EPR characterization of the new Qrc complex from sulfate reducing bacteria and its ability to form a supercomplex with hydrogenase and TpIc3

The Quinone-reductase complex (Qrc) is a respiratory complex with Type I cytochrome c(3):menaquinone reductase activity, recently described in sulfate-reducing bacteria. Qrc is related to the complex iron-sulfur molybdoenzyme family and to the alternative complex III. In this work we report a detailed characterization of the redox properties of the metal cofactors of Qrc using EPR spectroscopy, which allowed the determination of the reduction potentials of five out of six hemes c, one 3Fe-4S](1+/0) center and the three 4Fe-4S](2+/1+) centers. In addition, we show that Qrc forms a supercomplex with NiFe] hydrogenase and TpIc(3), its physiological electron donors.