Which molecule is an organic coenzyme?

• A. NAD/NADH
• B. FAD
• C. CoA
• D. Biotin

Answer: (A)

Coenzymes are organic molecules that assist enzymes in catalysis, often derived from vitamins. The molecule NAD/NADH is the classic organic coenzyme that acts as a universal electron carrier in many metabolic reactions. In its oxidized form, NAD+, it accepts a hydride from substrates during dehydrogenase steps, becoming NADH. Later, NADH donates those electrons to the electron transport chain to help drive ATP production. This redox cycling between NAD+ and NADH is central to energy metabolism, spanning glycolysis, the TCA cycle, and oxidative phosphorylation, and it reflects why NAD/NADH is the quintessential organic coenzyme.

FAD is another organic coenzyme with a redox role, often functioning within flavoproteins, but NAD/NADH is more broadly representative of electron transfer in many routes. Coenzyme A carries acyl groups rather than acting primarily as an electron carrier, and biotin serves as a carrier for CO2 in carboxylation reactions. NAD/NADH’s broad involvement in energetics and its well-known redox cycling make it the best answer here.