What is a rate limiting step ?

Pascal Hua pjh3142 at rigel.tamu.edu
Mon Nov 22 15:06:37 EST 1993

	This question might seem easy, but I found it not so obvious as I thought
about it more:

	What does rate limiting mean to you ?

It is used quite often, and not only in biology.  The example I am dealing
with is a common one, a metabolic pathway.

	The simplest way to see it, is that there is not RenoughS of the first
enzyme on the pathway and RplentyS of the following enzymes.  When more of
the first enzyme is made, the flux through the pathway increases in a
direct (not necessarily linear) function of this first enzyme.  Does it
mean that this first enzyme is used at saturation of its substrate(s) ?  If
not, the element that keeps those substrate(s) constant would be the real
rate limiting step by providing the right flux of substrate so as to keep
it constant !  If that flux does not change, an increase in the quantity
and consequently in flux of the first enzyme would have the effect of
lowering the concentration of its substrate until the activity was restored
to its original value !
	I feel that I am chasing the chicken and the egg story.  But I think it is
important, because what we consider as RregulatoryS enzymes (cooperative
binding, aloesteric effector) exhibit those effects mostly at
sub-saturating concentrations of substrates.  How can they be rate limiting
and take advantage of those characteristics ?

Pascal Hua                 pjh3142 at rigel.tamu.edu
Biochemistry & Biophysics
Texas A&M University.

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