The kinetics and KIE data indicate that their reduced catalytic rates compared to the parent enzymes, and differences between the chimeras themselves, do not arise from changes in the mechanism or to the rate-determining step of catalysis. Even moderate differences in WPD-loop sequence identity between the chimeras and native PTP1B result in significant effects on their state, affects catalytic turnover, as was similarly observed, for instance, in prior work on TIMbarrel proteins. By influencing the population of catalytically competent enzyme-substrate complexes, the pH profile of catalysis can be affected, as well as the limiting rate. Thus, variations in sequence of noncatalytic residues within the WPD-loop provides a means for nature to fine tune these enzymes, and likely contributes to the variation in rate and pH-dependency (broad versus narrow) in the PTP family despite strict conservation of the ionizable residues directly involved in the chemical steps. In another example, the change of the residue following the general acid from phenylalanine to glycine, the residue found in this position in WT YopH, switches the effect of Mg 2+ from an activator to an inhibitor.