Our simulations show that while the gripper interaction is stable in HisA throughout the simulations, in PriA, there is electrostatic repulsion between R85 and an additional active site arginine, R143, which causes instabilities in the catalytic loops (in particular loop 5, Figure ), as well as in the substrate positioning in the active site, such that the larger ProFAR is bound less stably in the PriA active site than in the HisA active site. This is in effect a ligand-gated effect, where interaction with the non-reactive phosphodianion (which is not present in the smaller substrate, PRA) facilitates the stability of catalytically important loop 5. Thus, the underlying principles driving loop stability are similar to those of other enzymes that are activated by ligandgated changes.