Lesson 4 of 4 / Enzymes and investigations
Competitive and non-competitive inhibition
Can adding substrate overcome this inhibitor?
I can compare inhibitor binding and effects on rate, including allosteric inhibition.
I can compare inhibitor structures and binding sites.Syllabus
Syllabus 9477, Core 1(r). Compare inhibitor structures and binding sites.
I can explain competitive, non-competitive and allosteric inhibition.Syllabus
Syllabus 9477, Core 1(s). Explain competitive, non-competitive and allosteric inhibition.
About 7 min
Make a guess. You are not marked.
An inhibitor slows an enzyme. You add a large excess of substrate and the rate recovers fully. What kind of inhibitor is it?
Show the answer
Competitive, because substrate outcompetes it.
A competitive inhibitor competes for the active site. With much more substrate, substrate wins most sites.
The key idea
A 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.
Allosteric enzymes behave differently from the simple models above. They usually have several subunits, each with its own active site, plus separate allosteric sites. When a substrate molecule binds to one subunit, the shape of the other subunits changes so that they bind substrate more easily. This is cooperative binding. A graph of rate against substrate concentration is therefore sigmoid (S-shaped) rather than the smooth hyperbola of a single-site enzyme: the rate rises slowly at low substrate, then steeply, then levels off.
An allosteric inhibitor binds at an allosteric site and holds the enzyme in its less active shape, so its active sites bind substrate less readily. The sigmoid curve shifts to the right and its rate at each substrate concentration falls. In end-product inhibition, the final product of a metabolic pathway is an allosteric inhibitor of an enzyme near the start of that pathway. When the product builds up, it slows its own production; when it is used up, inhibition is released and the pathway speeds up again. This is negative feedback. For example, ATP inhibits phosphofructokinase, an enzyme early in glycolysis, so a cell with plenty of ATP breaks down less glucose.
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.
Watch out for this
A student says: "All inhibitors are permanently attached and destroy enzymes." What is wrong with this?
Show the answer
Many inhibitors bind reversibly, and different binding mechanisms produce different kinetic effects.
Check your understanding
In the simple non-competitive model, adding much more substrate...
Show the answer
does not restore the original maximum rate.
Binding at a separate site reduces catalytic capacity even with abundant substrate.
Exam-style questions
Check your understanding
At low substrate, inhibitor X reduces the rate by half. At very high substrate, the rate with X equals the rate without it. Which conclusion is best supported?
Show the answer
X competes with the substrate for the active site.
Excess substrate outcompetes a competitive inhibitor, so the maximum rate is restored, as the data show.
Quick recall
Can adding substrate overcome this inhibitor?