# Explain how pH affects the rate of an enzyme-catalysed reaction.

> IB DP Biology Standard Level (2023 syllabus) — C1.1 Enzymes and metabolism · Explain · 4 marks

## Mark scheme (4 marks)

1. Each enzyme has an optimum pH at which the rate of reaction is greatest.
2. The shape of the active site depends on the pattern of ionic / hydrogen bonds that maintain the tertiary structure of the enzyme; changes in pH alter the ionisation of R-groups (amino acid side chains).
3. At pH values above or below the optimum, the active site changes shape so the substrate no longer fits / enzyme–substrate complementarity is reduced, lowering the rate of reaction.
4. At extreme pH values, denaturation occurs — the tertiary structure is permanently disrupted so the enzyme cannot function even if pH is restored.

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