![]() An exact correction to the "Cheng-Prusoff" correction. #Enzymex calculator freeUnder the conditions of some ligand-receptor (e.g., protein) binding studies, the free concentrations become sufficiently important to require modifications of these equations. 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. 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. 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. 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. 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. The outputs include tabulations of the Ki values under different kinetic schemes, extensive tabulations of the results and the corresponding equations. 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. This calculator will enable end users to judge the quality of the underlying assumptions for these calculations. ![]() Similar calculations can be performed for target molecule-ligand systems. 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. 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. 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. A much discussed problem in the literature (e.g. While these more time-consuming assays are usually done with the most promising candidates, accurate, initial estimates of Ki values would be beneficial. Thus, comparisons can be more readily made among different laboratories to characterize the inhibitors. 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. 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. Many functional assays seek a total inhibitor concentration that reduces these activities by 50% (IC50). Typically, high throughput screening assays are used initially to compare and down-select potential inhibitors of enzymatic activity or macromolecule-ligand binding. ![]() ![]() ![]() Some analyses of networks, pathways and metagenomics focus on identifying key proteins or polynucleic acids as targets for inhibitory compounds. ![]()
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