Component of the respiratory Qrc complex, that catalyzes the reduction of the menaquinone pool using electrons transferred from the reduced periplasmic cytochrome c3, and which is probably involved in sulfate respiration. Is likely essential for growth on H(2) or formate since the periplasmic hydrogenases and/or formate dehydrogenases act as primary electron donors for the Qrc complex. The function of the QrcB subunit is unknown; in the absence of a catalytic site, it may provide a structural scaffold for the other subunits. There is no molybdenum or tungsten pterin cofactor present in the Qrc complex, despite the similarity of QrcB to molybdopterin-containing oxidoreductases. The Qrc complex is composed of four subunits: QrcA, QrcB, QrcC and QrcD (PubMed:20498375). Can form a supercomplex with the [NiFe] hydrogenase HynA1 and the tetraheme Type I cytochrome c3 TpIc(3), its physiological electron donors (PubMed:21651911). Predicted to be exported by the Tat system. The position of the signal peptide cleavage has not been experimentally proven. Belongs to the prokaryotic molybdopterin-containing oxidoreductase family.