Biology · Introductory biology · Worked example
Tell competitive from noncompetitive inhibition
An enzyme is assayed at 1 to 32 mM substrate alone and with each of two inhibitors, A and B. Fitting each series gives Vmax = 20 µmol/min and Km = 4 mM without inhibitor, Vmax = 20 and Km = 12 mM with A, and Vmax = 10 and Km = 4 mM with B. How does each inhibitor act?
Compare each fit with the control
Put the parameters side by side. Inhibitor A leaves Vmax at 20 but triples Km; inhibitor B leaves Km at 4 but halves Vmax.
| Series | Vmax (µmol/min) | Km (mM) |
|---|---|---|
| No inhibitor | 20 | 4 |
| Inhibitor A | 20 | 12 |
| Inhibitor B | 10 | 4 |
Identify inhibitor A
An unchanged Vmax with a larger apparent Km is the signature of competitive inhibition: the inhibitor competes for the active site, so more substrate is needed to reach any given rate. In the simple model the apparent Km is Km(1 + [I]/Kᵢ), so here 1 + [I]/Kᵢ = 3.
Identify inhibitor B
A lower Vmax with Km unchanged is the signature of pure noncompetitive inhibition: the inhibitor binds away from the active site, and the enzyme it holds cannot work at any substrate level. Here Vmax is divided by 1 + [I]/Kᵢ = 2.
Confirm at high substrate
At [S] = 100 mM the competitive inhibitor is nearly outcompeted, but the noncompetitive one still halves the rate.
Know what the fit cannot show
These teaching data follow the model exactly, so the fits come out exact. Real rates are noisy, and a mechanism is best supported by several inhibitor concentrations and by binding evidence, not by one pair of fitted numbers.
Result
Inhibitor A is competitive: Km rises from 4 to 12 mM while Vmax stays at 20 µmol/min. Inhibitor B is noncompetitive: Vmax halves to 10 µmol/min while Km stays at 4 mM.
Your turn
A competitive inhibitor is present at half its Kᵢ, so [I]/Kᵢ = 0.5. If Km = 4 mM without it, what is the apparent Km, and what happens to Vmax?
Show the answer and explanation
The apparent Km is 6 mM; Vmax is unchanged.
Apparent Km = Km(1 + [I]/Kᵢ) = 4(1.5) = 6 mM. A competitive inhibitor does not change Vmax.
Keep exploring
The enzyme kinetics tool opens with all three series and compares each inhibitor’s fitted Vmax and Km with the control’s: 1 and 3 times for A, 0.5 and 1 times for B.
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