Catalytic subunit of succinate dehydrogenase (SDH) that is involved in complex II of the mitochondrial electron transport chain and is responsible for transferring electrons from succinate to ubiquinone (coenzyme Q). SDH1 and SDH2 form the catalytic dimer. Electrons flow from succinate to the FAD bound to SDH1, and sequentially through the iron-sulfur clusters bound to SDH2 and enter the membrane dimer formed by SDH3 and SDH4. a quinone + succinate = a quinol + fumarate Carbohydrate metabolism; tricarboxylic acid cycle; fumarate from succinate (eukaryal route): step 1/1. Component of complex II composed of four subunits: a flavoprotein (FP), an iron-sulfur protein (IP), and a cytochrome b composed of a large and a small subunit. Interacts with EMI5/SDH5; interaction is required for FAD attachment. Present with 10400 molecules/cell in log phase SD medium. Belongs to the FAD-dependent oxidoreductase 2 family. FRD/SDH subfamily.