What is the role of NAD+/NADH in metabolism?

• A. NAD+ accepts electrons (reduced to NADH) in catabolic reactions; NADH donates electrons to the electron transport chain to generate ATP.
• B. NAD+ donates electrons to the electron transport chain to generate ATP.
• C. NADH accepts electrons from the electron transport chain to generate ATP.
• D. NAD+ is only involved in anabolic reactions.

Answer: (A)

NAD+/NADH act as redox carriers that shuttle reducing equivalents in metabolism. In catabolic pathways, NAD+ accepts electrons (as a hydride) from substrates and becomes NADH, capturing energy in a portable form. NADH then delivers those electrons to the mitochondrial electron transport chain, where their energy is used to pump protons and drive ATP synthesis via oxidative phosphorylation. After donating electrons, NADH is converted back to NAD+, so the cycle can continue. This setup is essential for turning the energy from nutrient oxidation into ATP. It’s worth noting that NADPH serves a different role in biosynthesis, so NAD+ is not restricted to anabolic processes.