(b) : During Krebs' cycle when succinic acid is oxidised to fumaric acid then the precipitation of FAD as electron acceptor occurs. It is the only Krebs' cycle oxidation that does not employ a pyridine nucleotide. In this, succinate is dehydrogenated by succinate dehydrogenase. This enzyme removes two hydrogen ions and two electrons from succinate and these are then used to reduce FAD to FADH ‌2.