The IC50-to-Ki converter computes Ki values from experimentally determined IC50 values for inhibitors of enzymes that obey classic Michaelis-Menten kinetics and of protein-ligand interactions (Cer, Mudunuri, Stephens & Lebeda (2009)).
Please use the default enzyme concentration [E], substrate concentration [S], Michaelis-Menten constant Km, and IC50 to convert to Ki value OR enter your own value.
CLASSIC INHIBITOR EXAMPLE (botulinum neurotoxin type A)
- Ref Burnett et al. 2007
Some analyses of networks, pathways and metagenomics focus on identifying key proteins or polynucleic acids as targets for inhibitory compounds. Typically, high throughput screening assays are used initially to compare and down-select potential inhibitors of enzymatic activity or macromolecule-ligand binding. Many functional assays seek a total inhibitor concentration that reduces these activities by 50% (IC50). However, the IC50 value depends on concentrations of the enzyme (or target molecule), the inhibitor, and the substrate (or ligand) along with other experimental conditions. What is required is an accurate determination of the Ki value, an intrinsic, thermo-dynamic quantity that is independent of the substrate (ligand) but depends on the enzyme (target) and inhibitor. Thus, comparisons can be more readily made among different laboratories to characterize the inhibitors. While these more time-consuming assays are usually done with the most promising candidates, accurate, initial estimates of Ki values would be beneficial. A much discussed problem in the literature (e.g.. (Brandt et al. 1987 Biochem Med Metab Biol 37:344) is converting IC50 to Ki values because even the simplest types of inhibitory mechanisms (competitive, uncompetitive, non-competitive) will influence the calculation.
To help address this problem, our web server tool calculates Ki values from IC50 values using equations from the literature for enzyme-substrate and target-ligand interactions by different inhibitory mechanisms. Additional calculations are performed for tightly bound inhibitors of enzyme-substrate reactions in which free, rather than total, concentrations of the molecular species are calculated for non-classic Michaelis-Menten kinetics. Similar calculations can be performed for target molecule-ligand systems. This calculator will enable end users to judge the quality of the underlying assumptions for these calculations.
User-defined input values include total concentrations of the enzyme (or target molecule) and substrate (or ligand), the Km of the enzyme-substrate (or the Kd of the target-ligand) reaction and the IC50 value. The outputs include tabulations of the Ki values under different kinetic schemes, extensive tabulations of the results and the corresponding equations. Help buttons are available for Background, Assumptions, Literature, Links and Equations along with examples taken from the IC50_Ki_Converter host database that contains kinetic information on neurotoxin inhibitors.
It is assumed that all of the substrate and inhibitor binding reactions are reversible and that they all have a one-to-one stoichiometry, i.e., no multiple binding of inhibitor molecules or any form of cooperativity, or other complex mechanisms of inhibition such as partial or mixed types. It is also assumed that in the enzymatic reactions that enzyme autocleavage did not occur and that when substrates for fluorescence resonance energy transfer were used, appropriate corrections for inner filter effects were performed. Comparison of Km or IC50 values for a set of inhibitor candidates is only assumed to be valid when they are evaluated under identical experimental conditions. In most experimental studies of enzyme kinetics, the total concentrations of substrate and inhibitor used are in excess to make their free and total concentrations essentially the same. Under the conditions of some ligand-receptor (e.g., protein) binding studies, the free concentrations become sufficiently important to require modifications of these equations.
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