Reversible phosphorylation is a common post-translational modification seen in over 30% of eukaryotic proteins. Phosphatases work in tandem with kinases to regulate this process in a broad spectrum of organisms. Among the protein phosphatase families, protein tyrosine phosphatases (PTPs) are defined by the active site signature motif called the P-loop (HCX5R) that includes a cysteine residue required for catalysis. Among the subclass of classical, pTyr-specific PTPs, protein tyrosine phosphatase 1B (PTP1B) and Yersinia outer protein H (YopH) are the most studied. PTP1B is a human PTP whose best-known biological role is that of a negative regulator in the insulin signaling pathway, while YopH is the virulence factor of Yersinia pestis, responsible for the bubonic plague. PTPs catalyze dephosphorylation through a two-step mechanism involving the nucleophilic cysteine in the P-loop. Another catalytic residue, an aspartic acid, resides on a different conserved structural loop found among the classical PTPs. This is a mobile loop consisting of about a dozen residues called the WPD-loop, defined by the conserved residues tryptophan, proline, and aspartate found near its center. While mobile loops in proteins are common, PTPs are unusual in having a key catalytic residue residing on such a mobile element. This loop remains mobile upon substrate binding, but favors a closed conformation approximately 8 Å closer to the P-loop, bringing the aspartic acid into position to protonate the leaving group in the first step of catalysis (Figure ).