Explain why the tertiary structure of an enzyme is essential for its function, referring to the role of bonds that maintain 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 function depends on the precise three-dimensional shape of their active site. This shape is determined by the tertiary structure of the polypeptide chain.

Model answer (4 marks)

The tertiary structure gives the enzyme a specific three‑dimensional shape, creating an active site that is complementary to the substrate.

The shape is maintained by non‑covalent bonds – hydrogen bonds, ionic bonds, hydrophobic interactions – and covalent disulfide bonds.

If these bonds are broken (for example by heat or a change in pH) the protein unfolds (denatures) and the active site loses its shape.

Without a complementary active site the enzyme cannot bind the substrate, so enzyme‑substrate complexes cannot form and catalysis does not occur.

Examiner tips

  • Mention the active site’s complementarity first, then list at least one bond type, explain denaturation, and link loss of shape to loss of catalysis.
  • Use the word "tertiary structure" and specific bond names; keep the answer concise and structured.

Common mistakes

  • Failing to name a bond type that stabilises tertiary structure.
  • Not linking denaturation to loss of active‑site shape and therefore loss of catalysis.

Mark scheme (4 marks)

  1. The tertiary structure creates a specifically shaped active site that is complementary to the substrate
  2. Named bond(s) that maintain tertiary structure — at least one of: hydrogen bonds, disulfide bonds, ionic bonds, hydrophobic interactions
  3. If these bonds are disrupted (e.g. by heat or pH change) the tertiary structure is altered / lost (denaturation)
  4. Altered tertiary structure means the active site is no longer complementary to the substrate, so enzyme–substrate complexes cannot form and catalysis cannot occur

Key terms in this question

tertiary structure

Related

More Biological molecules questions

▶ Try answering this question with AI marking (free) →