Isoelectric point of proteins

Nick Fisher nef1002 at
Fri Sep 6 11:20:43 EST 1996

In article <32309593.5929 at>, Ole Diettrich
<diettric at> wrote:

> Dear netters,
> I wonder to what extend the isoelectric point of a polypeptide changes 
> between the native (folded) conformation and denatured (8 M Urea) 
> conditions. Are the, probably few, charged amino acids within the 
> molecule affected from the external pH ?
> Many thanks in advance,
> Ole

The difference in pI between a native structure and a denatured one (let's
assume it's adopted a completely random coil with no significiant secondary
structure) is probably larger than you think. It's not so much that
buried "charged" residues are suddenly exposed to the solvent but rather
that charged sidechains with abnormal pK's due to the local microenvironment
are going to going to shift back to the pK you would predict for them if
you had no knowledge of the proteins structure. For example, the
sidechains of Asp residues in a tightly clustered "acidic patch" are
likely to have a pK
somewhat higher than that of a single, sovent exposed Asp on a different 
part of the protein. I don't have any figures to hand, I'm afraid, but I
think you will find a difference in pK between native and denatured states for
the reason discussed above.

Nick Fisher
Biochemistry Dept.
Cambridge Univ.

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