# Explain how changes in pH affect the rate of an enzyme-catalysed reaction, with reference to the molecular basis of enzyme structure and active site function.

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

## Mark scheme (4 marks)

1. The active site of an enzyme has a specific three-dimensional shape determined by the sequence and interactions of amino acids, including R-group charges influenced by pH.
2. Changes in pH alter the ionisation state of acidic and basic R groups, disrupting ionic bonds and hydrogen bonds that maintain the tertiary structure of the enzyme.
3. Distortion or loss of the active site shape means substrate can no longer bind / enzyme–substrate complementarity is lost, reducing reaction rate.
4. At extreme pH values denaturation is irreversible; each enzyme has an optimum pH at which the active site conformation is best maintained and reaction rate is maximal.

## Key terms

- [active site](https://www.gradenine.co.uk/glossary/active-site)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-changes-in-ph-affect-5b8f004c) · Published by Druglandscape Ltd.