FAD is an important acceptor of electrons. FAD is the oxidized form of the molecule (lacks electrons). FADH2 is the reduced form (carries electrons). FAD and FADH2 function in many oxidation reactions, such as those catalyzed by succinate dehydrogenase and fatty acyl-CoA dehydrogenase. Electrons carried by FADH2 do not pass through complex I of the mitochondrial electron transport system and thus do not result in synthesis of as many ATPs in oxidative phosphorylation as electrons from NADH, which do pass through Complex I. It is estimated that each pair of electrons donated by FADH2 results in two ATPs, whereas each pair of electrons from NADH results in three ATPs.