Step by Step
1
The mechanism
A competitive inhibitor binds the SAME active site as the substrate, directly competing for that single binding location.
2
Effect on Km and Vmax
Km appears to increase (since more substrate is now needed to outcompete the inhibitor for the active site), while Vmax remains unchanged.
3
Why it can be overcome
Adding enough additional substrate can outcompete the inhibitor for the active site, eventually restoring the enzyme's full Vmax — this is the defining feature that distinguishes competitive from non-competitive inhibition.
4
A clinical example
Methotrexate competes directly with folate at the enzyme dihydrofolate reductase (DHFR) — a clinically important example of competitive inhibition used in cancer and autoimmune disease treatment.
Applied Walkthrough
1
A competitive inhibitor binds the exact same active site the substrate would normally occupy, directly competing with it for that single binding location.
2
This competition makes the enzyme appear to need more substrate to reach half-maximal velocity — an apparent increase in Km — even though Vmax itself remains completely unaffected.
3
Because the inhibitor and substrate are directly competing for the same site, simply adding enough extra substrate can outcompete the inhibitor, eventually restoring the enzyme's full Vmax — a defining, testable feature of competitive inhibition.
4
Methotrexate exemplifies this mechanism clinically, competing directly with folate at the enzyme dihydrofolate reductase — a competitive inhibition mechanism exploited therapeutically in treating certain cancers and autoimmune conditions.
Exam Application
Exams test whether you understand competitive inhibition's mechanism (same active site as substrate), its specific effect on Km (increases) and Vmax (unchanged), and whether you know it can be overcome by adding more substrate.
⚠ Common Trap
The most common trap is confusing competitive inhibition's effect on Km and Vmax with non-competitive inhibition's effect — competitive inhibition increases Km while leaving Vmax unchanged, and crucially CAN be overcome by adding more substrate, unlike non-competitive inhibition, covered in the next lesson.
✓ Quick Self-Check
1. Where does a competitive inhibitor bind, relative to the substrate?
The same active site.
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2. What happens to Km with competitive inhibition?
It increases (appears to).
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3. What happens to Vmax with competitive inhibition?
It remains unchanged.
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4. Can competitive inhibition be overcome, and how?
Yes — by adding enough additional substrate to outcompete the inhibitor.
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5. What is a clinical example of competitive inhibition?
Methotrexate competing with folate at dihydrofolate reductase (DHFR).
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