# Explain why the tertiary structure of an enzyme is essential for its function, and describe what happens to this structure when the enzyme is exposed to a high concentration of a heavy metal ion such as lead.

> OCR A-Level Biology A (H420) — 2.2 Biological molecules · Explain · 4 marks

> Industrial wastewater containing lead ions was accidentally released into a river. Researchers observed that fish in the affected area showed severely reduced metabolic activity, suggesting widespread enzyme inhibition.

## Mark scheme (4 marks)

1. The tertiary structure creates a specifically shaped active site that is complementary to the substrate
2. Tertiary structure is maintained by interactions between R groups, including hydrogen bonds, ionic bonds, disulfide bonds and hydrophobic interactions
3. Heavy metal ions disrupt / break the bonds maintaining tertiary structure (particularly ionic bonds and disulfide bonds)
4. This causes denaturation — the active site changes shape so the substrate can no longer bind / enzyme–substrate complexes cannot form

## Key terms

- [tertiary structure](https://www.gradenine.co.uk/glossary/tertiary-structure)
- [heavy metal ion](https://www.gradenine.co.uk/glossary/heavy-metal-ion)

## 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/explain-why-the-tertiary-structure-of-7748f2b5) · Published by Druglandscape Ltd.