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Specialty Chemistry Forums => Biochemistry and Chemical Biology Forum => Topic started by: mfiguera on February 03, 2012, 09:09:01 PM

Title: kcat/Km and kon
Post by: mfiguera on February 03, 2012, 09:09:01 PM

Hi,

I need a bit of help with enzyme kinetics:
I am studying kinetics of binding and reaction for a kinase. From steady state analysis I get kcat/Km and from the kinetics of the interaction I get kon. The issue I have is that I have read that kon cannot be lower than kcat/Km but that is what I see in my system (several fold difference). How is this possible? What can be causing this? What is the molecular or biochemical reason why kon cannot be lower than kcat/km?
Even though I found in textbooks that kcat/Km provides the lower limit for kon and that kcat/Km upper limit is set by the rate of diffusion, I am still confused, I don't "see" the molecular basis for kon not being lower than kcat/Km.
Thank you,
mf

Title: Re: kcat/Km and kon
Post by: Yggdrasil on February 03, 2012, 11:12:12 PM

Look up how K_M depends on k_on, k_off and k_cat.  This should show you why k_cat/K_M cannot be larger than k_on.

Conceptually, in an enzyme-catalyzed reaction, the enzyme and substrate must bind before they can react.  Therefore, the reaction cannot proceed faster than the product and substrate can bind.