Schiff base
Further research led to the isolation of an active site peptide sequence and identification of the active site lysine, Lys115, that is involved in the formation of the Schiff base intermediate. Additionally, later experiments led to the finding that maximum activity of the enzyme occurs at pH 5.95, suggesting that the pK of the ε-ammonium group of Lys115 is significantly perturbed in the active site. If the pK were not perturbed downward, the lysine residue would remain protonated as an ammonium cation, making it unreactive for the nucleophilic addition necessary to form the Schiff base.