Succinate-Q oxidoreductase (Complex II)

The second protein complex in the electron transport chain is named Succinate-Q oxidoreductase and is commonly labeled Complex II. This protein complex contains the enzyme succinate dehydrogenase, which catalyzes the conversion of succionate to fumarate in the TCA cycle and in the process energizes FAD to FADH₂. In Complex II the same enzyme produces fumarate from succionate and produces FADH₂. This energetic FADH₂ gives off two energetic electrons to a chain of Fe.S clusters that ultimately transfer them to coenzyme-Q (ubiquinone) to contribute to the flow of electrons in the electron transport chain. This adds to the electrons provided by Complex I, but Complex II does not pump any hydrogens to the intermembrane space.