and YopH is oriented towards the P-loop, where it protonates the leaving group oxygen of the bound substrate. However, in the tungstate-bound chimera structures, this side chain is pointing away from the active site, despite the lack of any significant change in the conformation of the WPD-loop backbone. This atypical aspartate sidechain conformation is facilitated by hydrogen bonding with Lys116 and Gln182, and by coordination with a magnesium ion from the crystallization solution. Magnesium is reported to have a modest activating effect on native PTP1B. This unproductive orientation provides another possible rationale for their reduced rates.