An enzyme that requires a tightly bound non-protein helper (e.g., haem in catalase) needs a
Aapoenzyme with no extra component
Bcoenzyme (loosely bound, vitamin-derived)
Cmetal ion such as Zn²⁺ or Mg²⁺
Dprosthetic group (covalently / tightly bound)
Answer & Solution
Correct answer: D. prosthetic group (covalently / tightly bound)
Cofactors come in three flavours: prosthetic groups (tightly bound, like haem), coenzymes (loosely bound, vitamin-derived like NAD), metal ions (Zn²⁺, Mg²⁺). The protein without its cofactor is the apoenzyme (inactive). Haem is the textbook prosthetic-group example.
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