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Hydrogen concentrations in methane-forming cells probed by the ratios of reduced and oxidized coenzyme F₄₂₀

Linda M. I. de Poorter

Department of Microbiology, Faculty of Science, Radboud University Nijmegen, Toernooiveld 1, NL-6525 ED, Nijmegen, The Netherlands

Correspondence
Jan T. Keltjens
J.Keltjens{at}science.ru.nl

Coenzyme F₄₂₀ is the central low-redox-potential electron carrier in methanogenic metabolism. The coenzyme is reduced under hydrogen by the action of F₄₂₀-dependent hydrogenase. The standard free-energy change at pH 7 of F₄₂₀ reduction was determined to be –15 kJ mol^–1, irrespective of the temperature (25–65 °C). Experiments performed with methane-forming cell suspensions of Methanothermobacter thermautotrophicus incubated under various conditions demonstrated that the ratios of reduced and oxidized F₄₂₀ were in thermodynamic equilibrium with the gas-phase hydrogen partial pressures. During growth in a fed-batch fermenter, ratios changed in connection with the decrease in dissolved hydrogen. For most of the time, the changes were as expected for thermodynamic equilibrium between the oxidation state of F₄₂₀ inside the cells and extracellular hydrogen. Also, methanol-metabolizing, but not acetate-converting, cells of Methanosarcina barkeri maintained the ratios of reduced and oxidized coenzyme F₄₂₀ in thermodynamic equilibrium with external hydrogen. The results of the study demonstrate that F₄₂₀ is a useful probe to assess in situ hydrogen concentrations in H₂-metabolizing methanogens.

Present address: Department of Biotechnology, Delft University of Technology, Delft, The Netherlands.

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				L. M. I. de Poorter, W. J. Geerts, and J. T. Keltjens Coupling of Methanothermobacter thermautotrophicus Methane Formation and Growth in Fed-Batch and Continuous Cultures under Different H2 Gassing Regimens Appl. Envir. Microbiol., February 1, 2007; 73(3): 740 - 749. [Abstract] [Full Text] [PDF]