Hill plot

deits at deits deits at deits
Mon Jan 16 11:05:00 EST 1995

In Article <maga-120195170139 at> "maga at vetbio.unizh.ch (Giovanni Maga)" says:
> From G. Maga, Biochemistry
> University Zuerich
> Zuerich (CH)
> e-mail: maga at vetbio.unizh.ch
> , if you obtain a non
> Mich.-Menten curve, but a sigmoidal one, can you use Hill plot as a test
> for real cooperativity? 
Perhaps the best way to answer this is to consider what we mean by
cooperativity.  I would be inclined to define it as an observed deviation
from Michaelis-Menten or classical binding curves - a strictly operational
definition.  In this case, the answer to your question is that any Hill
plot with n not = 1 exhibits 'real cooperativity.'  However, your question
probably is better phrased 'can you use a Hill plot as a test for allosteric
interactions'  which is one physical mechanism which may result in a 
cooperative effect.  In that case, the answer is no- there is no way to
prove the underlying mechanism from the kinetic phenomenon, which is of 
course the classical dilemma in kinetic analysis.
	Other mechanisms which might lead to cooperativity without invoking
an allosteric response and which I haven't seen mentioned in this thread
include hysteresis- a slow, substrate-induced conformation change in a 
enzyme that means that the steady state is not attained rapidly at all
substrate concentrations.  This causes a violation of the steady state
assumption, and can manifest itself as a cooperative interaction.  A good 
review of this topic can be found in Annuual Review of Biochemistry by
Carl Frieden in the mid-80's.  
				Tom Deits	
				deits at pilot.msu.e

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