Explain how the induced fit model of enzyme action differs from the lock-and-key model, and why induced fit better accounts for the catalytic activity of enzymes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
In the lock‑and‑key model the active site is rigid and the substrate fits exactly, with no change in enzyme shape.
In the induced‑fit model the active site changes shape when the substrate binds, moulding around it.
The conformational change positions catalytic residues or R‑groups correctly, or strains the substrate’s bonds, to facilitate the reaction.
Because a rigid fit would stabilise the substrate rather than destabilise its bonds, induced fit better lowers the activation energy and explains catalytic activity.
In the induced‑fit model the active site changes shape when the substrate binds, moulding around it.
The conformational change positions catalytic residues or R‑groups correctly, or strains the substrate’s bonds, to facilitate the reaction.
Because a rigid fit would stabilise the substrate rather than destabilise its bonds, induced fit better lowers the activation energy and explains catalytic activity.
Examiner tips
- Use the key terms ‘rigid’, ‘conformational change’, and ‘strain the substrate’
- Show the difference in how the active site behaves in each model
- Explain why a rigid fit cannot lower activation energy
Common mistakes
- Confusing the models by saying both involve shape change
- Failing to mention the role of catalytic residues or bond strain
- Using vague language such as ‘helps the enzyme’ without specifics
Mark scheme (4 marks)
- In the lock-and-key model, the active site is rigid/fixed in shape and the substrate fits exactly without any conformational change.
- In the induced fit model, the active site changes shape / undergoes a conformational change as the substrate binds, so the enzyme moulds around the substrate.
- The conformational change in induced fit places catalytic residues/amino acid R-groups in the correct orientation/position to facilitate the reaction / to strain or distort the substrate's bonds.
- Induced fit better accounts for catalysis because a perfectly rigid complementary fit (lock-and-key) would stabilise the substrate rather than destabilise / strain its bonds, so a static active site would not lower activation energy effectively.
Key terms in this question
induced fit model · lock-and-key model
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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