Which coenzyme is a key electron carrier in redox reactions?

• A. FAD and FADH2
• B. Coenzyme A
• C. NAD+ and NADH
• D. ATP

Answer: (C)

Redox reactions in metabolism rely on molecules that can shuttle electrons between reactions, and the most versatile and widely used carrier for this purpose is NAD+ when oxidized and NADH when reduced. NAD+ accepts two electrons and a proton (a hydride) from substrate molecules, becoming NADH. This stores energy that can later be released when NADH donates those electrons to the electron transport chain to drive ATP synthesis, regenerating NAD+ in the process. Because NAD+/NADH participate in a vast array of dehydrogenase reactions throughout catabolic and anabolic pathways, they function as the main electron carriers in many redox steps.

FAD/FADH2 are also electron carriers but are more limited to specific enzyme systems. Coenzyme A transports acyl groups, not electrons, and ATP serves as the energy currency rather than a direct electron carrier.