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I have just read this post at researchgate: "How to calculate experimental binding free energy from the IC50 value"

I am not be very sure about what the difference is between Ki and IC50. Could anybody explain?

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In biochemistry the Ki is the dissociation constant of a complex and molecules bound to it. It is a measure for the functional strength of the inhibitor. This can for example be an enzyme and its substrate, the Ki defines the stability of the complex.

The IC50 on the other hand is the halfmaximal inhibitory concentration of a substance on a biochemical process. This can be an inhibitor of an enzyme. See this figure (from the Wikipedia article on the IC50):

enter image description here

The Ki plays an important role in the calculation of the IC50, as it defines the strength of the enzyme-inhibitor complex.

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    $\begingroup$ I don't know if I'd call the Ki a measure of functional strength, I think it measures binding. It might be possible for a ligand to bind a protein tightly, giving it a low Ki, but not inhibit it, giving it a high IC50. But this situation probably only exists with allosteric binding sites, where a ligand can bind in a location separate from the active site. $\endgroup$ – user137 Dec 17 '14 at 21:27

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