# Explain how a change in pH away from an enzyme's optimum affects the rate of an enzyme-controlled reaction.

> WJEC A-Level Biology (Wales) — 1.4 Biological reactions and enzymes · Explain · 5 marks

> Enzymes are biological catalysts that speed up metabolic 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 / the rate of reaction is greatest at the optimum pH
2. As pH moves away from the optimum, the shape of the active site begins to change / is altered
3. This is called denaturation / the enzyme becomes denatured
4. The substrate can no longer fit into the active site / the active site is no longer complementary to the substrate
5. Therefore the rate of reaction decreases / fewer enzyme-substrate complexes form

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