# Explain how a change in pH above and below the optimum affects the action of an enzyme.

> Eduqas A-Level Biology — 1.4 Biological reactions and enzymes · Explain · 5 marks

> Enzymes are biological catalysts that control chemical reactions in living cells. Each enzyme has an active site with a specific shape that is complementary to its substrate.

## Mark scheme (5 marks)

1. Enzymes have an optimum pH at which they work best / fastest
2. As pH moves away from the optimum, the shape of the active site begins to change / is altered
3. This means the substrate can no longer fit into the active site / the active site is no longer complementary to the substrate
4. Fewer / no enzyme-substrate complexes can form
5. If the pH change is severe / extreme, the enzyme becomes denatured — this is a permanent change in the shape of the active site

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