3.1.7Inquiry Process
Enzyme Inhibition — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 107 of 160
Section 1 of 4Competitive Inhibition
Inhibitors reduce the rate of reaction by interfering with an enzyme, and may act at the active site or elsewhere.
Inhibitor shape
- The inhibitor has a shape similar to the substrate, so it can fit into the active site of the enzyme.
Competition
- Substrate and inhibitor compete for the same active sites, so the number of sites available to the substrate is reduced.
Reversible binding
- The inhibitor is not changed or used up;
- it binds and leaves repeatedly, so the effect depends on relative concentrations.
Effect of more substrate
- Raising substrate concentration lowers the effect of the inhibitor because the substrate wins the competition more often.
Example
- Malonate resembles succinate and competes with it for the active site of succinate dehydrogenase in the Krebs cycle.
Section 2 of 4Non-Competitive Inhibition
Binding site
- The inhibitor binds to an allosteric site elsewhere on the enzyme, not to the active site itself.
Shape change
- Binding alters the tertiary structure, so the active site changes shape and the substrate no longer fits as well.
No competition
- Since substrate and inhibitor bind at different places, adding more substrate cannot fully overcome the inhibition.
Lower maximum rate
- The maximum rate is lower because a fraction of the enzyme molecules is out of action at any moment.
Example
- Cyanide binds to cytochrome c oxidase in the electron transport chain and blocks aerobic respiration.
| Feature | Competitive | Non-competitive |
|---|---|---|
| Binds at | Active site | Allosteric site |
| Inhibitor resembles substrate? | Yes | No |
| Effect of extra substrate | Reduces inhibition | Little effect |
| Maximum rate reached | Same as without inhibitor | Lower than without inhibitor |
Section 3 of 4Interpreting the Graphs
Interpreting the Graphs
No inhibitor
- Rate rises with substrate concentration and levels off at the maximum rate when all active sites are occupied.
Competitive
- At low substrate concentration the rate is lower, but at high substrate concentration the curve reaches the same maximum.
Non-competitive
- The whole curve is lower and levels off at a lower maximum rate, however much substrate is added.
Uses
- Many drugs and poisons are enzyme inhibitors, and metabolic regulation also relies on inhibition.
Section 4 of 4Exam Checklist & Tips
3.1.7 – Key Points for the Exam
- Inhibitor A molecule that reduces the activity of an enzyme.
- Competitive inhibitor An inhibitor that binds to the active site, competing with the substrate.
- Non-competitive inhibitor An inhibitor that binds to an allosteric site and changes the shape of the enzyme.
- Allosteric site A site other than the active site where a molecule can bind to the enzyme.
Common Mistakes
- Saying that all inhibitors denature the enzyme; many inhibitors bind reversibly and do not denature it.
- Claiming that extra substrate overcomes non-competitive inhibition, when it does not.
- Writing that a competitive inhibitor lowers the maximum rate; it only slows the approach to it.
- Confusing the allosteric site with the active site.
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