This led us to consider the possibility that these chimeras might exhibit inhibition by Mg 2+ , in contrast with the accelerating effect of Mg 2+ ion for WT PTP1B. The interaction of Asp181 with Mg 2+ ion is not seen in WT PTP1B tungstate-bound structure, nor with the corresponding Asp356 in tungstate-bound YopH. To assess whether this unproductive conformation of Asp181 occurs in solution, and whether the presence of magnesium ion contributes to its population, kinetic data were collected on Chimera 4 in the presence of Mg 2+ and showed an inhibitory effect (Figure ). The inhibition data were fitted to the competitive, uncompetitive, and mixed inhibition models. The mixed inhibition model gave the best fit results and yielded a Ki value of 4.8 ± 0.2 mM. The structural data explain why this nonproductive conformation is unique to the chimeras.