Subject: What is a rate limiting step ?

From: [email-masked] (Pascal Hua)
Date: 22 Nov 1993 20:06:10 GMT

Hello,
	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 ÒenoughÓ of the first
enzyme on the pathway and ÒplentyÓ 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 ÒregulatoryÓ 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                 [email-masked]
Biochemistry & Biophysics
Texas A&M University.

Cross-posted to:
bionet.biology.computational
bionet.cellbiol
bionet.general
bionet.info-theory
bionet.metabolic-reg
bionet.molbio.proteins