In Silico Approach to Support that p-Nitrophenol Monooxygenase from Arthrobacter sp. Strain JS443 Catalyzes the Initial Two Sequential Monooxygenations.

p-Nitrophenol (PNP), used primarily for manufacturing pesticides and dyes, has been recognized as a priority environmental pollutant. It is therefore important to reduce the input of this toxicant into the environment and to establish approaches for its removal from the contaminated sites. PNP monooxygenase, a novel enzyme from Gram-positive bacteria like Arthrobacter sp. and Bacillus sp., that comprises two components, a flavoprotein reductase and an oxygenase, catalyzes the initial two sequential monooxygenations to convert PNP to trihydroxybenzene. Accurate and reliable prediction of this enzyme-substrate interactions and binding affinity are of vital importance in understanding these catalytic mechanisms of the two sequential reactions. As crystal structure of the enzyme has not yet been published, we built a homology model for PNP monooxygenase using crystallized chlorophenol 4-monooxygenase from Burkholderia cepacia AC1100 (3HWC) as the template. The model was assessed for its reliability using PROCHECK, ERRAT and ProSA. Molecular docking of the physiological substrates, PNP and 4-nitrocatechol (4-NC), was carried out using Glide v5.7 implemented in Maestro v9.2, and the binding energies were calculated to substantiate the prediction. Docking complexes formed by molecular level interactions of PNP monooxygenase-PNP/4-NC without or with the cofactors, FAD and NADH, showed good correlation with the established experimental evidence that the two-component PNP monooxygenase catalyzes both the hydroxylation of PNP and the oxidative release of nitrite from 4-NC in B. sphaericus JS905. Furthermore, molecular dynamics simulations performed for docking complexes using Desmond v3.0 showed stable nature of the interactions as well.