Competitive Noncompetitive Uncompetitive No inhibitor (dashed reference)
Read the intercepts, not the slopesOn a Lineweaver-Burk plot the y-intercept is 1/Vmax and the x-intercept is −1/Km. A line that meets the y-axis at the same point has an unchanged Vmax; a line that meets the x-axis at the same point has an unchanged Km. Two glances answer the question.
Mixed inhibitionA mixed inhibitor binds both free enzyme and the ES complex, but not equally. Vmax always falls; Km rises or falls depending on which form it prefers. Noncompetitive is simply the special case where it binds both exactly equally and Km does not move.
Why uncompetitive looks backwardsIt binds only the enzyme-substrate complex, so it removes ES from the equilibrium and pulls more enzyme into binding substrate. Apparent affinity improves (Km falls) while activity drops, which is why it is the only pattern where Km and Vmax move in the same direction.
Only three numbers ever change: where the curve saturates, where it reaches half-saturation, and whether more substrate can rescue it. Competitive is the only one substrate can out-compete, which is why it is the only one that leaves Vmax alone.
A competitive inhibitor looks enough like the real substrate to bind the active site, but it cannot be catalyzed. When the inhibitor is sitting in the active site, the enzyme cannot do its job. When a real substrate arrives, it has to wait its turn - or, if there is a lot of substrate, it just outcompetes the inhibitor.
That word “outcompete” is the entire concept. Competitive inhibition can be reversed by adding more substrate. This is why Vmax does not change: with enough substrate, the enzyme still hits its full speed. Only the apparent Km goes up - it takes more substrate to reach half-Vmax because some of the substrate is being wasted fighting the inhibitor for active sites.
Effect on Km and Vmax
| Parameter | Change | Why |
|-----------|--------|-----|
| Km (apparent) | Increases | You need more substrate to reach half-Vmax because some substrate is wasted displacing inhibitor |
| Vmax | Unchanged | At infinite [S], substrate always wins; the enzyme still reaches its full speed |
| Binding site | Active site only | Inhibitor binds where substrate does |
Lineweaver-Burk Signature
On Lineweaver-Burk, competitive inhibition produces lines that all cross at the y-intercept. The y-intercept (1/Vmax) is fixed because Vmax is unchanged. The x-intercept (-1/Km) moves rightward (closer to zero), showing that Km has increased.
Real Drug Examples
Competitive inhibitors make excellent drugs because the body can flush them out, and more substrate restores normal function.
Statins (atorvastatin, simvastatin) competitively inhibit HMG-CoA reductase, the rate-limiting enzyme of cholesterol synthesis.
Methotrexate is a folate analog that competitively inhibits dihydrofolate reductase, used in cancer chemotherapy and autoimmune disease.
Ethanol is given as a treatment for methanol poisoning because it competes with methanol for alcohol dehydrogenase, slowing the production of toxic formaldehyde.
How does a competitive inhibitor affect Vmax and Km?
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Vmax is unchanged. Apparent Km increases. Because the inhibitor binds the active site but can be displaced by substrate, adding enough substrate still brings the enzyme to full speed - only more substrate is required to reach half-max velocity.
Methotrexate competes with folate at dihydrofolate reductase. On a Lineweaver-Burk plot with and without methotrexate, where will the two lines meet?
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They will meet on the y-axis at the same y-intercept (1/Vmax). This is the hallmark of competitive inhibition - Vmax is preserved, so the y-intercepts are identical, while the x-intercepts (reflecting Km) shift.
Why is ethanol given as a treatment for methanol poisoning?
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Ethanol competitively inhibits alcohol dehydrogenase, the same enzyme that converts methanol to toxic formaldehyde. By saturating the enzyme with ethanol, you slow methanol oxidation long enough for the body to clear it unchanged. It is a deliberate therapeutic competitive inhibition.
