Explain how hydrophobic interactions and disulfide bonds each contribute to the stability of the tertiary structure of a protein.

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

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Hydrophobic R‑groups cluster together in the interior of the protein, away from the aqueous environment, minimising the free energy of the system and stabilising the folded conformation.
Disulfide bonds are covalent bonds formed between the sulfhydryl (-SH) groups of two cysteine residues, covalently cross‑linking distant parts of the polypeptide chain and holding the tertiary structure in a fixed, stable conformation.

Examiner tips

Use the exact terms ‘hydrophobic R‑groups’, ‘free energy’, ‘disulfide bonds’, ‘covalent cross‑link’, and link each to stability.
Show the cause–effect relationship: clustering → lower free energy → stability; cross‑link → fixed conformation → stability.
Keep each point short and to the point – 1 mark each.

Common mistakes

Confusing hydrophobic interactions with hydrogen bonds; writing ‘hydrogen bonds’ instead of ‘hydrophobic interactions’.
Failing to mention that disulfide bonds are covalent and involve cysteine residues; writing ‘ionic bonds’ or ‘non‑covalent’ instead.

Mark scheme (4 marks)

Hydrophobic R-groups cluster together in the interior of the protein, away from the aqueous environment.
This clustering minimises the free energy of the system / is thermodynamically favourable, thereby stabilising the folded conformation.
Disulfide bonds are covalent bonds formed between the sulfhydryl (-SH) groups of two cysteine residues.
Disulfide bonds covalently cross-link distant parts of the polypeptide chain, holding the tertiary structure in a fixed, stable conformation.

Key terms in this question

tertiary structure · hydrophobic interactions · disulfide bond