Explain how the induced fit model accounts for the specificity and catalytic activity of enzymes.

IB DP Biology Higher Level (2023 syllabus) — C1.1 Enzymes and metabolism · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The active site is flexible, not a rigid lock.
1. When a substrate binds, its R‑groups interact with amino‑acid residues, inducing a conformational change.
2. This change positions catalytic residues optimally around the substrate.
3. The new geometry strains the substrate’s bonds, lowering the activation energy and increasing catalytic activity.
4. Only a substrate with the correct shape and chemistry can trigger this precise change, giving the enzyme its specificity.

Examiner tips

Use the word ‘induced’ to link substrate binding to conformational change; mention catalytic residues and activation energy; explain specificity as a consequence of the fit.
Keep each point short and to the point; use correct terminology (active site, conformational change, catalytic residues).

Common mistakes

Describing the active site as a rigid lock; forgetting to mention catalytic residues or activation energy; claiming all substrates fit the enzyme, ignoring specificity.

Mark scheme (4 marks)

The active site of an enzyme is not a rigid, pre-formed complementary shape but is flexible/dynamic, changing conformation upon substrate binding.
Interaction between the substrate and R-groups/amino acid residues of the active site induces a conformational change that optimally positions catalytic residues around the substrate.
The conformational change creates mechanical strain/distortion in the substrate's bonds, lowering the activation energy of the reaction.
Specificity is explained because only a substrate with the correct molecular geometry/complementary chemical properties can trigger the precise conformational change, so non-complementary molecules cannot induce a productive fit.

Key terms in this question

induced fit model · specificity