Coenzymes that are tightly bound to the enzyme are called?

• A. Prosthetic group
• B. Cofactor
• C. Apoenzyme
• D. Holoenzyme

Answer: (A)

Tightly bound nonprotein components that are essential for enzyme activity are called prosthetic groups. This term refers to a cofactor (often an organic molecule) that remains firmly attached to the enzyme as part of its structure, forming the active holoenzyme. In contrast, many coenzymes bind loosely and can dissociate during catalysis, acting more like cosubstrates. Examples include heme in certain cytochromes and biotin in carboxylases.