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

Coenzyme F-420 is the central low-redox-potential electron carrier in methanogenic metabolism. The coenzyme is reduced under hydrogen by the action of F-420-dependent hydrogenase. The standard free-energy change at pH 7 of F-420 reduction was determined to be -15 kJ mol(-1), irrespective of the temperature (25-65 degrees C). Experiments performed with methane-forming cell suspensions of Methanothermobacter thermautotrophicus incubated under various conditions demonstrated that the ratios of reduced and oxidized F-420 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 F-420 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 F-420 in thermodynamic equilibrium with external hydrogen. The results of the study demonstrate that F-420 is a useful probe to assess in situ hydrogen concentrations in H-2-metabolizing methanogens.