Explain how the amino acid sequence of a polypeptide determines the final three-dimensional shape of the protein.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The amino‑acid sequence (primary structure) is encoded by the gene and is determined by the sequence of codons in the mRNA, giving each protein a unique arrangement of R‑groups.
These R‑groups interact with each other and with water. Hydrogen bonds, ionic bonds, hydrophobic interactions and disulfide bonds between R‑groups drive the folding of the polypeptide.
Local, repetitive interactions between the backbone –NH and –C=O groups (independent of the R‑groups) form secondary structures such as α‑helices and β‑pleated sheets; the pattern of these structures is also dictated by the sequence.
The overall tertiary (and quaternary) shape is the lowest‑free‑energy conformation for that specific sequence, so a given sequence consistently and spontaneously folds into the same functional three‑dimensional shape.
These R‑groups interact with each other and with water. Hydrogen bonds, ionic bonds, hydrophobic interactions and disulfide bonds between R‑groups drive the folding of the polypeptide.
Local, repetitive interactions between the backbone –NH and –C=O groups (independent of the R‑groups) form secondary structures such as α‑helices and β‑pleated sheets; the pattern of these structures is also dictated by the sequence.
The overall tertiary (and quaternary) shape is the lowest‑free‑energy conformation for that specific sequence, so a given sequence consistently and spontaneously folds into the same functional three‑dimensional shape.
Examiner tips
- Mention the gene → mRNA → codon sequence to show primary structure; include R‑group interactions; state secondary structures; finish with lowest‑free‑energy tertiary shape.
- Use the exact terms: primary, secondary, tertiary, quaternary, R‑group, hydrogen bond, ionic bond, hydrophobic interaction, disulfide bond, lowest free‑energy conformation.
Common mistakes
- Confusing the role of R‑groups with backbone interactions; forgetting to mention secondary structures; using vague terms like ‘folding’ without specifying the forces involved.
Mark scheme (4 marks)
- The amino acid sequence (primary structure) is encoded by the gene / determined by the sequence of codons in mRNA, so each protein has a unique sequence of R-groups.
- The R-groups interact with each other and with water; these interactions (hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bonds) between R-groups drive the folding of the polypeptide.
- Local, repetitive interactions between backbone –NH and –C=O groups (not R-groups) produce secondary structures such as α-helices and β-pleated sheets, the pattern of which is also dictated by the sequence.
- The overall tertiary (and quaternary) shape is the lowest free-energy conformation for that specific sequence; therefore a given sequence consistently and spontaneously folds into the same functional three-dimensional shape.
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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