For comparison, in our simulations of PriA in complex with substrate PRA, we sample an active site conformation in which the R143 side chain (loop 5 residue) forms salt bridges with the side chains of with D130 and D175, and the arginine acts as a "shield" dampening the electrostatic repulsion between the D130 side chain and the anthranilate carboxylate group of PRA. This interaction also stabilizes the catalytic aspartic (D175), placing it in an optimal position for catalysis (Figure and Table ), as shown in previous studies . This PriA conformation is similar to the "TrpF-active" conformation observed in the SeHisA(dup13-15/D10G/Q24L/G102A) crystal structure (PDB ID: 5AB3 , Figure , with manual placement of PRA in the active site), where the arginine is close to residue D129. While we observe this conformation in our PriA simulations, we do not observe the formation of a corresponding interaction in our simulations of wild-type HisA in complex with PRA, the negative charge repulsion between PRA and the D129 side chain destabilizes the position of the substrate in the active site (Figure ), as well as the stability of the loop 6 carrying the key catalytic aspartic acid side chain (Figure ). We do, however, observe a similar interaction with the R169 side chain in the SeHisA(L169R) variant, with interactions with D129 and, in this case, a salt bridge interaction with the anthranilate carboxylate group of PRA (Figure and Table ), consistent with experimental work that demonstrated that the introduction of the L169R substitution in HisA induces TrpF activity (Table ). Finally, in the case of TrpF in complex with PRA, we observe a salt-bridge interaction between E184 and the side chain of R36 on loop 3 (Figure and Table ), and we can see that as for HisA and PriA, that R36 is again acting as a "shield" avoiding possible negative repulsion interactions between the substrate and the negatively charged side chain. However, we do not observe clear interactions between the R36 side chain and the substrate PRA (Figure and Table , with the fraction of simulation time in which this interaction is observed being <0.1).