Isoelectric Point calculations

David J. States states at
Mon Mar 25 00:30:43 EST 1996

> In article <4hok83$315 at>, Eugene A Kapp
> <100277.2367 at CompuServe.COM> wrote:
> > I have found that the predicted pI calculations from two WWW
> > sites differ by approximately 0.4 for a given protein sequence.
> > Expasy ( and the genome
> > sequencing site at ( are
> > therefore using slightly different pKR values.
> > Could someone suggest the correct pKR values or add some light to
> > the situation.

The programs may be using different reference values, but the
tertiary structure, hydrogen bonding neighors, surface exposure
of titratable groups, temperature, and ionic strength of the solution 
all need to be considered in attempting to predict the isoelectric 
point or ionization state of a protein.  The literature on the
subject is extensive going back to Tanford in the 50's.

+- 0.5 pK units is probably not a bad estimate for the accuracy
of primary sequence based estimates.

David J. States
Institute for Biomedical Computing / Washington University in St. Louis

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