Two distinct, membrane-bound, FAD-containing enzymes are responsible for the catalysis of fumarate and succinate interconversion; fumarate reductase is used in anaerobic growth, and succinate dehydrogenase is used in aerobic growth. Anchors the catalytic components of the fumarate reductase complex to the cell inner membrane, binds quinones (PubMed:10373108). The QFR enzyme complex binds 2 quinones in or near the membrane; 1 near the [3Fe-4S] cluster (QP is proximal to the [3Fe-4S] cluster, on the cytoplasmic side of the membrane) while QD (the distal cluster) is on the other side of the membrane. It is not clear if both of the quinol-binding sites are functionally relevant (PubMed:10373108, PubMed:11850430). Part of an enzyme complex containing four subunits: a flavoprotein (FrdA), an iron-sulfur protein (FrdB), and two hydrophobic anchor proteins (FrdC and FrdD). Belongs to the FrdC family.