The stimulation of carbon dioxide reduction to methane by addition of 2-(methylthio)ethanesulfonate (CH3-S-CoM) to cell extracts of Methanobacterium thermoautotrophicum was investigated. Similar stimulation of CO2 reduction by CH3-S-CoM was found for cell extracts of Methanobacterium bryantii and Methanospirillum hungatei. The CH3-S-CoM requirement could be met by the methanogenic precursors formaldehyde, serine, or pyruvate, or by 2-(ethylthio)ethanesulfonate (CH3CH2-S-CoM), but not by other coenzyme M derivatives. Efficient reduction of CO2 to CH4 was favored by low concentrations of CH3-S-CoM and high concentrations of CO2. Sulfhydryl compounds were identified as effective inhibitors of CO2 reduction. Both an allosteric model and a free-radical model for the mechanism of CO2 activation and reduction are discussed.
