A challenge in analyzing the OPH active site is that the native substrate remains unknown, and only structures with substrate analogs exist. While paraoxon (diethyl p-nitrophenyl phosphate) is considered a near-native substrate of OPH , it is not possible to obtain a co-crystal structure of the WT-OPH with the bound paraoxon because of the likelihood of hydrolysis. A co-crystal structure of OPH with diethyl 4-methylbenzylphosphonate (DMBP), which is a substrate analog sharing a high similarity with paraoxon, is available . Consequently, most of the researchers used the binding mode of DMBP as a starting structure in their studies in order to model paraoxon ).