Sodium ion-dependent hydrogen production in Acidaminococcus fermentans

Acidaminococcus fermentans is able to ferment glutamate to ammonia, CO2, acetate, butyrate, and H-2. The molecular hydrogen (approximately 10 kPa; E' = -385 mV) stems from NADH generated in the 3-hydroxy-butyryl-CoA dehydrogenase reaction (E degrees' = -240 mV) of the hydroxyglutarate pathway. In contrast to growing cells, which require at least 5 mM Na-+, a Na-+-dependence of the H-2-formation was observed with washed cells. Whereas the optimal glutamate fermentation rate was achieved already at 1 mM Na+, H-2 formation commenced only at > 10 mM Na+ and reached maximum rates at 100 mM Na+. The acetate/butyrate ratio thereby increased from 2.0 at 1 mM Na+ to 3.0 at 100 mM Na+. A hydrogenase and an NADH dehydrogenase, both of which were detected in membrane fractions, are components of a model in which electrons, generated by NADH oxidation inside of the cytoplasmic membrane, reduce protons outside of the cytoplasmic membrane. The entire process can be driven by decarboxylation of glutaconyl-CoA, which consumes the protons released by NADH oxidation inside the cell. Hydrogen production commences exactly at those Na+ concentrations at which the electrogenic H+/Na+-antiporter glutaconly-CoA decarboxylase is converted into a Na+/Na+ exchanger.