Explain how the sequence of amino acids in a protein determines its specific three-dimensional shape, and why this shape is important for the protein's function. Use your knowledge of protein structure to support your answer.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Enzymes are proteins that act as biological catalysts. A change in just one amino acid in an enzyme can destroy its ability to catalyse a reaction.
Model answer (5 marks)
The sequence of amino acids is encoded by the DNA and therefore is fixed for each protein. This primary sequence determines how the polypeptide chain folds through interactions such as hydrogen bonds, ionic bonds, hydrophobic interactions and disulphide bridges, giving the protein a unique tertiary structure. The tertiary structure gives the protein a specific three‑dimensional shape that is essential for its function. In an enzyme the shape creates an active site that is complementary in size, shape and chemical properties to the substrate, allowing catalysis. If one amino acid is changed, the pattern of interactions is altered, the chain folds differently, the active site is distorted and the enzyme can no longer catalyse the reaction.
Examiner tips
- Show the link from DNA to primary sequence, then to tertiary structure and function; use terms ‘primary’, ‘tertiary’, ‘active site’.
- Explain how a single amino acid change can alter folding and activity.
- Use the word ‘complementary’ when describing the active site.
Mark scheme (5 marks)
- The sequence (order) of amino acids in a protein is determined by the genetic code / DNA
- The sequence of amino acids causes the protein chain to fold into a specific three-dimensional shape
- Each protein has a unique / specific shape (tertiary structure) that is essential for its function
- For an enzyme, the specific shape forms an active site that is complementary to the substrate
- If the amino acid sequence changes, the shape of the protein changes and it can no longer function correctly
Key terms in this question
three-dimensional shape · function
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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