Explain how the structure of an enzyme allows it to break down only one specific type of substrate. In your answer, refer to the active site and the 'lock and key' hypothesis.

WJEC A-Level Biology (Wales) — 1.1 Chemical elements and biological molecules · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

The enzyme has a specific active site that is shaped to fit only one particular substrate.
The active site is complementary in shape to that substrate, allowing the substrate to enter and form an enzyme‑substrate complex.
Only this one substrate can fit the active site, so the enzyme cannot act on other molecules – this is the ‘lock and key’ hypothesis.
When the substrate binds, the enzyme catalyses its breakdown, the products are released, and the enzyme is unchanged and can be reused.

Examiner tips

  • Use the phrase ‘specific shape’ for the active site. Mention the enzyme‑substrate complex. State the lock‑and‑key idea. Show that the enzyme is unchanged after the reaction.

Common mistakes

  • Confusing the active site with the whole enzyme. Saying the enzyme can act on many substrates. Forgetting that the enzyme is unchanged after the reaction.

Mark scheme (5 marks)

  1. The enzyme has an active site with a specific shape
  2. The active site is complementary to the shape of one specific substrate
  3. The substrate enters the active site to form an enzyme-substrate complex
  4. Only one substrate can fit the active site, so the enzyme cannot break down other substrates — linked to the 'lock and key' hypothesis
  5. The substrate is broken down and the product is released, and the enzyme is unchanged / can be reused

Key terms in this question

active site · substrate

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