A complex generally comprising an FAD-containing component that also binds the carboxylate substrate (A subunit), a component that contains three different iron-sulfur centers [2Fe-2S], [4Fe-4S], and [3Fe-4S] (B subunit), and a hydrophobic membrane-anchor component (C, or C and D subunits) that is also the site of the interaction with quinones. The enzyme is found in the inner mitochondrial membrane in eukaryotes and the plasma membrane of bacteria and archaea, with the hydrophilic domain extending into the mitochondrial matrix and the cytoplasm, respectively. Under aerobic conditions the enzyme catalyses succinate oxidation, a key step in the citric acid (TCA) cycle, transferring the electrons to quinones in the membrane, thus linking the TCA cycle with the aerobic respiratory chain (where it is known as complex II). Under anaerobic conditions the enzyme functions as a fumarate reductase, transferring electrons from the quinol pool to fumarate, and participating in anaerobic respiration with fumarate as the terminal electron acceptor. The enzyme interacts with the quinone produced by the organism, such as ubiquinone, menaquinone, caldariellaquinone, thermoplasmaquinone, rhodoquinone etc. Some of the enzymes contain two heme subunits in their membrane anchor subunit. These enzymes catalyse an electrogenic reaction and are thus classified as EC
7.1.1.12, succinate dehydrogenase (electrogenic, proton-motive force generating).
