jphilo at amgen.com
Tue Mar 18 16:46:36 EST 1997
Jen Robertson wrote:
> I am confused about the physical difference between "high" and "low"
> affinity receptor populations. Is the difference between the two
> conformational or something more involved?
> Jen Robertson
To my knowledge no one has ever adequately answered this question. One
view is that the "high affinity" receptor population is already
dimerized, but somehow it is incapable of signal transduction even
though it is already dimerized.
In my view, you are asking the wrong question. The real question is:
"Is the evidence that there are two populations with different
affinities correct?" The evidence is that when people study EGF binding
by Scatchard analysis they get a concave upward Scatchard plot, which
they then interpret as indicating the presence of two populations.
However, this interpretation ignores the question of whether a Scatchard
analysis makes any sense for this system. What is almost universally
ignored is the true stoichiometry of the interaction. Scatchard
analysis is fine (tho' certainly not optimal by today's standards for
non-linear analysis) for a simple 1:1 binding interaction. However, for
ANY system where you believe ligand binding leads to receptor
dimerization a Scatchard analysis is fundamentally, thermodynamically
just plain WRONG, and what one measures are apparent AVIDITIES, not
thermodynamic binding affinities.
Furthermore, when the receptor is being dimerized a correct
thermodynamic analysis predicts that a Scatchard curve MUST be
non-linear even when all receptors are behaving identically. This is
easiest to see if we think in terms of binding energy: if the receptor
is being dimerized then the APPARENT binding energy of ligand to
receptor will be the energy of the simple initial 1:1 binding event plus
some portion of the binding energy responsible for receptor
It is also predicted that the apparent affinity from a Scatchard
analysis will vary with the number of receptors per unit area on the
cell surface (a prediction which is supported by data for EPO binding to
its receptor, for one).
The problems with Scatchard analysis for these systems have been known
for many years and even discussed in texts, yet most workers in the
field just ignore them.
John Philo, Protein Chemistry/Biophysics, Amgen
*** Disclaimer: These are the opinions of the poster not Amgen Inc.***
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