Computational insight into biotransformation of halophenols by cytochrome P450: Mechanism and reactivity for epoxidation.

Halophenols (XPs) have aroused great interests due to their high toxicity and low biodegradability. Previous experimental studies have shown that XPs can be catalytically transformed into epoxides and haloquinones by cytochrome P450 enzymes (CYPs). However, these metabolites have never been detected directly. Moreover, the effects of the reaction site and the type and number of halogen substituents on the biotransformation reactivity of halophenols still remain unknown. In this work, we performed density functional theory (DFT) calculations to simulate the CYP-mediated biotransformation of 36 XPs with mono-, di-, and tri-halogen (F, Cl, and Br) substitutions to unravel the mechanism and relevant kinetics of XPs epoxidation. The whole epoxidation process consists of initial rate-determining O-addition and subsequent ring-closure steps. The simulation results show that the epoxidation in low-spin (LS) state is kinetically preferred over that in high-spin (HS) state, and the formation of epoxide metabolite is strongly exothermic. For all XPs, the epoxidation reactivity follows the order of ortho/para O-addition > meta O-addition. Moreover, the O-addition with higher energy barriers roughly corresponds to chlorophenols and fluorophenols with more halogen atoms. Compared with dichlorophenols, the additional ortho-Cl substitution on trichlorophenols can slightly increase the energy barriers of meta O-addition. By contrast, the additional inclusion of an ortho-Cl to monochlorophenols enhances the meta O-addition reactivity of dichlorophenols. Overall, the present work clarifies the biotransformation routes of XPs to produce epoxides, and identifies the key factors affecting the epoxidation reactivity, which are beneficial in understanding comprehensively the metabolic fate and toxicity of XPs.

Binding and Metabolism of Brominated Flame Retardant ß-1,2-Dibromo-4-(1,2-dibromoethyl)cyclohexane in Human Microsomal P450 Enzymes: Insights from Computational Studies.

The emerging brominated flame retardant, 1,2-dibromo-4-(1,2-dibromoethyl)cyclohexane (TBECH), has recently attracted strong interest due to its extensive detection in the environment and potential toxicological effects on humans. Previous in vitro experiments have shown that the technical mixture of TBECH and the pure ß-isomer (ß-TBECH) can be metabolized by cytochrome P450 enzymes (CYPs) into multiple metabolites, but the specific CYP isoforms involved in TBECH metabolism and the relevant metabolic regioselectivity remain unknown. Here, we, for the first time, investigated the binding patterns and affinities of ß-TBECH in human CYPs 1A2, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1, and 3A4, through molecular dynamics (MD) simulations. The binding affinities of ß-TBECH in CYPs, which are estimated by the calculated binding free energies, follow the order of 2A6 > 2C9 > 2B6 > 2E1 > 3A4 ≈ 2C19 ≈ 1A2 > 2D6. Although all CYPs are important ß-TBECH receptors, only 2A6, 2C19, 2E1, and 3A4 are responsible for metabolizing ß-TBECH. Specially, 2A6 and 2E1 may selectively hydroxylate the C1 and C7 sites of ß-TBECH, while 2C19 and 3A4 show metabolic preference for C7- and C8-hydroxylations, respectively. The three hydroxylation routes proposed by the further density functional theory (DFT) calculations generate C1-, C7-, and C8-hydroxylated metabolites, while the latter two may further undergo debromination to yield the respective ketone and aldehyde as additional metabolites. The results provide meaningful insight into the binding and metabolism of ß-TBECH by human CYPs, which is helpful for understanding the metabolic fate and toxicity mechanism of this chemical.