How do dehydrogenases and oxidoreductases differ in cofactor usage (NAD+/NADP+) and cellular roles?

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Cofactor choice in dehydrogenases determines whether the reaction contributes to energy production or to biosynthesis. Dehydrogenases are oxidoreductases that transfer electrons from a substrate to a redox cofactor. The two main cofactors, NAD+ and NADP+, are not interchangeable because they support different cellular needs: NAD+ accepts electrons to form NADH, which primarily feeds into the electron transport chain to generate ATP in catabolic (energy-producing) pathways. NADP+ accepts electrons to form NADPH, which provides reducing power for anabolic (biosynthetic) reactions and for antioxidant defenses. The balance of NADH/NAD+ versus NADPH/NADP+ pools helps steer reversible reactions in the direction that fits the cell’s current state. Some dehydrogenases do use other cofactors like FAD or FMN, especially in the mitochondrial respiratory chain, but many rely on NAD+ or NADP+. Therefore, it’s not correct to claim NAD+ is never used or that all dehydrogenases use FAD only.