Sodium ion-dependent hydrogen production in Acidaminococcus fermentans

Abstract

Acidaminococcus fermentans is able to ferment glutamate to ammonia, CO₂, acetate, butyrate, and H₂. The molecular hydrogen (approximately 10 kPa; E′ = –385 mV) stems from NADH generated in the 3-hydroxybutyryl-CoA dehydrogenase reaction (E°′ = –240 mV) of the hydroxyglutarate pathway. In contrast to growing cells, which require at least 5 mM Na⁺, a Na⁺-dependence of the H₂-formation was observed with washed cells. Whereas the optimal glutamate fermentation rate was achieved already at 1 mM Na⁺, H₂ 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 glutaconyl-CoA decarboxylase is converted into a Na⁺/Na⁺ exchanger.