Why, do you suppose, the coenzyme for the reaction in the citric acid cycle that is catalyzed by succinate dehydrogenase is FAD and not NAD⁺?

Verified step by step guidance

1. Understand the role of succinate dehydrogenase in the citric acid cycle. It catalyzes the oxidation of succinate to fumarate.
2. Recognize that this reaction involves the removal of two hydrogen atoms from succinate, which are transferred to a coenzyme.
3. Consider the standard reduction potentials of FAD and NAD⁺. FAD is more suitable for reactions involving the removal of two hydrogen atoms, as it can accept two electrons and two protons, forming FADH₂.
4. Note that NAD⁺ typically accepts only one hydride ion (one proton and two electrons), which is not ideal for this reaction.
5. Conclude that FAD is the preferred coenzyme for this reaction because it can effectively participate in the oxidation of succinate by accepting two hydrogen atoms, which aligns with the reaction's requirements.

Key Concepts

Citric Acid Cycle

The citric acid cycle, also known as the Krebs cycle, is a series of enzymatic reactions that occur in the mitochondria, where acetyl-CoA is oxidized to produce energy. It plays a crucial role in cellular respiration, generating ATP, NADH, and FADH₂, which are essential for the electron transport chain. Understanding this cycle is vital for grasping how energy is produced in aerobic organisms.

Succinate Dehydrogenase

Succinate dehydrogenase is an enzyme that catalyzes the oxidation of succinate to fumarate in the citric acid cycle. It is unique because it is the only enzyme that participates in both the citric acid cycle and the electron transport chain, directly linking these two metabolic pathways. This enzyme uses FAD as a coenzyme, which is crucial for its function in facilitating the transfer of electrons.

FAD vs. NAD⁺

FAD (flavin adenine dinucleotide) and NAD⁺ (nicotinamide adenine dinucleotide) are both important coenzymes involved in redox reactions. FAD is typically used in reactions that involve the formation of double bonds, such as the oxidation of succinate to fumarate, while NAD⁺ is more commonly involved in reactions that produce alcohols or aldehydes. The choice of coenzyme can influence the reaction pathway and the type of products formed.