Differences in D-amino acid metabolism mediated by D-amino acid dehydrogenase in autotrophic and heterotrophic cultures of the Sulfur-Oxidizing Bacterium Starkeya novella

The sulfur-oxidizing bacterium Starkeya novella (S. novella) is a facultative chemoautotroph. D-Amino acid dehydrogenase (DAD) is a membrane-bound flavoenzyme that dehydrogenates D-amino acids to produce 2-oxoacids, ammonia, and hydrogen to generate electrons. In Helicobacter pylori, these electrons are transferred to cytochromes in the respiratory chain. This study aimed to assess electron transfer from DAD to cytochromes in S. novella. Under heterotrophic conditions, when D-amino acids were used as substrates, the cytochrome system was reduced by the electrons produced by DAD, and oxygen consumption was observed. This suggests that D-amino acids are used as respiratory substrates and are involved in ATP synthesis. Under autotrophic conditions, NAD⁺ was reduced by electrons dehydrogenated by DAD. This suggests that NAD⁺ is used for carbonic acid fixation. These results suggest that the DAD of S. novella may play different roles under autotrophic and heterotrophic conditions.