In the case of YopH-based chimeras, transposing the WPD-loop of PTP1B onto the YopH scaffold resulted in two chimeric proteins with unusual structural properties, Chimeras 2 and 3 . Crystal structures of both enzymes show their WPD-loop in a "wide-open" conformation, facilitated by the extension of the adjacent α4 helix, which pulls the WPD-loop out into this conformation (Figure ). While Chimera 2 is catalytically inactive, curiously, Chimera 3 retains some catalytic activity, albeit at a diminished rate compared to either parent enzyme (Table ). In addition, enhanced sampling (Hamiltonian replica exchange ) simulations indicate that although this wide-open structure has never been observed in any crystal structure of wild-type YopH, it is sampled even in the wild-type enzyme, although as a rare event . Representative structures of YopH with wide-open WPD-loops show a similar α4extension to that observed in the chimeras, suggesting that the chimeras did not generate this wideopen conformation, but rather that it is intrinsic also to the wild-type sequence, and the chimeras merely stabilized it sufficiently to be observed in crystal structures.