Biodegradation of Cosmetics Products: A Computational Study of Cytochrome P450 Metabolism of Phthalates

Abstract

Cytochrome P450s are a broad class of enzymes in the human body with important functions for human health, which include the metabolism and detoxification of compounds in the liver. Thus, in their catalytic cycle, the P450s form a high-valent iron(IV)-oxo heme cation radical as the active species (called Compound I) that reacts with substrates through oxygen atom transfer. This work discusses the possible degradation mechanisms of phthalates by cytochrome P450s in the liver, through computational modelling, using 2-ethylhexyl-phthalate as a model substrate. Phthalates are a type of compound commonly found in the environment from cosmetics usage, but their biodegradation in the liver may lead to toxic metabolites. Experimental studies revealed a multitude of products and varying product distributions among P450 isozymes. To understand the regio-and chemoselectivity of phthalate activation by P450 isozymes, we focus here on the mechanisms of phthalate activation by Compound I leading to O-dealkylation, aliphatic hydroxylation and aromatic hydroxylation processes. We set up model complexes of Compound I with the substrate and investigated the reaction mechanisms for products using the density functional theory on models and did a molecular mechanics study on enzymatic structures. The work shows that several reaction barriers in the gas-phase are close in energy, leading to a mixture of products. However, when we tried to dock the substrate into a P450 isozyme, some of the channels were inaccessible due to unfavorable substrate positions. Product distributions are discussed under various reaction conditions and rationalized with valence bond and thermodynamic models.

Biotransformation of Phthalate Plasticizers and Bisphenol A by Marine-Derived, Freshwater, and Terrestrial Fungi

“…P450s also relevant to mammal PE metabolism (Choi et al, 2012(Choi et al, , 2013Girvan and Munro, 2016;Cantú Reinhard and De Visser, 2017). Cytochrome P450-catalyzed oxidations are commonly substrate-specific, as well as regio-, and frequently stereoselective, with different isoenzymes being responsible for the respective type of reaction, its selectivity, and hence also product variability (Choi et al, 2012;Syed and Yadav, 2012;Girvan and Munro, 2016;Cantú Reinhard and De Visser, 2017). The enormous catabolic versatility of certain fungal lineages is partly based on corresponding inventories of multiple cytochrome P450encoding genes (Harms et al, 2011;Syed and Yadav, 2012;Syed et al, 2013).…”

“…The successive enzymatic hydrolysis of phthalate diesters to the corresponding monoesters and then to PA is the most common PE transformation pathway in bacteria under both aerobic and anaerobic conditions, having also been reported for fungi (Lee et al, 2007;Liang et al, 2008;Ahuactzin-Perez et al, 2016Ren et al, 2018). The alternative breakdown pathway involving alkyl chain removal via O-dealkylation is known to be catalyzed by cytochrome P450 monooxygenase systems in mammals, however it is not well understood in fungi and prokaryotes (Lee et al, 2007;Liang et al, 2008;Girvan and Munro, 2016;Cantú Reinhard and De Visser, 2017). Additionally, phthalates with longer side chains than diethyl phthalate (DEP) may first undergo alkyl chain shortening via β-oxidation, a process initiated by cytochrome P450 mediated hydroxylation, prior to de-esterification occurring (Amir et al, 2005;Liang et al, 2008).…”

“…Thus, hydrolytic enzymes (e.g. cutinases, esterases, lipases) and intracellular oxidative cytochrome P450s can be considered as primary candidates for initiating the biocatalytic breakdown of PEs in fungi Ghaly et al, 2010;Okamoto et al, 2011;Ahuactzin-Perez et al, 2016;Cantú Reinhard and De Visser, 2017;Ren et al, 2018), with major evidence for the latter remaining to be established. Aside from enzymatic removal, biosorption via physio-chemical processes such as adsorption, absorption, and ion interactions may also contribute and must be accounted for when investigating PE fate within the environment (Liang et al, 2008;Gadd, 2009;Net et al, 2015).…”

A Comparative Review on the Catalytic Mechanism of Nonheme Iron Hydroxylases and Halogenases

“…Furthermore, a variety of spectroscopic techniques have studied short-lived intermediates of CPO through ultraviolet-visible (UV-Vis) absorption [53], resonance Raman [54], electron paramagnetic resonance (EPR) [55,56], and Mössbauer [55] spectroscopic measurements. At the same time, computational studies using density functional theory (DFT) as well as quantum mechanics/molecular mechanics (QM/MM) has given insight into the structure and activity of Compound I [38,[50][51][52][57][58][59][60]. Thus, Compound I of CPO reacts with halide anions, typically Cl − or Br − , to form hypohalous products in a one-step reaction mechanism [61,62].…”

“…The V-HPOs are found in macroalgae, fungi and bacteria, and are further classified in accordance with the most electronegative halide that they react with. As such, vanadium One of these intermediates is compound I, which has been both spectroscopically characterized [43][44][45], and studied by computational modelling [46][47][48][49][50][51][52]. Furthermore, a variety of spectroscopic techniques have studied short-lived intermediates of CPO through ultraviolet-visible (UV-Vis) absorption [53], resonance Raman [54], electron paramagnetic resonance (EPR) [55,56], and Mössbauer [55] spectroscopic measurements.…”