CCL: IC50 deltaG

Hello cclers,
Thank you very much for updating us on IC50, Ki, DeltaG.
However, I have a very naive question that I couldnt get clarified for so long. My apologies if the question looks silly.
Scenario: We are working on developing / Identifying new chemical compounds that can Inhibit a particular enzyme. We however, wanted to initiate the project by studying already discovered chemical compounds known to inhibit that enzyme. The next step is to do a computational prediction based on that information with either QSAR or Pharmacophore or Dcokgins studies.
In various publications we have accessed so far, the biological activity against the same target enzyme, but of various chemical compounds is reported as units of IC50, Ki, Kiapp etc.,values. If we need to take into an account for all the ligands, known to inhibit the target enzyme we are studying, it makes an absolute requirement that all the activities are reported as either IC50 or Ki. Upon further research, we came across a KI and IC50 conversion facility through Cheng-Prusoff equation  explained clearly in :
However, for the interconvertion, there are no reported radioligand concentration and kd values that are required by the above mentioned formula.
Question: How do we report the biological activities of various chemical compounds in various publications as  a single consistent value? i.e., IC50 or Ki. when there is no information available for using the cheng-prusoff equation?
Any help is highly appreciated..

On Mon, Jun 2, 2008 at 10:35 PM, Michael K. Gilson gilson(_) <owner-chemistry[A]> wrote:

Sent to CCL by: "Michael K. Gilson" [gilson,,]
No, you can't assume IC50 is approximately Kd and use DG = RT ln IC50.

IC50 is the concentration of inhibitor that leads to half-maximal inhibition of the targeted enzyme
or receptor.  If it is a competitive inhibitor, then it is competing with substrate (in the case of
an enzyme) or with a labelled ligand (in the case of a receptor). For a given value of Ki, the value
of IC50 will still vary depending upon how tightly the substrate or labeled ligand binds the
protein, and also upon its concentration.  The higher the affinity of the substrate or labeled
ligand and the higher its concentration, the more inhibitor will be needed to have an effect, and
hence the higher IC50 will be -- even though Ki is unchanged.

The relationship is given by the Cheng-Prusoff equation.  See, e.g.,

However, if two inhibitors are assayed for same substrate or labeled ligand and same concentration
thereof, then Delta Delta G = -RT ln r, where r is the ratio of the IC50 values.

(The analysis is different if the inhibitor is not competitive.)


R D wrote:
Sent to CCL by: "R  D" [rafi4dd+/]

I need a clarification on fundamentals. I want to compare experimental IC50 values to calculated binding free energy values.
Can I assume IC50 is approx = Kd and use DeltaG = RTlnIC50.

for example if I want to convert an IC50 value of 1microM it will be equivalent to delta G of -8.2Kcal/mol. Am I right?

Are we using DeltaG = RTlnIC50 (instead of -RTlnIC50) because we are focused on dissociation of receptor-ligand complex rather than association of receptor ligand complex.
I apologise if the question is silly, but any reply will be greatly helpful.

Thank you.>


Michael K. Gilson, M.D., Ph.D.
CARB Fellow and Professor
Center for Advanced Research in Biotechnology
University of Maryland Biotechnology Institute
9600 Gudelsky Drive
Rockville, MD 20850
Voice: 240-314-6217
Fax:   240-314-6255
Lab Page:

E-mail to subscribers: CHEMISTRY[A] or use:

E-mail to administrators: CHEMISTRY-REQUEST[A] or use

Before posting, check wait time at:

Job: Conferences:

Search Messages:  (login: ccl, Password: search)