The molecular components involved in energy metabolism of deep-sea Epsilonproteobacteria were characterized in the mesophilic hydrogen- and sulfur-oxidizing chemolithoautotroph Sulfurovum sp. NBC37-1. Previous whole-genome analysis of strain NBC37-1 identified key genes likely to be associated with both sulfur reduction (psr gene families) and oxidation (two sox gene clusters). However, the sox gene clusters showed unique organizations and low homologies to those in other bacteria. Therefore, the biochemical mechanism of inorganic sulfur metabolism has been uncertain. Enzymatic activity measurements and partial protein purification indicated that the Sox enzyme system was constitutively expressed, whereas the expression of sulfur-reduction enzymes varied depending on the culture conditions. The operative Sox system in strain NBC37-1 required membrane components. The molecular basis of energy metabolism reported in this study provides important insight into how deep-sea Epsilonproteobacteria change their energy metabolism in response to variable physical and chemical conditions in mixing zones between hydrothermal fluid and ambient seawater.