A Membrane-bound Multienzyme, Hydrogen-oxidizing, and Sulfur-reducing Complex from the Hyperthermophilic Bacterium Aquifex aeolicus

Aquifex aeolicus is a hyperthermophilic, chemolithoautotrophic, hydrogen-oxidizing, and microaerophilic bacterium growing at 85 °C. We have shown that it can grow on an H2/S° medium and produce H2S from sulfur in the later exponential phase. The complex carrying the sulfur reducing activity (electron transport from H2 to S°) has been purified and characterized. It is a membrane-bound multiprotein complex containing a [NiFe] hydrogenase and a sulfur reductase connected via quinones. The sulfur reductase is encoded by an operon annotated dms (dimethyl sulfoxide reductase) that we have renamed sre and is composed of three subunits. Sequence analysis showed that it belongs to the Me2SO reductase molybdoenzyme family and is similar to the sulfur/polysulfide/thiosulfate/tetrathionate reductases. The study of catalytic properties clearly demonstrated that it can reduce tetrathionate, sulfur, and polysulfide, but cannot reduce Me2SO and thiosulfate, and that NADPH increases the sulfur reducing activity. To date, this is the first characterization of a supercomplex from a bacterium that couples hydrogen oxidation and sulfur reduction. The distinctive feature in A. aeolicus is the cytoplasmic localization of the sulfur reduction, which is in accordance with the presence of sulfur globules in the cytoplasm. Association of this sulfur-reducing complex with a hydrogen-oxygen pathway complex (hydrogenase I, bc1 complex) in the membrane suggests that subcomplexes involved in respiratory chains in this bacterium are part of supramolecular organization.