Computational Insight into Biotransformation Profiles of Organophosphorus Flame Retardants to Their Diester Metabolites by Cytochrome P450

Abstract

Biotransformation of organophosphorus flame retardants (OPFRs) mediated by cytochrome P450 enzymes (CYPs) has a potential correlation with their toxicological effects on humans. In this work, we employed five typical OPFRs including tris(1,3-dichloro-2-propyl) phosphate (TDCIPP), tris(1-chloro-2-propyl) phosphate (TCIPP), tri(2-chloroethyl) phosphate (TCEP), triethyl phosphate (TEP), and 2-ethylhexyl diphenyl phosphate (EHDPHP), and performed density functional theory (DFT) calculations to clarify the CYP-catalyzed biotransformation of five OPFRs to their diester metabolites. The DFT results show that the reaction mechanism consists of Cα-hydroxylation and O-dealkylation steps, and the biotransformation activities of five OPFRs may follow the order of TCEP ≈ TEP ≈ EHDPHP > TCIPP > TDCIPP. We further performed molecular dynamics (MD) simulations to unravel the binding interactions of five OPFRs in the CYP3A4 isoform. Binding mode analyses demonstrate that CYP3A4-mediated metabolism of TDCIPP, TCIPP, TCEP, and TEP can produce the diester metabolites, while EHDPHP metabolism may generate para-hydroxyEHDPHP as the primary metabolite. Moreover, the EHDPHP and TDCIPP have higher binding potential to CYP3A4 than TCIPP, TCEP, and TEP. This work reports the biotransformation profiles and binding features of five OPFRs in CYP, which can provide meaningful clues for the further studies of the metabolic fates of OPFRs and toxicological effects associated with the relevant metabolites.

Keywords: P450 enzyme; biotransformation; density functional theory calculations; molecular dynamics simulations; organophosphorus flame retardant.

Figure 1

Free energy profiles for (a) C₁-hydroxylation of TDCIPP to IM_1-OH by Cpd I and (b) O-dealkylation of IM_1-OH to BDCIPP, along with the optimized geometries of relevant reaction species in HS and LS states. Bond lengths are given in angstroms, while relative energies are given in kcal/mol.

Figure 2

Free energy profiles for (a) C₁-hydroxylation of TCIPP to IM_1-OH by Cpd I and (b) O-dealkylation of IM_1-OH to BCIPP, along with the optimized geometries of relevant reaction species in HS and LS states. Bond lengths are given in angstroms, while relative energies are given in kcal/mol.

Figure 3

Free energy profiles for (a) C₁-hydroxylation of TCEP to IM_1-OH by Cpd I and (b) O-dealkylation of IM_1-OH to BCEP, along with the optimized geometries of relevant reaction species in HS and LS states. Bond lengths are given in angstroms, while relative energies are given in kcal/mol.

Figure 4

Free energy profiles for (a) C₁-hydroxylation of TEP to IM_1-OH by Cpd I and (b) O-dealkylation of IM_1-OH to DEP, along with the optimized geometries of relevant reaction species in HS and LS states. Bond lengths are given in angstroms, while relative energies are given in kcal/mol.

Figure 5

Free energy profiles for (a) C₁-hydroxylation of EHDPHP to IM_1-OH by Cpd I and (b) O-dealkylation of IM_1-OH to DPHP, along with the optimized geometries of relevant reaction species in HS and LS states. Bond lengths are given in angstroms, while relative energies are given in kcal/mol.

Figure 6

Averaged binding conformations of (a) TDCIPP, (b) TCIPP, (c) TCEP, (d) TEP, and (e) EHDPHP in CYP3A4 extracted from the last 20 ns MD trajectories. The key residues contributing to OPFRs binding are shown in the images, and corresponding binding energy contributions are given in kcal/mol and shown in the brackets. Distances are given in angstroms.