# Explain why an enzyme stops working when the pH moves far from its optimum.

> Cambridge International IGCSE Biology (0610) — 5.1 Enzymes · Explain · 4 marks

## Mark scheme (4 marks)

1. Change in pH alters the shape of the active site
2. Substrate can no longer fit into / bind to the active site
3. No enzyme-substrate complexes can form
4. The enzyme has been denatured / the change is permanent

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