Explain how the structure of an enzyme's active site determines its specificity for a particular substrate.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The active site has a specific, complementary shape that fits only one substrate or a small group of structurally similar substrates.
The amino‑acid residues that line the active site form non‑covalent interactions – hydrogen bonds, ionic bonds and van der Waals forces – with particular chemical groups on the substrate.
If a molecule does not have the correct shape or the required functional groups it cannot bind to the active site, so the enzyme will not catalyse a reaction – this is substrate specificity.
The induced‑fit model shows that the active site can change shape slightly when the substrate binds, optimising the fit, lowering the activation energy and further ensuring specificity.
The amino‑acid residues that line the active site form non‑covalent interactions – hydrogen bonds, ionic bonds and van der Waals forces – with particular chemical groups on the substrate.
If a molecule does not have the correct shape or the required functional groups it cannot bind to the active site, so the enzyme will not catalyse a reaction – this is substrate specificity.
The induced‑fit model shows that the active site can change shape slightly when the substrate binds, optimising the fit, lowering the activation energy and further ensuring specificity.
Examiner tips
- Use the phrase "specific, complementary shape" to hit the first point. Mention the types of non‑covalent interactions to cover the second point. Explain the consequence of a mismatch to address the third point. Include the induced‑fit model to demonstrate the final point and show understanding of dynamic binding.
Common mistakes
- Confusing the active site with the whole enzyme – only the active site determines specificity. Forgetting to mention the types of non‑covalent interactions. Using vague terms like "good fit" without specifying shape or chemical groups.
Mark scheme (4 marks)
- The active site has a specific/complementary shape (tertiary structure) that matches only one substrate or a small group of structurally similar substrates.
- The amino acid residues lining the active site form non-covalent interactions (e.g. hydrogen bonds, ionic interactions, van der Waals forces) with specific chemical groups on the substrate.
- If a molecule lacks the correct shape or chemical groups it cannot bind to the active site, so the enzyme will not catalyse a reaction with that molecule / substrate specificity is a result of this selective binding.
- The induced-fit model describes how the active site can change shape slightly upon substrate binding to optimise the fit, further ensuring specificity and lowering the activation energy of the reaction.
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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