Question

Determine the PI of peptide AEGK (Ala-Glu-Gly-Lys).

Answer 1

PI is the average of the pKa values representing the protonation and deprotonation of the neutral form of the peptide.

The pH range where the peptide has a net charge of 0 is between pH 4.2 and pH 9.7.

Below pH 4.2, which represents the pKa value for the glutamic acid carboxyl group, the peptide gains a net charge of +1.

Above pH 9.7, which represents the pKa value for peptide N-terminus, the peptide has a net charge of -1.

Therefor, PI = (4.2+9.7)/2 = 6.95