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Isoelectric Points

Stephen P. Driska PhD driska at astro.ocis.temple.edu
Fri Mar 22 15:02:39 EST 1996

	Wouldn't this be some sort of weighted average?  In other
words, if you had a protein with 20 Asp's and 3 Arg's, wouldn't the pI
change more by deletion of one of the few Arg's than by deletion of one
of the many Asp's?

	A simple yes or no answer is OK for me; don't feel like you
have to explain it.  This may be one of those mysteries (like
split-level parking decks, intertwined helical stairways, and stacking
gels) that I will never REALLY understand.


T. Chyau Liang (tliang at utmmg.med.uth.tmc.edu) wrote:
: pI is the average of the two pKa values that link the equilibria between
: (+1 <==>0) and (0 <==> -1) charged states of the protein. In a
: hypothetical situation, if one has ionizations of all carboxyl groups
: (which have pKa around 4.5) in the negatively charged states and all
: arginines (which have pKa around 12.5) on the other side of the
: equilibrium, by changing one residue (from Arg to Asp/Glu or Asp/Glu to
: Arg), one expect to have a maximal change in pI of 4.0 (half of 12.5-4.5).

: T. Chyau Liang, Ph.D.
: U. Texas-Houston

: In article <314D858A.735 at oci.utoronto.ca>, RAB <rab at oci.utoronto.ca> wrote:

: > Bob Hoesch wrote:
: > > 
: > > What kinds of calculations and what sorts of parameters need to be
: taken into
: > > consideration in trying to answer the following questions?---
: > > 
: > > 1) What is the largest theoretical isoelectric point shift due to a
: single AA
: > > substitution?  (Does anyone know of any observed measurements?)
: > 
: > One example I worked with, the removal of a C-terminal Lysine from pae 
: > lectin results in a pI shift from 7.2 to 6.1
: > 
: > Flip

Steve Driska, Physiology Department, Temple University Medical School
Philadelphia, PA 19140 USA (215) 707-3283 
driska at astro.ocis.temple.edu  
If I could think of something witty and clever to say, it would go right 
here ----->>    .........................

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