Enzymes are proteins that act as biological catalysts. Explain why an enzyme stops working when the temperature is raised significantly above its optimum, referring to the structure of the protein.

WJEC A-Level Chemistry (Wales) — 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).

Enzymes are proteins found in all living organisms. Each enzyme has an active site with a specific shape that allows it to bind to its substrate. At temperatures well above the optimum, the enzyme no longer functions.

Model answer (5 marks)

Enzymes are proteins that are denatured when the temperature is raised well above their optimum.
1. The high temperature breaks the non‑covalent bonds (hydrogen bonds, ionic bonds, hydrophobic interactions) that maintain the protein’s three‑dimensional structure.
2. As these bonds break, the protein unfolds and its tertiary structure is lost.
3. The active site, which relies on the precise three‑dimensional arrangement of amino‑acid residues, changes shape or disappears.
4. Because the active site no longer has the correct geometry, the substrate can no longer bind to it.
5. Without substrate binding, the enzyme can no longer catalyse the reaction.

Examiner tips

  • Use the term ‘denatured’ early; link temperature to bond breakage. Show the chain: high T → bond break → shape change → loss of active site → no catalysis. Include the word ‘substrate’ to demonstrate understanding of the enzyme–substrate interaction.

Common mistakes

  • Confusing denaturation with inhibition; forgetting to mention bond breakage. Using vague terms like ‘changes shape’ without linking to loss of active site. Omitting the substrate‑binding step or the final loss of catalytic activity.

Mark scheme (5 marks)

  1. The enzyme is denatured (at high temperature)
  2. The bonds holding the protein's three-dimensional shape are broken
  3. The three-dimensional shape of the protein changes / the active site changes shape
  4. The substrate can no longer fit into / bind to the active site
  5. So the enzyme can no longer catalyse the reaction

Key terms in this question

enzyme

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