Explain why the tertiary structure of an enzyme is essential for its function, and describe what happens to this structure when the enzyme is exposed to a high concentration of a heavy metal ion such as lead.

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).

Industrial wastewater containing lead ions was accidentally released into a river. Researchers observed that fish in the affected area showed severely reduced metabolic activity, suggesting widespread enzyme inhibition.

Model answer (4 marks)

The tertiary structure of an enzyme gives the enzyme a specific three‑dimensional shape, creating an active site that is complementary in shape and chemical properties to its substrate. This shape is maintained by interactions between side chains, such as hydrogen bonds, ionic bonds, disulfide bonds and hydrophobic interactions. When a high concentration of a heavy metal ion such as lead is present, the metal ion binds to thiol or carboxyl groups and disrupts these stabilising interactions, especially ionic and disulfide bonds. The loss of these interactions causes the protein to unfold or change conformation – a process called denaturation. The active site then loses its correct shape, so the substrate can no longer bind and enzyme–substrate complexes cannot form, leading to reduced metabolic activity.

Examiner tips

  • Use the term "tertiary structure" and "active site" early. Mention the specific stabilising interactions. Explain how lead disrupts these bonds. State that denaturation changes the active site and stops catalysis.

Common mistakes

  • Confusing secondary with tertiary structure. Forgetting to mention disulfide bonds. Saying the enzyme is "inactivated" without explaining the loss of the active site shape.

Mark scheme (4 marks)

  1. The tertiary structure creates a specifically shaped active site that is complementary to the substrate
  2. Tertiary structure is maintained by interactions between R groups, including hydrogen bonds, ionic bonds, disulfide bonds and hydrophobic interactions
  3. Heavy metal ions disrupt / break the bonds maintaining tertiary structure (particularly ionic bonds and disulfide bonds)
  4. This causes denaturation — the active site changes shape so the substrate can no longer bind / enzyme–substrate complexes cannot form

Key terms in this question

tertiary structure · heavy metal ion

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