# A student investigates the activity of the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen. The student adds a competitive inhibitor to separate reaction mixtures, keeping all other variables constant. Explain how the presence of a competitive inhibitor reduces the rate of the catalase-catalysed reaction.

> OCR A-Level Biology A (H420) — 2.4 Enzymes · Explain · 4 marks

> Catalase is found in almost all living cells and has one of the highest turnover numbers of any known enzyme. A competitive inhibitor has a molecular shape complementary to the active site of catalase.

## Mark scheme (4 marks)

1. The competitive inhibitor has a complementary shape to the active site of catalase
2. The competitive inhibitor competes with hydrogen peroxide (substrate) for binding to the active site
3. Fewer enzyme–substrate complexes are formed per unit time
4. Therefore the rate of reaction is reduced because less product is formed per unit time

## Key terms

- [competitive inhibitor](https://www.gradenine.co.uk/glossary/competitive-inhibitor)
- [catalase](https://www.gradenine.co.uk/glossary/catalase)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](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/a-student-investigates-the-activity-of-4757b38e) · Published by Druglandscape Ltd.