sentences:  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.