-!- The enzyme is found in the inner mitochondrial membrane in eukaryotes and the plasma membrane of many aerobic or facultative bacteria. -!- It catalyzes succinate oxidation in the citric acid cycle and transfers the electrons to quinones in the membrane, thus constituting a part of the aerobic respiratory chain (known as complex II). -!- In vivo the enzyme uses the quinone found in the organism - eukaryotic enzymes utilize ubiquinone, bacterial enzymes utilize ubiquinone or menaquinone, and archaebacterial enzymes from the Sulfolobus genus use caldariellaquinone. -!- Cf. EC 1.3.5.4.