For OPH:H254S, the same interactions were observed compared to WT:OPH. Only one hydrogen bond, detected between the mutated residue H254S and a water molecule in the binding site, appears after 60 ns of MD simulations on the mutant complex (Figure ). It is worth noting that this water molecule is one of the two crystal water molecules that are involved in the coordination of one of the Zn(II) ions in the active site. This result suggests that the induced mutation, H254S, contributes to the stabilization of the Zn(II) coordination in the active site, and could perhaps improve hydrolytic activity against paraoxon.