The short answer is no.
Because 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 your
hypothetical protein the pI would occur when 3 Asp are ionized (the other
17 are unionized). Therefore, the pI will be the average of the pKa values
responsible for the ionization of the 3rd and 4th Asp. In real protein
each Asp will have a slightly different pKa, but these pKa don't differ by
much. Deleting one Asp or Arg will shift the pI by an unappreciable amount
in this case.
I hope I haven't made it mroe confusing.
T. Chyau Liang, Ph.D.
In article <4iv10v$m5e at cronkite.ocis.temple.edu>,
driska at astro.ocis.temple.edu (Stephen P. Driska PhD) wrote:
> 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?