Building upon this finding, Westheimer et al. directly measured the pKa of Lys115 in the active site using 5-nitrosalicylaldehyde (5-NSA) . Reaction of 5-NSA with acetoacetate decarboxylase and subsequent reduction of the resulting Schiff base with sodium borohydride led to the incorporation of a 2-hydroxy-5-nitrobenzylamino reporter molecule in the active site (Figure 2). Titration of the enzyme with this attached reporter group revealed that the pKa of Lys115 is decreased to 5.9 in the active site. These results were the basis for the proposal that the perturbation in the pKa of Lys115 was due to its proximity to the positively charged ε-ammonium group of Lys116 in the active site. A nearby positive charge could cause unfavorable electrostatic repulsions that weaken the N-H bond of Lys115. Westheimer et al.'s proposal was further supported by site-directed mutagenesis studies. When Lys116 was mutated to cysteine or asparagine, the pKa of Lys115 was found to be significantly elevated to over 9.2, indicating that positively charged Lys116 plays a critical role in determining the pKa of Lys115. Although a crystal structure was not yet solved to provide structural evidence, this proposal was widely accepted and cited as a textbook example of how the active site can be precisely organized to perturb a pKa and affect reactivity.