Here's why:
* Lysine is a diprotic amino acid: It has two ionizable groups that can donate protons (H+).
* The two ionizable groups are:
* The carboxyl group (-COOH) at the end of the molecule.
* The amino group (-NH3+) on the side chain.
Each of these groups has a specific pKa value representing the pH at which it is half-ionized.
Therefore, lysine has two pKa values: one for the carboxyl group and one for the amino group on the side chain.
