In the holoenzyme, the non-protein organic molecule that activates the apoenzyme is called a coenzyme.

• A. Apoenzyme
• B. Coenzyme
• C. Holoenzyme
• D. Proenzyme

Answer: (B)

The active enzyme is formed when the protein part (apoenzyme) binds its nonprotein helper, turning into the holoenzyme. If that helper is an organic molecule, it’s called a coenzyme. Coenzymes often derive from vitamins and provide the chemical groups needed for catalysis (for example, NAD+, FAD, or coenzyme A). This binding changes the apoenzyme into the fully active enzyme, capable of catalysis.

Apoenzyme refers to the enzyme protein by itself without any bound cofactor, so it is inactive. Holoenzyme is the complete, active enzyme with its cofactor attached. A proenzyme (zymogen) is an inactive precursor that requires a chemical modification to become active, not simply binding a cofactor.