been described that use ethanol, propanol and 2-propanol, 2-butanol and cyclopentanol as electron donors for CO2 reduction to 

In Methanogenium thermophilum, which can use both H2 and ethanol as electron donor for CO2 reduction, the energetics and the mechanism of ethanol oxidation was 

Several reactions of CO2 reduction to CH4 by methanogens resemble those of CO2 reduction to by acetogens. Most acetogens, which are anaerobic bacteria (eubacteria), catalyze in their energy metabolism the formation of acetate from CO2 and H2 according to the following equation  

Acetate is synthesized via the acetyl-CoA pathway (for recent reviews see refs. [190-192]) one molecule CO2 is reduced via formyl-tetrahydrofolate (formyl-H4F) and methylene—H4F to methyl-H4F, a second one is reduced to a enzyme-bound carbonyl. The methyl group is condensed with the carbonyl group and coenzyme A (CoA) to acetyl-CoA. Acetyl-CoA is then converted to acetate, and ATP is formed in the acetate kinase reaction. 

CO2 reduction to acetate and CO2 reduction to CH4 involve two energetically analogous sites (1) the endergonic reduction of CO2 to the formate level ( CO2 activation ), and (2) the exergonic conversion of a coenzyme-bound formaldehyde to the methanol level. 

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