Electron transport from FADH₂:

Succinate dehydrogenase catalyzes the oxidation of succinate to fumarate in the citric acid cycle. A succinate dehydrogenase hold bound FAD which is decreased to FADH₂   in the reaction.   Re-oxidation of the FADH₂   occurs through succinate-Q reductase (Complex II), the integral protein of the inner mitochondrial membrane.  The Succinate  dehydrogenase  is component  of  this  complex  but  it also contains FeS clusters. In during re-oxidation of FADH₂ and the two electrons pass from the FADH₂ to the FeS clusters and are then passed on to ubiquinone. They then enter the major electron transport chain and cause H⁺ ions to be pumped out of the mitochondrion as for the oxidation of NADH. Moreover, succinate-Q reductase itself is not an H⁺ pump since the free energy modify of the whole reaction is too small. The FADH₂ of other flavoproteins, like as mitochondrial glycerol 3-phosphate dehydrogenase in the glycerol 3-phosphate shuttle and fatty acyl CoA dehydrogenase in fatty acid oxidation that also feed their electrons into the electron transport chain at ubiquinone.