The preference for either Glu197 or Glu441 being protonated could be influenced by certain factors. First, a particular organization of the local electrostatic field is expected to play a role. Second, there may be changes in the available conformational spaces for residues induced by binding a particular ligand. These considerations led us to quantify the proton transfer dynamics for all stable states along the reaction process (Figure -13) for both human cholinesterases. ). In hAChE, the balance of states is generally shifted more to the protonated external Glu202 (hBChE: Glu197) (Figure ). The only hAChE state that prefers a protonated internal Glu450 (hBChE: Glu441) is the covalently inhibited one, much like the DEP-hBChE. This state, alongside with the pre-reaction complex (ES) and both tetrahedral intermediates (TI1 and TI2), still feature a dynamic interconversion between both protonation variants. However, the free enzyme state, the acyl-enzyme, and the final product state (EP) have a proton almost exclusively on their external Glu.