pI is the average of the two pKa values that link the equilibria between the
(+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.
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
> > consideration in trying to answer the following questions?---
> > 1) What is the largest theoretical isoelectric point shift due to a
> > 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