sentences:  Additionally, later experiments led to the finding that maximum activity of the enzyme occurs at pH 5.95, suggesting that the pKa of the ε-ammonium group of Lys115 is significantly perturbed in the active site.If the pKa 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.Building upon this finding, Westheimer et al.