Furthermore, although the isomerization of both ProFAR and PRA proceeds through the same Amadori rearrangement (Figure ), ProFAR (the native substrate of HisA) is a much larger molecule, including the presence of a second phosphate group. This group forms hydrogen bonding interactions with the side chains of R83 from loop 3 and S103 from loop 4 of SeHisA, and with the corresponding side chains of R85 and T105 in MtPriA (Figure ). Although neither of these residues are on the primary mobile loops of either enzyme (Figure ), nevertheless, the interactions with the second phosphate group of ProFAR are very similar to analogous interactions in other enzymes activated by ligand-gated conformational changes, suggesting that loops 3 and 4 in SeHisA and MtPriA may similarly act as "gripper loops" allowing these enzymes to attain relevant catalytically active conformations for the isomerization of the larger substrate. This effect would clearly not be present when the smaller substrate, PRA, is bound to the active site as this substrate lacks a non-reactive phosphodianion group to interact with these loops. the active site tryptophan that forms stacking interactions with the larger substrate ProFAR in SeHisA and MtPriA (W145 in both enzymes), as well as the key gripper residues that interact with the distal phosphate group of the larger substrate/product in SeHisA and MtPriA (R83 and S103 in SeHisA and R85 and T105 in MtPriA). Key hydrogen bonding interactions are also highlighted, using the distances (Å) found in the corresponding crystal structures. Note that for clarity, N7D and A176D reversions were applied in SeHisA in complex with substrate ProFAR (and this reversion was also applied in our simulations, as outlined in the Methodology section).