# Explain how the properties of the R-groups of amino acids contribute to the stabilisation of tertiary protein structure.

> IB DP Biology Higher Level (2023 syllabus) — B1.2 Proteins · Explain · 4 marks

## Mark scheme (4 marks)

1. Hydrophobic R-groups are non-polar and cluster towards the interior of the protein, away from the aqueous environment, stabilising the structure by reducing contact with water.
2. Polar or charged R-groups can form hydrogen bonds with each other or with water at the protein surface, contributing to tertiary structure stabilisation.
3. Oppositely charged R-groups (e.g. one acidic and one basic side chain) can form ionic bonds / electrostatic interactions that hold distant parts of the polypeptide together.
4. Cysteine R-groups contain a sulfhydryl (-SH) group that can form covalent disulfide bonds with other cysteine residues, providing strong stabilisation of tertiary (or quaternary) structure.

## Key terms

- [R-group](https://www.gradenine.co.uk/glossary/r-group)

## Related

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-properties-of-the-abd84ac4) · Published by Druglandscape Ltd.