Explain why the tertiary structure of an enzyme is essential for its function, and describe one way in which a permanent change to this structure would affect the enzyme's ability to catalyse a reaction.

OCR A-Level Biology A (H420) — 2.2 Biological molecules · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Enzymes are globular proteins whose function depends on maintaining a precise three-dimensional shape. Certain environmental conditions or chemical agents can alter this shape irreversibly.

Model answer (4 marks)

The tertiary structure gives the enzyme its specific three‑dimensional shape, which defines the geometry of the active site.

Because the active site is complementary to the substrate, the enzyme can form an enzyme‑substrate complex.

A permanent change to the tertiary structure denatures the enzyme, altering the shape of the active site.

The enzyme‑substrate complex can no longer form, so the reaction rate falls to zero – no catalysis occurs.

Examiner tips

  • Use the term ‘tertiary structure’ and ‘active site’ – these are key words. Explain the complementarity of active site and substrate. Show the causal chain: change → denaturation → loss of active site → no complex → no reaction. Keep the answer concise – 4 points only.

Common mistakes

  • Using ‘secondary structure’ instead of tertiary. Failing to mention that the active site is complementary to the substrate. Saying the enzyme is ‘inactivated’ without explaining the loss of the active site or the reaction rate.

Mark scheme (4 marks)

  1. The tertiary structure determines the shape of the active site
  2. The active site is complementary to the substrate, allowing an enzyme-substrate complex to form
  3. A permanent change to tertiary structure denatures the enzyme, altering the shape of the active site
  4. The enzyme-substrate complex can no longer form, so the rate of reaction decreases to zero / no catalysis occurs

Key terms in this question

tertiary structure

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