Lesson 4 of 4 / Enzymes and investigations
Competitive and non-competitive inhibition
Can adding substrate overcome this inhibitor?
In this lesson: Compare inhibitor binding and effects on rate, including allosteric inhibition.
About 7 min
The key ideaA competitive inhibitor competes at the active site; a non-competitive model reduces functional catalytic capacity through another binding site.
Explore the idea
Can more substrate restore the rate?
Control rate: 83.3. Inhibited rate: 62.5. Increasing substrate reduces the relative effect of reversible active-site competition. Both curves approach 100 units, although neither must exactly reach its limit at a finite concentration.
Green curve: control. Orange: inhibited. Dotted horizontal line: inhibited model limit. Original illustrative rate models with enzyme amount, temperature and pH fixed; the numbers are not measurements. The binding drawing is one possible complex, while the curve describes a population of enzymes.
Where does allosteric inhibition fit?
An allosteric site is separate from the active site. Binding there can alter conformation and activity. This describes communication within the protein; it does not require every allosteric inhibitor to follow the ideal pure non-competitive curve drawn here. A curve suggests a model, while binding evidence tests the molecular explanation.
Explanation
A competitive inhibitor has properties that allow it to bind at the active site and compete with substrate. It need not be identical to the substrate, but must have suitable shape and chemistry. Increasing substrate concentration can reduce its relative occupancy and approach the uninhibited maximum rate in a reversible competitive model.
A non-competitive inhibitor binds at a site distinct from the active site. Its binding alters enzyme conformation or catalytic function, reducing the effective catalytic capacity. In the simple pure non-competitive model, increasing substrate cannot restore the original maximum rate.
An allosteric site is a regulatory site separate from the active site. Binding there can alter activity, including inhibition. Allosteric regulation describes a mechanism of conformational communication; it is not a guarantee that every real enzyme follows one idealised kinetic curve.
Interpret a family of rate-substrate curves by comparing plateaus and behaviour at low substrate. Keep enzyme concentration, temperature and pH constant. A lower plateau supports reduced catalytic capacity, but experimental evidence about binding is needed before identifying an unknown inhibitor with certainty.
Step by step
- 1
Locate binding
Active site or a separate regulatory site?
- 2
Predict substrate competition
Ask whether substrate can displace the inhibitor from the same site.
- 3
Read the maximum
Compare the high-substrate plateaus under controlled conditions.
Worked example
Work through the evidence
An inhibitor lowers rate at low substrate, but the curve approaches the control plateau at high substrate. Which simple model fits?
One way to explain it
Reversible competitive inhibition: abundant substrate increasingly outcompetes inhibitor at the active site, allowing the same maximum rate to be approached.
Why this answer works
- A shared maximum is important evidence.
- This is a model-based interpretation, not proof of a molecular binding location by itself.
Is this true? "All inhibitors are permanently attached and destroy enzymes."
Many inhibitors bind reversibly, and different binding mechanisms produce different kinetic effects.