a quinone + succinate = a quinol + fumarate a menaquinone + succinate = a menaquinol + fumarate Binds 1 FAD covalently per subunit. 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 FAD-dependent oxidoreductase 2 family. FRD/SDH subfamily.