Figure shows the pH-dependent catalytic activity with the substrate pNPP for WT SHP-1 and the variants H422Q, E427A, and S418A. The parameter kcat/KM reflects the part of the overall mechanism up to and including the first irreversible step, formation of the phosphoenzyme intermediate, the first step in Figure . The overall rate-determining step in the SHP-1 reaction, reflected in kcat, is the hydrolysis of this intermediate, in which D421 acts as a base to deprotonate a nucleophilic water molecule. The pH dependency of kcat/KM is broader in the variants, reminiscent of the variability of pH dependencies within the PTP family. For example, YopH catalysis shows a narrower pH profile for compared to PTP1B. These PTPs share catalytic residues, but differ in residues neighboring the catalytic ones and within their WPD-loops. In the present case, the broader pH profile reflects the fact that the E427A variant has higher catalytic efficiency at low and high pH. The acidic limbs of the kcat pH-rate profiles and the maximal activities showed modest changes between the variants and the native enzyme, but the basic limbs show more significant effects. Each of the variants has higher turnover numbers above pH 6 than the native enzyme. At pH 7, H422Q and E427A have turnover numbers approximately 5-fold higher than WT SHP-1.