Explain reversible competitive and non-competitive inhibition
Competitive inhibitors compete at the active site; non-competitive inhibitors bind elsewhere and reduce productive enzyme activity.
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
Where does a competitive inhibitor bind?
What you will learn
- Explain reversible competitive inhibition using active-site competition
- Explain reversible non-competitive inhibition through binding away from the active site
Competitive inhibitor occupies the active site
A competitive inhibitor has a sufficiently similar fit to bind at the enzyme’s active site, competing with substrate. While the inhibitor occupies the site, substrate cannot bind there to form the productive enzyme–substrate complex.
Because binding is reversible, increasing substrate concentration can make substrate more likely to occupy active sites and reduce the inhibitor’s effect. The exact pattern depends on conditions; do not infer mechanism from one rate value.
Non-competitive inhibitor binds elsewhere
A non-competitive inhibitor binds at a site other than the active site. This interaction changes enzyme activity or active-site shape so that fewer enzyme molecules form productive complexes.
Adding more substrate does not simply displace an inhibitor that binds elsewhere. Both forms of reversible inhibition lower activity, but their binding locations and responses to substrate concentration differ.
Pause and explain
What distinguishes a non-competitive inhibitor in the syllabus model?
Worked example
An inhibitor binds away from the active site and reduces product formation even when substrate is abundant. Identify the likely type and explain.
Show the worked solution
- Binding away from the active site is non-competitive.
- The binding changes enzyme activity rather than competing for the substrate site.
Answer This is consistent with non-competitive inhibition. The inhibitor binds away from the active site and reduces productive enzyme activity; adding substrate does not directly displace it from that separate site.
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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.
Explain how a competitive inhibitor reduces reaction rate.
Give me a hint
Describe where it binds and the consequence for substrate.
Compare my reasoning
- It binds reversibly at the active site.
- Substrate is less likely to bind productively while the site is occupied.
A competitive inhibitor binds reversibly at the active site and competes with substrate. When it occupies the site, a substrate molecule cannot bind there to form the productive enzyme–substrate complex, so rate falls.
Look for these in your work
- Active-site competition stated
- Effect on productive complexes explained
A reversible inhibitor binds at a separate site and changes the active site’s shape. Explain the effect.
Give me a hint
Follow the chain from separate-site binding to product formation.
Compare my reasoning
- Binding changes enzyme activity or active-site shape.
- Fewer productive enzyme–substrate complexes form.
The inhibitor binds away from the active site and changes enzyme activity or active-site shape. Fewer substrate molecules bind productively, so product forms more slowly.
Look for these in your work
- Separate site named
- Rate consequence explained
A competitive inhibitor’s effect becomes smaller when substrate concentration is raised. Explain why this is consistent with competition.
Give me a hint
Consider which molecule is more likely to occupy the shared site.
Compare my reasoning
- Substrate and inhibitor compete for the active site.
- More substrate makes productive substrate occupancy more likely.
Substrate and inhibitor compete for the active site. Raising substrate concentration makes substrate more likely to occupy the site, so the inhibitor has less effect on the measured rate under those conditions.
Look for these in your work
- Shared binding site identified
- Substrate-concentration effect explained
Common mistakes
- Saying both inhibitor types bind at the active site
- Saying a competitive inhibitor permanently destroys the active site
- Assuming adding substrate directly displaces a non-competitive inhibitor
What can you explain now?
Where does a competitive inhibitor bind?
Compare with the explanation
At the active site, competing with substrate
It competes for the substrate-binding location.
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Make it stick
Where do competitive and non-competitive inhibitors bind, and how does substrate concentration affect competitive inhibition?
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