Fig. 4.

Quantitative fractional ionization model of hydrogen bonding groups within the KSI D40N active site with bound phenols of increasing solution pK_a. (A) Fractional ionization values (estimated uncertainty ± 0.15) were derived from the ¹³C NMR data as explained in the main text and SI Materials and Methods. The data were globally fit (R² values of individual fits = 0.92–0.97) with the equilibrium proton transfer model given in Methods to give best-fit values of 8.6 for parameter a, 0.1 for parameter b (the slope of the linear dependence of Y16 acidity on phenol pK_a), and 9.6 for the apparent pK_a of Y57. (B) Schematic depiction of the ionization states present within the active site hydrogen bond network whose fractional populations shift as a function of increasing phenol pK_a.