An apoenzyme is defined as an enzyme that is

• A. Active with cofactors
• B. Inactive without cofactors
• C. Cofactors bound covalently
• D. Substrates of the enzyme

Answer: (B)

An apoenzyme is the protein part of an enzyme that cannot catalyze a reaction on its own because it lacks the necessary nonprotein cofactor. When the cofactor binds, the enzyme becomes active, forming the holoenzyme. So the apoenzyme is inactive without cofactors. The other descriptions describe the active enzyme or a permanently attached cofactor, not the protein alone, and thus don’t fit the definition.