X-ray data indicate that SHP-1 shares a mobile WPD-loop with other classical PTPs. It has been inferred that SHP-1 and other PTPs with mobile WPD-loops exhibit the same correlation between loop dynamics and catalysis as seen in PTP1B and YopH. This study investigated the pH-dependent enzymatic activities of several point variants of the catalytic subunit of SHP-1 that were reported to exhibit elevated turnover rates and efficiencies at pH 5 in solutions containing 40% glycerol. Glycerol raises both kcat and KM of SHP-1 catalysis and increases susceptibility to proteolysis, effects tentatively ascribed to relaxation of structural components. We were interested in whether these variant SHP-1 proteins had higher activities in general and how their pH-dependencies of catalysis compared to the native enzyme. Our kinetic studies were carried out in aqueous solution, more reflective of biological conditions, and to avoid complicating effects of glycerol. Glycerol is a potent viscosigen, and the co-solvent will also affect pH by altering the pKa of buffering species, and affect the pKa and nucleophilicity of catalytic residues. These kinetic studies are supported by molecular dynamics simulations of wild-type and variant SHP-1 in the unliganded and phosphoenzyme intermediate states, to explore the impact of the substitutions on the dynamics of the WPD-loop. Curiously, our computational results show minimal impact of the mutations on loop dynamics, suggesting that the observed shifts in pH-dependency are due to chemical effects (primarily through modulating the pKa of the catalytic aspartic acid), in contrast to YopH where altered pH-dependencies were shown to be due to altered dynamics of the WPDloop. 9