Which molecule is the other organic coenzyme listed besides NAD/NADH?

• A. Pyridoxal phosphate
• B. Biotin
• C. FAD
• D. NAD+

Answer: (A)

Coenzymes derived from vitamins play specific roles in enzyme reactions. NAD/NADH is a well-known redox coenzyme, while the other organic coenzyme here is pyridoxal phosphate, the active form of vitamin B6. It acts as a versatile coenzyme in amino acid metabolism, especially in transamination, decarboxylation, and other transformations, by carrying amino groups and forming temporary Schiff bases with amino acids. This distinct function in amino acid processing differentiates it from the others listed—biotin (carboxylation), FAD (redox reactions but a different vitamin source), and NAD+ (the paired NAD/NADH). Thus pyridoxal phosphate best fits as the other organic coenzyme alongside NAD/NADH.