Enzymes are proteins that act as biological catalysts. Explain why an enzyme stops working when it is heated to a very high temperature, referring to the structure of proteins in your answer.

Eduqas A-Level Chemistry — 4.7 Amino acids, peptides and proteins · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Proteins have a specific three‑dimensional shape that is essential for activity.
High temperature breaks the weak bonds (hydrogen, ionic, hydrophobic) that maintain this shape.
The enzyme becomes denatured, meaning its structure is permanently altered.
The active site changes shape so the substrate can no longer fit or bind.
Consequently the enzyme can no longer catalyse the reaction.

Examiner tips

  • Use the term "denatured" to show understanding of irreversible change.
  • Link the loss of structure directly to loss of active‑site function.
  • Show the chain of reasoning: temperature → bond breakage → shape change → loss of activity.
  • Mention that the change is irreversible to earn full marks.

Common mistakes

  • Confusing denaturation with simple heat inactivation without noting irreversible shape change.
  • Failing to mention the active site specifically.
  • Using vague terms like "damage" instead of "breaks the bonds" or "denatures".

Mark scheme (5 marks)

  1. Proteins have a specific three-dimensional shape / folded structure
  2. High temperature breaks the bonds that hold the protein in its shape
  3. The protein / enzyme becomes denatured (shape is permanently changed)
  4. The active site changes shape so the substrate can no longer fit / bind
  5. Therefore the enzyme can no longer catalyse the reaction

Key terms in this question

catalyst

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