# Explain why an enzyme stops working when the pH moves far from its optimum, and describe how this differs from the effect of temporarily lowering the temperature on the same enzyme.

> AQA A-Level Biology (7402) — 3.1.4 Proteins and Enzymes · Explain · 5 marks

## Mark scheme (5 marks)

1. Each enzyme has an optimum pH at which it works best / has maximum activity
2. pH far from optimum changes / breaks the bonds holding the active site in shape
3. The active site changes shape / is denatured so the substrate no longer fits
4. Denaturation is permanent / the enzyme cannot return to its original shape once denatured
5. Lowering temperature reduces enzyme activity / slows the rate of reaction but does NOT denature the enzyme, so activity returns when temperature is raised back / the effect is reversible

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