Question

Refer to Table 14-4 to explain why FAD rather than NAD+ is used

in the succinate dehydrogenase reaction.

Short answer

The NAD+ reduction potential is too low to oxidize succinate.

Step-by-step solution

Reaction of Succinate dehydrogenase with FAD

In the succinate dehydrogenase reaction, succinate is oxidized into fumarate.But oxidation can only be done in paired reaction; the reduction has to occur simultaneously.

All the dehydrogenase enzymes acting during the citric acid cycle use NAD as the final electron acceptor except for the succinate dehydrogenase. The succinate dehydrogenase enzyme uses covalently bound FAD (also called SDH-bound FAD) instead of NAD⁺ as a final electron acceptor for the catalysis of the reaction of succinate oxidation into fumarate.

Reduction of the potential of NAD+ and succinate

From the given table, we can find that the NAD+ reduction potential is$(-0.315)$ lower than the succinate, which is $(0.031)$. If NAD+ is used in this dehydrogenase reaction, it will result in the oxidation of NAD+ instead of succinate.

FAD has a reduction potential lower on its own (-0.219), but it is bound to an enzyme that can oxidize succinate.Also, it can use up both the protons generated during the conversion of succinate to fumarate. Thus, FAD is used in the reaction involving succinate dehydrogenase.