The dihydrolipoate, covalently bound to a lysine residue of the complex, is then transfered to the dihydrolipoyl dehydrogenase (E3) active site, where it undergoes a flavin-mediated oxidation, similar in chemistry to e.g. thioredoxin reductase. First, FAD oxidizes dihydrolipoate back to its lipoate (disulfide) resting state, producing FADH. Then, the substrate NAD oxidizes FADH back to its FAD resting state, producing NADH and H.