Vanadate is notable for its ability to adopt a trigonal bipyramidal geometry, providing a structural transition state analog (TSA) for phosphoryl transfer. The vanadate-bound crystal structures show an apical interaction with the sulfur of cysteine and a trigonal bipyramidal geometry, an analog of the transition state for the second step shown in Figure . The active sites in the vanadate-bound complexes of Chimeras 3 and 4 are highly superimposable, with an RMSD of 0.07 Å when aligning 8 P-loop residues, H214-R221.The vanadium-sulfur distance is 2.6 Å and the apical V-O distance is 1.8 Å. These compare well to the corresponding distances in the vanadate structure of WT PTP1B, where the same TS analog has a V-S distance of 2.5 Å and V-O (apical) distance of 2.1 Å. These results are consistent with conclusions from KIE data that the transition states for the chimera-catalyzed reactions are similar to the native enzyme.