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3.1.8. Charges and
isoelectric point of a protein
Most proteins have a known amino acid sequences thanks to sequencing of
the corresponding DNAs. It is further possible to roughly estimate the charge
profile of the protein, including determination of the isoelectric point (pI), which
is defined as the pH where the protein
The strategy is simple:
1. Identify the
charged
amino acids (+ C,N terminal ends) of the protein
2. Determine the
number
of each type (assume position plays no role)
3. Assign pK
a
for each type (again ignoring influence of other amino acids)
4. Select a pH-interval (hopefully) covering pI
5. Estimate α and the total charge contribution from each AA at each pH using
the HH equation (simplified: 1% - 10% - 50% - 90% - 99% rule)
6. Make a table (Ovalbumin example below)
6. pI lies in the interval where the net charge changes from - to +
Example: Ovalbumin