Enzyme Inhibition Ki Calculator (Cheng–Prusoff IC50 Conversion)

Enzyme Inhibition Ki Calculator (Cheng–Prusoff IC50 Conversion) | Online Free Tool | Elysia Tools

Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category: Education
Input types: select, number
Output type: json
Sample coverage: 4
API ready: Yes

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.

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

1Choose IC50 → Ki or Ki → IC50 mode and select competitive, uncompetitive, or pure noncompetitive inhibition.
2Enter the IC50 or Ki value, its unit in nM or µM, and the substrate concentration and Km in a shared unit.
3The calculator applies the matching Cheng–Prusoff relationship using the [S]/Km or Km/[S] ratio.
4Review 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.

Examples

1. Competitive IC50 to Ki conversion at twice Km

Enzyme assay researcher

Background

An inhibitor screen reports an IC50 of 100 nM. The assay uses 50 µM substrate, while the substrate Km is 25 µM.

Problem

Estimate the competitive inhibition constant while accounting for the substrate concentration.

How to use

Select IC50 → Ki, choose Competitive, enter IC50 = 100 nM, [S] = 50 µM, and Km = 25 µM.

Inhibition type: competitive; IC50: 100 nM; substrate concentration: 50 µM; Km: 25 µM.

Outcome

The [S]/Km ratio is 2, so Ki = 100 ÷ (1 + 2) = 33.33 nM.

2. Uncompetitive IC50 to Ki′ conversion

Biochemistry student

Background

An inhibitor that binds the enzyme–substrate complex produces an IC50 of 60 nM at 10 µM substrate. The substrate Km is 30 µM.

Problem

Calculate the uncompetitive inhibition constant Ki′.

How to use

Select IC50 → Ki, choose Uncompetitive, enter IC50 = 60 nM, [S] = 10 µM, and Km = 30 µM.

FAQ

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. Tight-binding cases should be analyzed with the Morrison equation.

Inhibition type: uncompetitive; IC50: 60 nM; substrate concentration: 10 µM; Km: 30 µM.

3. Predicting competitive IC50 from a known Ki

Assay planner

Background

A known competitive inhibitor has Ki = 20 nM. A planned assay will use 50 µM substrate with Km = 25 µM.

Problem

Predict the IC50 expected under the planned substrate conditions.

How to use

Select Ki → IC50, choose Competitive, enter Ki = 20 nM, [S] = 50 µM, and Km = 25 µM.

Inhibition type: competitive; Ki: 20 nM; substrate concentration: 50 µM; Km: 25 µM.

Outcome

The factor is 1 + [S]/Km = 3, giving a predicted IC50 of 20 × 3 = 60 nM.

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Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Overview

When to use

How it works

Use cases

Examples

1. Competitive IC50 to Ki conversion at twice Km

2. Uncompetitive IC50 to Ki′ conversion

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3. Predicting competitive IC50 from a known Ki