Enzyme Inhibition Calculator
Calculate enzyme inhibition constants Ki from IC50 values. Use Cheng-Prusoff equation: Ki = IC50 / (1 + [S]/Km) for competitive inhibition. Determine inhibitor type and potency.
🧪 Pharmacology📐 Ki = IC50 / (1 + [S]/Km)🧬 Biology
IC50 (nM)
Substrate [S] (uM)
Michaelis constant Km (uM)
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Enzyme Inhibition Calculator | — | Ki = IC50 / (1 + [S]/Km) | nM or uM |
Step-by-Step Examples
Example 1
Kinase Inhibitor
A drug has IC50=50nM vs a kinase with Km=10uM for ATP, assayed at [ATP]=10uM.
- Ki = 50 / (1 + 10/10) = 50 / 2 = 25 nM
- Correction factor: 2x (assay at [S]=Km)
- True inhibitory constant Ki = 25 nM
✓ Ki = 25 nM
Example 2
Protease Inhibitor
IC50=200nM for a protease, Km=500uM, assay at [S]=100uM.
- Alpha = 1 + 100/500 = 1.2
- Ki = 200/1.2 = 167 nM
- Small correction since [S] << Km
✓ Ki = 167 nM
Example 3
High Substrate Assay
IC50=1000nM, Km=5uM, assay at [S]=50uM (10x Km).
- Alpha = 1 + 50/5 = 11
- Ki = 1000/11 = 90.9 nM
- Large correction! True Ki much lower than IC50
✓ Ki = 90.9 nM
Real-World Applications
Drug Discovery
Ki determines drug potency independent of assay conditions, enabling compound comparison.
SAR Studies
Structure-activity relationships use Ki to rank inhibitors during medicinal chemistry optimization.
Therapeutic Index
Ki/Kd ratios vs off-target enzymes determine selectivity for safe drug candidates.
Mechanistic Studies
Ki reveals inhibitor binding mode and thermodynamics when combined with temperature studies.
Common Mistakes to Avoid
⚠️
Confusing IC50 with Ki
IC50 depends on assay conditions ([S], [E], time). Ki is a true constant. Always convert IC50 to Ki using Cheng-Prusoff equation.
⚠️
Wrong inhibition mode assumed
Cheng-Prusoff applies to competitive inhibition. For noncompetitive inhibition: Ki = IC50. For uncompetitive: Ki = IC50 x (1 + Km/[S]).
⚠️
Ignoring enzyme concentration
At [E] > Ki, the Morrison equation (tight-binding) must be used. If IC50 ≈ [E], use Morrison Ki for accurate results.
Frequently Asked Questions
What is the difference between IC50 and Ki? ▾
IC50 is the concentration that inhibits 50% activity under specific assay conditions. Ki is the thermodynamic dissociation constant of inhibitor from enzyme — independent of conditions.
When does Cheng-Prusoff not apply? ▾
For non-competitive inhibitors (Ki = IC50), uncompetitive inhibitors (Ki = IC50 x Km/([S]+Km)), and tight-binding inhibitors where [I] ≈ [E].
What is a good Ki for a drug? ▾
Drugs typically need Ki < 100 nM for potency. Clinical drugs often have Ki in the picomolar to low nanomolar range (e.g., statins: Ki ~1nM for HMG-CoA reductase).
How does selectivity relate to Ki? ▾
Selectivity ratio = Ki(off-target) / Ki(on-target). A ratio >100x is generally considered selective.
What is the Morrison tight-binding equation? ▾
vi/v0 = 1 - ([E]+[I]+Ki - sqrt(([E]+[I]+Ki)^2 - 4[E][I])) / (2[E]). Used when [I] is comparable to [E].