sentences:  '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.