# Explain why an enzyme works best at its optimum pH and why changing the pH above or below this value reduces the rate of reaction.

> Edexcel GCSE Biology (1BI0) — 1.4 Enzymes · Explain · 4 marks

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

## Mark scheme (4 marks)

1. At the optimum pH, the active site has the correct/complementary shape to fit the substrate
2. Changing the pH above or below the optimum alters the shape of the active site
3. This is because the bonds (holding the enzyme's shape) are broken/changed by the altered pH
4. The substrate can no longer fit / bind to the active site, so fewer enzyme-substrate complexes form and the rate of reaction decreases

## Key terms

- [optimum pH](https://www.gradenine.co.uk/glossary/optimum-ph)

## Related

- [Revision notes for Edexcel GCSE Biology (1BI0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-an-enzyme-works-best-675dacef) · Published by Druglandscape Ltd.