In wild type OPH, the carboxylate group of Asp 253 forms a hydrogen bond with the imidazole of His 230 (Figure ). This hydrogen bond orients His 230 for an optimal coordination of one of the Zn(II) in the active site. We computationally mutated the aspartate residue at position 253 to a glutamate to Then, we performed MD simulations on the WT-OPH complexed with paraoxon and the D253E mutant complexed with paraoxon to evaluate whether the substitution of the aspartate's side chain by a longer side chain would have an impact on the number and the stability of the interactions between the OPH active site residues. A structural analysis of our MD simulations showed that, in the mutant form D253E, the active site residues establish four additional stable hydrogen bonds compared to the WT OPH. As shown on Figure , and based on the distance between atoms that could form potential hydrogen bonds during MD simulations, we concluded that four interactions are more stable during the simulations of the D253E mutant complex. These interactions occur between: i) Thr 54 and His 55, ii) His 57 and Trp 302, iii) Thr 200 and His 201, and iv) Leu 271 and His 254. His 55, His 57, and His 201 take part in the Zn(II) ligation while His 254 is not involved in the Zn(II) coordination. However, His 254 plays a role in the stability of the OPH active site by stacking with His 230, which guides and stabilizes the Zn(II)-His 230 interaction.