Competitive Inhibition Calculator
Calculate apparent Km with competitive inhibitor. Competitive inhibitors increase apparent Km (Km_app = Km x alpha) while leaving Vmax unchanged. Find alpha factor and inhibited velocity.
🧪 Pharmacology📐 Km_app = Km x (1 + [I]/Ki)🧬 Biology
Km (uM)
Inhibitor [I] (uM)
Ki (uM)
Substrate [S] (uM)
Vmax (uM/s)
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Competitive Inhibition Calculator | — | Km_app = Km x (1 + [I]/Ki) | uM |
Step-by-Step Examples
Example 1
Classic Competitive Inhibition
Glucose/fructose compete for hexokinase. Km for glucose=0.1mM. Fructose at 5mM with Ki=5mM.
- Alpha = 1 + 5/5 = 2.0
- Km_app = 0.1 x 2 = 0.2 mM
- Vmax unchanged (10 uM/s)
- At [glucose]=0.1mM: v = 10x0.1/(0.2+0.1) = 3.3 uM/s vs v0=5 uM/s
✓ Km_app = 0.2 mM, 33% inhibition
Example 2
Statin Drug Example
HMG-CoA reductase: Km=0.4mM. Statin Ki=0.001mM at 0.01mM concentration.
- Alpha = 1 + 0.01/0.001 = 11
- Km_app = 0.4 x 11 = 4.4 mM
- Statin effectively displaces HMG-CoA at physiological concentrations
- Vmax unchanged
✓ Km_app = 4.4 mM, potent competitive inhibition
Example 3
Excess Inhibitor
At [I]=100xKi, enzyme is nearly fully inhibited.
- Alpha = 1 + 100 = 101
- Km_app = Km x 101 = very high
- To achieve half-max velocity, need [S] = 101xKm
- Practically, enzyme appears inactive
✓ Km_app increases 101-fold at [I]=100xKi
Real-World Applications
Drug Design
Competitive inhibitors mimic substrate structure to block active site — most enzyme-targeting drugs.
Metabolic Control
Competitive inhibition is a key regulatory mechanism in metabolic pathways (feedback inhibition).
Antiviral Therapy
HIV protease inhibitors and neuraminidase inhibitors compete with viral enzyme substrates.
Pharmacokinetics
Competitive inhibition explains drug-drug interactions at metabolizing enzymes (CYP450).
Common Mistakes to Avoid
⚠️
Thinking competitive inhibition lowers Vmax
Competitive inhibitors NEVER change Vmax. Only Km_app increases. With enough substrate, the inhibitor can always be outcompeted.
⚠️
Forgetting alpha depends on [I]
Alpha changes with inhibitor concentration. Report Ki, not alpha, as the fundamental constant.
⚠️
Mixing up competitive vs noncompetitive
Competitive: Vmax same, Km increases. Noncompetitive: Km same, Vmax decreases. Uncompetitive: both decrease proportionally.
Frequently Asked Questions
Can competitive inhibition be overcome with substrate? ▾
Yes! Add more [S] and you can outcompete the inhibitor, restoring Vmax. This is the defining feature of competitive inhibition.
What makes a molecule a competitive inhibitor? ▾
Usually structural similarity to the substrate, allowing binding to the same active site. Examples: methotrexate/DHFR, statins/HMG-CoA reductase.
Is competitive inhibition reversible? ▾
Classic competitive inhibition is reversible (equilibrium binding). Irreversible inhibitors (covalent) are mechanistically different.
What is the alpha factor? ▾
Alpha = 1 + [I]/Ki. It's the factor by which Km increases. At [I]=Ki, alpha=2 (Km doubles). At [I]=0, alpha=1 (no inhibition).
How does temperature affect Ki? ▾
Ki typically decreases with temperature (tighter binding at lower T for enthalpy-driven interactions). Always report Ki with temperature.