Why is FAD, rather than NAD+, reduced in the succinate dehydrogenase reaction of the Tricarboxylic Acid Cycle?

Question

FADH₂ is produced in the conversion of Succinate to Fumarate in the tricarboxylic acid cycle. Why is this so? Why not NADH?

Answer 1

The conversion of succinate to fumarate does not release enough energy to reduce NADH. When succinate is converted to fumarate, two hydrogens are removed resulting in a double bond. The energy from this reaction is not enough to reduce NAD+. Likewise the enzyme that catalyzes this reaction, succinate dehydrogenase, is complex II on the electron transport chain which utilizes FAD and FADH2.