For the computational part of this work, we used chain A of OPH from the PDB crystal structure 1HZY ). Chain A contains 330 amino acid residues and two Zn(II) in the active site. The first Zn(II) is coordinated by His 201, His 230, and two water molecules, while the second Zn(II) is coordinated by His 55, His 57, Asp 301, and a water molecule (Figure ). Also, we removed the carboxylic acid group originally added on the Lys 169 and treated residue 169 as a regular lysine. Based on the experimental conditions, we used the H++ server () to protonate the protein at pH 9. The resulting protein structure was used to prepare the different OPH mutants examined here -D253E, D253E-H254R, D253E-I255G, H254S, and I255S -using Chimera (University of California, San Francisco) ). Then, we used the MCPB.py program developed by to determine the parameters of the OPH metal binding site. Merz-Kollman charges were obtained using Gaussian09 and the B3LYP/6-31G* level of theory.