Compare the lock-and-key and induced-fit hypotheses
The two models explain enzyme specificity; induced fit includes a substrate-related change in active-site shape.
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons teach enzyme concepts and experimental reasoning. Written practice is self-checked; it does not demonstrate supervised practical competence or replace teacher-reviewed assessment.
Before you begin
- Recognise that proteins have specific three-dimensional shapes and that reaction rates can be compared using controlled measurements.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which hypothesis describes a binding-related active-site shape change?
What you will learn
- Explain enzyme specificity using the lock-and-key hypothesis
- Describe induced fit and distinguish the two models
Specificity depends on complementary interactions
An enzyme is specific because only substrates with suitable shape and chemical properties can bind productively at its active site. A substrate that does not fit or interact appropriately is unlikely to form the required enzyme–substrate complex.
The lock-and-key hypothesis treats the active site as having a complementary shape before substrate binding. The substrate is likened to a key that fits a particular lock. This is a useful model of specificity, not a literal mechanism.
Induced fit includes a shape change
In the induced-fit hypothesis, substrate binding causes the active site to change shape slightly so that it fits the substrate more closely. The altered enzyme–substrate interaction helps catalysis.
Both hypotheses explain specificity and enzyme–substrate binding. The distinction is whether the active site is treated as already complementary (lock and key) or changes shape on binding (induced fit). Real enzyme action involves chemical interactions; diagrams simplify them.
Pause and explain
Why is an enzyme specific?
Worked example
A diagram shows an active site changing shape as the correct substrate binds. Which model does it represent?
Show the worked solution
- A shape change caused by binding is the defining idea of induced fit.
- A permanently complementary, unchanged site is lock and key.
Answer The diagram represents induced fit: substrate binding causes the active site to change shape so that it fits the substrate more closely.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State one feature of the lock-and-key hypothesis.
Give me a hint
Describe the active site before binding.
Compare my reasoning
- The active site has a complementary shape.
- A suitable substrate fits it to form the enzyme–substrate complex.
The lock-and-key hypothesis treats the active site as having a shape complementary to a particular substrate, which can bind to form an enzyme–substrate complex.
Look for these in your work
- Initial active-site shape described
- Specific binding stated
Distinguish induced fit from lock and key.
Give me a hint
Focus on whether active-site shape changes on binding.
Compare my reasoning
- Lock and key treats the active site as already complementary.
- Induced fit proposes that substrate binding changes the site shape slightly.
In lock and key, the active site is treated as already complementary to the substrate. In induced fit, substrate binding causes a slight shape change so the active site fits more closely.
Look for these in your work
- Both models represented
- Shape-change distinction explicit
A second molecule binds an enzyme weakly but does not form products. Explain why this observation is consistent with specificity.
Give me a hint
Binding alone does not guarantee productive catalysis.
Compare my reasoning
- The molecule may not have the right shape or chemical interactions.
- Without a productive enzyme–substrate complex, products need not form.
The molecule may bind weakly but lack the shape or chemical complementarity needed for a productive enzyme–substrate complex. Specificity concerns effective binding and catalysis, not simply any contact with the enzyme.
Look for these in your work
- Weak contact distinguished from productive binding
- Specificity applied
Common mistakes
- Saying the lock-and-key model includes a binding-induced shape change
- Treating the models as literal mechanical locks
- Saying any substrate can bind any active site
What can you explain now?
Which hypothesis describes a binding-related active-site shape change?
Compare with the explanation
Induced fit
Induced fit includes a shape change on substrate binding.
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Make it stick
How does induced fit differ from lock and key, and what does specificity mean?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.