Two distinct, membrane-bound, FAD-containing enzymes are responsible for the catalysis of fumarate and succinate interconversion; fumarate reductase is used during anaerobic growth, and succinate dehydrogenase is used during aerobic growth. 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). a quinone + succinate = a quinol + fumarate a menaquinone + succinate = a menaquinol + fumarate Binds 1 [2Fe-2S] cluster. Binds 1 [3Fe-4S] cluster. Binds 1 [4Fe-4S] cluster. 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 succinate dehydrogenase/fumarate reductase iron-sulfur protein family.