Drybench Assay.

Ki from IC50 for a competitive inhibitor (Cheng–Prusoff)

Ki from an enzyme-assay IC50 you type, the substrate concentration it was measured at and the substrate's Km, with the assumptions stated.

Converts an enzyme-assay IC50 to Ki for reversible competitive inhibition at steady state, from the substrate concentration the IC50 was measured at and the substrate's Km under the same conditions. All three are typed from your own assay; nothing is taken from a curve fitted here.

This page is deliberately not linked from the app's results: a cell-viability or reporter C50 is not an enzyme IC50.

If you measured rates at several inhibitor and substrate concentrations, fit Ki directly in the inhibition analysis.

IC50 from your enzyme assay

Substrate concentration [S] at which the IC50 was measured

Km of the substrate under the same conditions

IC50 interval, if you have one (optional)

IC50 interval, low (nM) (optional)

IC50 interval, high (nM) (optional)

Type the IC50 interval your dose-response fit reported, in the IC50's unit; both ends, or neither.

Answer

50 nMKi from IC50, competitive (Cheng–Prusoff)

[S]/Km = 1; Ki = IC50 / (1 + [S]/Km).

Assumptions: Reversible competitive inhibition at steady state; the IC50 measured at the stated [S]; enzyme far below Ki, with no tight-binding correction.

Not for

Not for cell-based or reporter IC50s; not for noncompetitive, uncompetitive or mixed inhibition; not for tight-binding inhibitors; not for receptor-binding IC50s.

The relation for a competitive inhibitor is Cheng and Prusoff's (1973), Biochem Pharmacol 22:3099–3108.

Worked example

[S] equal to Km halves the IC50: IC50 100 nM at [S] = Km = 50 µM gives Ki = 50 nM.
[S] in mM and Km in µM are converted exactly: IC50 37 nM, [S] 2.5 mM, Km 800 µM give Ki = 37 / (1 + 3.125) = 8.97 nM.
[S] molar against Km in µg/mL is refused, not converted: no number is given.

Method

Ki from IC50, competitive (Cheng–Prusoff). Cheng Y, Prusoff WH (1973), Biochemical Pharmacology 22:3099–3108Checked against hand-worked exact values (Python fractions) on 6 cases, Ki within 1e-12 relative and the four-figure display exact, and 5 refusals that give no number: a missing value, [S] and Km in different dimensions, a zero Km and a negative [S] (10 October 2026).

Limits

For research and education only, and not medical advice: a C50 estimated here describes this assay under these conditions and is not a binding constant.

Verification