Explain why a protease enzyme cannot break down starch.

Cambridge International IGCSE Biology (0610) — 5.1 Enzymes · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A protease has an active site that is specifically shaped to bind a protein substrate. The active site’s shape is complementary to the peptide bonds of a protein, not to the glucose chains of starch. Consequently starch cannot bind to or enter the protease’s active site, so no enzyme–substrate complex is formed and the starch is not hydrolysed.

Examiner tips

  • Use the word ‘specifically shaped’ and ‘complementary’ to show understanding of active site specificity.
  • Explain that lack of binding prevents complex formation and therefore no reaction.
  • Keep the answer concise – 4 marks can be earned with 3–4 short sentences.

Common mistakes

  • Confusing the role of the protease with that of amylase; students may say the enzyme is inactive rather than explaining binding.
  • Using vague terms like ‘doesn’t work’ instead of ‘cannot bind’ or ‘no complex forms’.

Mark scheme (4 marks)

  1. Each enzyme has a specifically shaped active site
  2. The shape of the active site is complementary to its substrate (protein), not to starch
  3. Starch cannot bind to / enter the active site of protease
  4. Therefore no enzyme-substrate complex can form, so starch is not broken down

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