Explain why the tertiary structure of an enzyme is essential for its function, referring to the role of R groups in maintaining this structure.

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 specific three-dimensional shape is critical to their activity. The tertiary structure of an enzyme arises from interactions between the R groups of its constituent amino acids.

Model answer (4 marks)

The tertiary structure of an enzyme determines the shape of its active site, which must be complementary in shape to the substrate for binding to occur.

The shape of the tertiary structure is maintained by interactions between the R groups of the amino acids, such as hydrogen bonds, ionic bonds, hydrophobic interactions and disulfide bridges.

If these R‑group interactions are disrupted (e.g. by heat, pH or chemicals) the protein unfolds – a process called denaturation – and the active site loses its specific shape.

Without a correctly shaped active site the enzyme–substrate complex cannot form, so the catalytic reaction does not take place.

Examiner tips

  • Use the term ‘active site’ and explain its complementarity to the substrate; mention specific R‑group interactions; describe denaturation and loss of activity; keep answer concise and use exam terminology.

Common mistakes

  • Confusing tertiary structure with quaternary structure; not naming specific R‑group interactions; failing to link loss of structure to loss of catalytic activity.

Mark scheme (4 marks)

  1. The tertiary structure forms the active site, which is complementary in shape to the substrate
  2. Named interaction(s) between R groups that maintain tertiary structure
  3. If these R group interactions are disrupted, tertiary structure is lost (denaturation) and the active site changes shape
  4. Without the complementary active site, the enzyme–substrate complex cannot form and catalysis does not occur

Key terms in this question

tertiary structure · R groups

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