Membrane-anchoring 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). The heme is bound between the two transmembrane subunits. Carbohydrate metabolism; tricarboxylic acid cycle. Forms part of complex II containing four subunits: a 70 kDa flavoprotein (FP), a 27 kDa iron-sulfur protein (IP), a cytochrome B and a membrane-anchoring protein. Belongs to the cytochrome b560 family.