I am aware of the method of determining product inhibition constants and concurrently the "type" of inhibition by comparing the Michaelis Menten graphs generated in the presence of different inhibitor concentrations.
Is there are an easier method available for an approximation of the Ki that agrees well with the traditional method? Has anyone seen whether direct measurement of relative activity (at one substrate concentration only) vs. changing inhibition concentration gives comparable results to the traditional method? (i.e. the inhibitor concentration at 50% relative activity = ki).
What about the method produced by Stephen G. WALEY "A quick method for the determination of inhibition constants" in Journal of Biochemistry ?
The Cheng-Prusoff equation relates IC50 of an inhibitor to Ki for steady-state mechanisms. Each kinetic mechanism and inhibitor mechanism has a different equation, so you need to know both the rate equation for the kinetic mechanism and inhibitor mechanism to derive it. You also need to know the values of all the relevant kinetic constants from the rate equation. The derivation itself is just algebra. The original paper shows several examples.
Cheng Y, Prusoff WH (December 1973). "Relationship between the inhibition constant (KI) and the concentration of inhibitor which causes 50 per cent inhibition (I50) of an enzymatic reaction". Biochem Pharmacol 22 (23): 3099–108. doi:10.1016/0006-2952(73)90196-2. PMID 4202581
Hey Adam, Thanks for the answer. I wasn't aware of the Cheng-Prusoff equation that linked IC50 to Ki. What is your view on the webserver that tries to save you time on the algebra :
IC50-to-Ki: a web-based tool for converting IC50 to Ki values for inhibitors of enzyme activity and ligand binding
R. Z. Cer,1 U. Mudunuri,1 R. Stephens,1 and F. J. Lebeda2,*
I looked at the web site you mentioned. I like that it can be used to deal with tight-binding inhibitors that violate the assumption of steady-state kinetics that the enzyme concentration is much lower than the ligand concentrations. It appears to be limited to single-substrate enzymes. It would not be possible to use it for multi-substrate enzymes, which are very common.
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