Final answer:
FAD is used as the oxidizing agent by succinate dehydrogenase during the conversion of succinate to fumarate because it can participate in different oxidation mechanisms and is covalently bound to the enzyme. Also, the energy from the electrons is sufficient for FAD reduction and allows direct electron transfer to the electron transport chain.
Step-by-step explanation:
In the redox reaction catalyzed by succinate dehydrogenase, FAD acts as the oxidizing agent rather than NAD because of its association with the enzyme and its role in the electron transport chain. Succinate is oxidized to fumarate, and in this process, FAD is reduced to FADH₂.
Since FAD is more versatile, it can participate in oxidation reactions using different mechanisms, including the reduction involving attachment to succinate dehydrogenase through a histidine residue. Furthermore, the energy contained in the electrons transferred from succinate is insufficient to reduce NAD but is adequate to reduce FAD. Additionally, unlike NADH, FADH₂ remains attached to succinate dehydrogenase, which is located on the inner mitochondrial membrane, allowing the direct transfer of electrons to the electron transport chain, thus bypassing complex I. This is significant as fewer ATP molecules are produced in this pathway, due to the lower energy released from FADH₂ compared to from NADH.