An investigator studying epigenetic mechanisms isolates histone proteins, the structural motifs involved in DNA binding and regulation of transcription. The peptide bonds of histone proteins are hydrolyzed and one type of amino acid is isolated. At normal body pH, this amino acid has a net charge of +1. The investigator performs titration of this amino acid and obtains the graph shown. The isolated amino acid is most likely which of the following:
- Lysine is a basic amino acid (as are arginine and histidine) with three ionizable groups: an α-amino group, an α-carboxyl group, and an ε-amino group.
- At body pH (7.35–7.45), the amino groups are protonated (NH3+) and the carboxyl group is deprotonated (COO‑), giving lysine a net charge of +1.
- The titration curve showed above mirrors the ionization of the three groups with changing pH: at a very low pH (high concentration of H+ ions) all three groups are protonated.
- After adding more OH–equivalents, first the carboxyl group gets deprotonated (first plateau, pKa1)), then the α-amino group (second plateau, pKa2) and finally the ε-amino group (third plateau, pKa3).