FIGURE 4.
The pH dependence of the ETPT in cNOR wild type and variants. The rates of the ETPT for cNOR wild type and the constructed variants that were affected at pH 7.5 are plotted as a function of pH: wild type (black; circles for the data from Ref. 8 and crosses for the data with the slightly altered purification protocol), K54AC (red triangles), E58QC (blue diamonds). The WT data (from Ref. 8) were fitted to a pKa of 6.61 ± 0.05 and a kH of 244 ± 7 s−1 (black line); K54AC data were fitted to a pKa = 6.4 ± 0.1 and a kH = 247 ± 11 s−1 (red line); and E58QC data were fitted to a pKa = 5.8 ± 0.1 and a kH = 49 ± 2 s−1 (blue line). However, data points for K54AC did not follow the fit around pH 7–7.5, possibly because the one-exponential fit is an oversimplification for this mutant. As a comparison, also plotted (as a dotted red line) is the diffusion rate, assuming kdiff of ∼2 × 108 m−1 s−1 (see “Results” for details). The error bar at pH 7.0 for K54AC indicates that at this pH the range of possible fits is rather large.