# Explain how the amino acid sequence of a polypeptide determines the final three-dimensional shape of the protein.

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

## Mark scheme (4 marks)

1. 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.
2. 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.
3. 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.
4. 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.

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