The protein portion of an enzyme is called:

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Question

The protein portion of an enzyme is called:

Accepted Answer

Correct Answer: D — ApoenzymeStep-by-step Solution:Many enzymes require additional non-protein components for their catalytic activity. Understanding the terminology:Apoenzyme: The protein portion of an enzyme alone (without its cofactor) — inactive by itself. 'Apo' means 'without.' The apoenzyme provides the structural framework and determines substrate specificity.Cofactor: The non-protein component required for enzyme activity. Cofactors can be inorganic ions (e.g., Fe²⁺ for catalase, Zn²⁺ for carbonic anhydrase, Mg²⁺ for hexokinase, Cu²⁺ for cytochrome oxidase) or organic molecules (coenzymes).Coenzyme: An organic (carbon-containing) cofactor that is loosely bound to the apoenzyme. Many coenzymes are vitamins or derived from vitamins. Examples: NAD⁺ (from niacin/vitamin B3), FAD (from riboflavin/vitamin B2), Coenzyme A (from pantothenic acid), pyridoxal phosphate (from vitamin B6).Prosthetic group: An organic cofactor that is tightly/covalently bound to the apoenzyme — it remains permanently attached. Examples: heme group in hemoglobin and cytochromes; biotin in pyruvate carboxylase; FAD in succinate dehydrogenase.Holoenzyme: The complete, active enzyme = Apoenzyme + Cofactor (whether inorganic ion, coenzyme, or prosthetic group).Extended Explanation:Summary table: Apoenzyme (protein part, inactive alone) + Cofactor (non-protein) = Holoenzyme (active enzyme). Cofactors include: Inorganic ions (metal activators — Fe²⁺, Zn²⁺, Mg²⁺, Cu²⁺, Mn²⁺, K⁺) and Organic cofactors (prosthetic groups if tightly bound; coenzymes if loosely bound). Coenzymes function as carriers of chemical groups, electrons, or hydrogen: NAD⁺/NADH — carries hydrogen/electrons; FAD/FADH₂ — carries hydrogen; Coenzyme A — carries acyl groups; ATP — carries phosphate groups; Biotin — carries CO₂. The apoenzyme-cofactor distinction is fundamental to understanding enzyme regulation and the role of vitamins in metabolism (vitamin deficiencies impair enzyme function because vitamins serve as coenzyme precursors).Key Concepts: Enzyme structure, apoenzyme, holoenzyme, cofactors, coenzyme, prosthetic group, enzyme activity.

The protein portion of an enzyme is called:

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The protein portion of an enzyme is called:

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