Isoelectric point for amino acids?

Question

How do you calculate the isoelectric point for an amino acid, in particular, amino acids with acidic or basic side chains that tend to have three pKas? Thanks.

Answer 1

For simple amino acids, the pI is pK1+pK2/2. If you have three pKs, e.g. in lysine, it really boils down to an average of the pKs for the amine groups. Thus, if lysine is fully protonated, the carboxyl group will not have a charge, and the 2 amine groups will be positive. Thus, you have 2 positive charges, and the pI would be approximated as pKaminegroup1 + pKaminegroup2/2.