Explain how the properties of the R-groups of amino acids contribute to the stabilisation of tertiary protein structure.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Hydrophobic R‑groups are non‑polar and tend to cluster in the protein interior, away from water, reducing contact with the aqueous environment and stabilising the fold.
Polar or charged R‑groups can form hydrogen bonds either with each other or with water molecules at the protein surface, helping to maintain the tertiary structure.
Oppositely charged R‑groups (e.g. an acidic side chain and a basic side chain) can form ionic bonds or electrostatic interactions that link distant parts of the polypeptide chain.
Cysteine R‑groups contain a sulfhydryl (-SH) group that can form covalent disulfide bonds with another cysteine, providing strong stabilisation of the tertiary (and sometimes quaternary) structure.
Polar or charged R‑groups can form hydrogen bonds either with each other or with water molecules at the protein surface, helping to maintain the tertiary structure.
Oppositely charged R‑groups (e.g. an acidic side chain and a basic side chain) can form ionic bonds or electrostatic interactions that link distant parts of the polypeptide chain.
Cysteine R‑groups contain a sulfhydryl (-SH) group that can form covalent disulfide bonds with another cysteine, providing strong stabilisation of the tertiary (and sometimes quaternary) structure.
Examiner tips
- Use the exact terms ‘hydrophobic’, ‘hydrogen bonds’, ‘ionic bonds’, ‘disulfide bonds’.
- Show how each interaction contributes to stability (e.g. ‘reduces contact with water’).
- Include both intra‑protein and protein‑water interactions.
Mark scheme (4 marks)
- 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.
- Polar or charged R-groups can form hydrogen bonds with each other or with water at the protein surface, contributing to tertiary structure stabilisation.
- 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.
- 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 in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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