Regarding cofactors in holoenzymes, which statement is true?

• A. Inactive without cofactors
• B. They are required for activity and bound to the enzyme
• C. Unrelated to enzyme activity
• D. They are substrates

Answer: (B)

Cofactors are non-protein parts that enable enzymes to catalyze reactions. When a cofactor is bound to the enzyme, the complex is the holoenzyme, the active form. Without the cofactor, the protein part (the apoenzyme) is typically unable to catalyze the reaction, so the activity depends on having that bound component. Cofactors can be inorganic ions like Mg2+ or organic molecules called coenzymes, and they work by helping shape the active site, participating in chemical steps, or stabilizing transition states. That’s why the statement that cofactors are required for activity and bound to the enzyme is the true one. They are not substrates and they are not unrelated to enzyme activity.