In WT YopH, the Lys loop providing one of the Mg 2+ -coordinating residues is absent, and no other residue is in close proximity for hydrogen bonding with this nonproductive conformation of Asp181. In WT PTP1B, residue 182 is phenylalanine, which lacks a hydrogen bond acceptor for this conformation of the Asp sidechain. Hence, the network for coordinating a magnesium ion can only occur in the chimeras. Only Chimera 4 was tested, but Mg 2+ inhibition is expected in both Chimeras 3 and 4 based on their structural data. This is another example of how changes to residues in the WPD-loop that are not directly catalytic can indirectly affect catalytic characteristics with potential regulatory consequences.