Which statement correctly describes apoenzyme and holoenzyme?

Which statement correctly describes apoenzyme and holoenzyme?

• A. Apoenzyme is the active form
• B. Apoenzyme is inactive until a cofactor binds to form a holoenzyme
• C. The holoenzyme is always inactive
• D. Cofactors never bind to enzymes

Answer: (B)

A key idea is that enzymes can exist in two forms: the apoenzyme, which is the protein part without a bound non-protein factor, and the holoenzyme, which is the complete, active enzyme when a cofactor is bound. The apoenzyme alone cannot catalyze reactions because it lacks the necessary component provided by the cofactor. When the cofactor binds, the enzyme becomes the holoenzyme, the active form capable of catalysis. So describing the apoenzyme as inactive until a cofactor binds to form the holoenzyme accurately captures their relationship. The other statements don’t fit: the apoenzyme is not the active form, the holoenzyme is not always inactive, and cofactors do bind to enzymes to enable activity (they can be metal ions or organic molecules).

A key idea is that enzymes can exist in two forms: the apoenzyme, which is the protein part without a bound non-protein factor, and the holoenzyme, which is the complete, active enzyme when a cofactor is bound. The apoenzyme alone cannot catalyze reactions because it lacks the necessary component provided by the cofactor. When the cofactor binds, the enzyme becomes the holoenzyme, the active form capable of catalysis. So describing the apoenzyme as inactive until a cofactor binds to form the holoenzyme accurately captures their relationship. The other statements don’t fit: the apoenzyme is not the active form, the holoenzyme is not always inactive, and cofactors do bind to enzymes to enable activity (they can be metal ions or organic molecules).