# Enzyme Inhibition Ki Calculator (Cheng–Prusoff IC50 Conversion) IC50 ↔ Ki conversion for competitive, uncompetitive, and pure noncompetitive inhibition with mechanism notes, potency bands, and tight-binding caveats. Educational use only. > Canonical page: https://elysiatools.com/en/tools/enzyme-inhibition-ki - **Category:** Education - **Keywords:** cheng prusoff calculator, ki ic50 conversion, inhibition constant calculator, competitive inhibitor ki, uncompetitive inhibition, noncompetitive inhibition, ic50 substrate dependence, morrison equation tight binding, enzyme inhibitor potency, drug discovery calculator, biochemistry calculator ## Overview The Enzyme Inhibition Ki Calculator converts between measured IC50 and Ki for competitive, uncompetitive, and pure noncompetitive inhibition using Cheng–Prusoff relationships. Enter the inhibitor value, inhibition type, substrate concentration, and Km to estimate the corresponding constant. Educational use only. ## Inputs - **Calculation Mode** (select): Derive Ki from a measured IC50, or predict the IC50 a known Ki will give at your substrate concentration. - **Inhibition Type** (select): Mechanism of inhibition — it decides which Cheng–Prusoff variant applies. - **IC50** (number): Measured IC50 in the unit selected below (IC50 → Ki mode). - **Ki** (number): Known inhibition constant in the unit selected below (Ki → IC50 mode). - **IC50/Ki Unit** (select): Concentration unit shared by the IC50/Ki input and the output. - **Substrate Concentration \[S\]** (number): Substrate concentration in the assay, same unit as Km (below). - **Km** (number): Michaelis constant for the substrate in the assay. - **Substrate/Km Unit** (select): Shared unit of \[S\] and Km (only their ratio matters). - **Decimal Places** (number) ## When to use - Convert a measured IC50 into Ki or Ki′ while accounting for substrate concentration and Km. - Predict the IC50 expected from a known Ki under a defined assay substrate concentration. - Compare inhibitor potency while keeping the inhibition mechanism and concentration units explicit. ## How it works - Choose IC50 → Ki or Ki → IC50 mode and select competitive, uncompetitive, or pure noncompetitive inhibition. - Enter the IC50 or Ki value, its unit in nM or µM, and the substrate concentration and Km in a shared unit. - The calculator applies the matching Cheng–Prusoff relationship using the \[S\]/Km or Km/\[S\] ratio. - Review the calculated value, mechanism notes, and potency band; use caution for tight-binding inhibitors, which require the Morrison equation. ## Use cases - Interpret enzyme inhibitor screening results by converting IC50 values into mechanism-specific Ki estimates. - Plan assay conditions by predicting how substrate concentration may change the observed IC50. - Prepare educational or experimental calculations involving competitive, uncompetitive, and pure noncompetitive inhibition. ## Frequently asked questions ### What is the difference between IC50 and Ki? IC50 is the inhibitor concentration that reduces measured activity by 50% under specified assay conditions. Ki estimates inhibition affinity and depends on the inhibition model. ### Which formula is used for competitive inhibition? For competitive inhibition, Ki = IC50 ÷ (1 + [S]/Km). ### Which formula is used for uncompetitive inhibition? For uncompetitive inhibition, Ki′ = IC50 ÷ (1 + Km/[S]). ### How is pure noncompetitive inhibition calculated? For pure noncompetitive inhibition, Ki = IC50, so the conversion is independent of substrate concentration. ### Can this calculator analyze tight-binding inhibitors? No. The calculator assumes non-tight, reversible equilibrium inhibition. 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