Enzyme Competitive Inhibition Calculator

In enzyme pharmacology, Ki (inhibition constant) and IC50 (half-maximal inhibitory concentration) are related but distinct measures of how potently a drug blocks an enzyme — and converting between them requires knowing the assay conditions. Enter your Ki or IC50 value, substrate concentration [S], and Michaelis constant Km into the Enzyme Competitive Inhibition Calculator to convert in either direction. Secondary outputs include the fold difference (IC50/Ki) and the [S]/Km ratio, which shows how far your assay substrate concentration sits from the enzyme's half-saturation point.

Conversion Direction *

M

Ki value (M) when converting Ki→IC50, or IC50 value (M) when converting IC50→Ki

M

Concentration of substrate in the assay

M

Km value of the enzyme-substrate system

Results

Calculated Value

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Fold Difference (IC50/Ki)

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[S]/Km Ratio

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Frequently Asked Questions

What is Ki to IC50 conversion?

Ki to IC50 conversion uses the Cheng-Prusoff equation to interconvert between the inhibition constant (Ki) and half-maximal inhibitory concentration (IC50) for competitive enzyme inhibition. This conversion is essential for comparing inhibitor potencies and analyzing enzyme kinetics data.

What is the Cheng-Prusoff equation?

The Cheng-Prusoff equation is IC50 = Ki × (1 + [S]/Km), where [S] is substrate concentration and Km is the Michaelis constant. It relates the IC50 measured in competition assays to the true inhibition constant Ki.

When should I use this calculator?

Use this calculator when analyzing competitive enzyme inhibition data, comparing inhibitor potencies across different assay conditions, or converting between Ki and IC50 values for pharmacological studies.

Why is IC50 typically higher than Ki?

IC50 is typically higher than Ki because competitive inhibitors must overcome substrate binding. The higher the substrate concentration relative to Km, the more inhibitor is needed to achieve 50% inhibition, making IC50 larger than Ki.

What does the [S]/Km ratio tell me?

The [S]/Km ratio indicates how the substrate concentration compares to the enzyme's affinity. When [S]/Km is high, more inhibitor is needed for the same effect, and IC50 becomes much larger than Ki.

How accurate is this conversion?

The conversion is accurate for true competitive inhibition where the inhibitor and substrate bind to the same site. For mixed or noncompetitive inhibition, this equation may not apply accurately.

What units should I use?

All concentrations (Ki, IC50, [S], and Km) should be in the same molar units. The calculator uses molarity (M), but you can input values in any consistent molar unit like μM or nM.

Can I use this for noncompetitive inhibition?

No, this calculator is specifically designed for competitive inhibition. For noncompetitive inhibition, IC50 equals Ki regardless of substrate concentration, so no conversion is needed.