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 heavy metal ions.

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 catalytic activity depends entirely on maintaining a precise three-dimensional shape. Industrial processes sometimes expose biological systems to heavy metal ions such as lead or mercury, which are known to disrupt protein structure.

Model answer (4 marks)

The tertiary structure gives the enzyme its specific three‑dimensional shape, creating an active site that is complementary to the substrate.
The shape is maintained by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions between side chains.
When a high concentration of heavy metal ions such as Pb²⁺ or Hg²⁺ is present, these ions bind to cysteine or histidine residues and break the bonds that stabilise the tertiary structure.
The enzyme denatures, the active site loses its precise shape, and it can no longer bind the substrate or catalyse the reaction.

Examiner tips

  • State the role of tertiary structure in forming the active site. Explain the types of bonds that stabilise this structure. Describe how heavy metals disrupt these bonds and the consequence for activity.

Common mistakes

  • Confusing tertiary with quaternary structure. Failing to mention specific bond types. Saying the enzyme is ‘inactivated’ without explaining denaturation.

Mark scheme (4 marks)

  1. The tertiary structure creates the specific shape of the active site, which is complementary to the substrate
  2. Tertiary structure is maintained by bonds between R groups, including hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions
  3. Heavy metal ions disrupt / break the bonds maintaining tertiary structure, causing the enzyme to denature
  4. The active site changes shape so it is no longer complementary to the substrate, and the enzyme cannot catalyse the reaction

Key terms in this question

tertiary structure · heavy metal ions

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