Mn(II)-Mn(III)-Mn(IV) redox cycling inhibits the removal of methylparaben and acetaminophen mediated by horseradish peroxidase: New insights into the mechanism
Graphical abstract
Introduction
Methylparaben (MeP) and acetaminophen (AAP) are important precursors of pharmaceuticals and personal care products (PPCPs) (Peng et al., 2021; Tan et al., 2021). MeP as a typical antibiotic has been widely used in foodstuffs, cosmetics and pharmaceuticals (Calafat et al., 2010; Guo and Kannan, 2013; Liao and Kannan, 2014; Liao et al., 2013), which increase its potential release, and lead to its wide detection in the environmental media (Haman et al., 2015; Wang and Kannan, 2016; Yoom et al., 2018). AAP is one of human derived pharmaceuticals extensively used for reducing fever and relieve pain (Bessems and Vermeulen, 2001). Numerous studies have demonstrated that AAP was partially metabolized with significant fraction excreted via urine and feces, ending up in sewage (Gong et al., 2020; Roberts and Thomas, 2006). Therefore, AAP is deemed to be one of the most frequently detected pharmaceuticals in surface water (Lu et al., 2009). In fact, during the production of AAP, MeP could be added as additives, so they are often detected together in soils, sludge and wastewater (Devault et al., 2020; Folarin et al., 2019). Due to their estrogenic activity, several studies have demonstrated that MeP and AAP may cause adverse effects to ecosystems and human beings (Nagar et al., 2020; Taboureau et al., 2020). Therefore, it is important to find a green and effective way to remove these two pollutants.
For potential remediation applications, it is desirable to identify an approach that can effectively remove PPCPs under naturally relevant conditions. Enzyme-mediated oxidation, which is considered to be an efficient and specific method without unreasonably long reaction times or overly intensive energy consumption, has been widely used to remove micropollutants, especially phenolic organics (Aldhahri et al., 2021; Gao et al., 2020). Horseradish peroxidase (HRP) is a representative of common extracellular peroxidases ubiquitously existed in aquatic environment. The peroxidase activity in lakes was reported in the range of 0.06– 4.71 mmol L−1 h−1 (ABTS) (Yang et al., 2017). HRP contains iron (III)-porphyrin catalytic center. In the presence of hydrogen peroxide, HRP could mediate single electron oxidation of substrate having phenolic groups to yield phenoxy radicals, and then the phenoxy radicals generated can couple with each other to form insoluble polymers that can be removed by sedimentation or filtration (Bansal and Kanwar, 2013; Li et al., 2017). Previous studies have demonstrated that HRP is able to catalyze the removal of phenolic pollutants, such as tetrabromobisphenol A, bisphenol A, 17α-ethinylestradiol, triclosan and diclofenac (Lu et al., 2015; Maryskova et al., 2021). Under real environmental conditions, however, numerous factors may affect the performance of HRP toward the removal of pollutants, which should be taken into consideration prior to a rational design.
Either in natural or water treatment process, degradation of pollutants is strongly impacted by metal ions (Calvin and Melchior, 1948). Previous studies revealed that transition metal ions can coordinate with donor residues at active sites to activate enzymes (Nazari et al., 2005). On the contrary, such coordination may hinder the interaction of substrate with enzymes and inhibit the reaction (Nazari et al., 2005). Manganese (Mn) is the third most abundant transition metal and plays an important role in geochemical cycles. In nature, Mn exists in three oxidation states, namely Mn(II), Mn(III) and Mn(IV), in which Mn(II) is the more often existent state (Liu et al., 2021; Ma et al., 2020; Qian et al., 2019). In addition, the concentration of Mn2+ was approximately 16 mg L−1 in battery plant wastewater and ranged from 3.5 to 17.2 mg L−1 in tannery effluents (Jiang et al., 2020; Kabir et al., 2017). Previous studies have demonstrated that the structural effects of dissolved Mn(II) when it reacts with manganese oxides involved both adsorption and comproportionation with Mn(IV), producing solid-phase Mn(III), which played an important role in the transformation of pollutants (Hinkle et al., 2017; Wang et al., 2014). For example, Wang et al. reported the ability of Mn(III) complexes to oxidize UO2, providing insights into Mn-mediated oxidation of other redox-active contaminants (Wang et al., 2014). However, the impacts of Mn(II) on the removal of phenolic pollutants mediated by HRP were rarely reported. Although Mahmoudi et al. found that Mn2+ inhibited the catalytic performance of HRP, the mechanism of inhibition was not clear (Mahmoudi et al., 2003). Additionally, our previous study found that slight differences in the structure of substrates may significantly affect their conversion rates mediated by enzymes (Shi et al., 2016). Therefore, it is speculated that Mn2+ may pose different impacts on the transformation of substrates (i.e. MeP and AAP) mediated by enzyme, and the underlying mechanism requires further study.
In this study, we addressed the influence of dissolved Mn(II) on the removal of MeP and AAP mediated by HRP in detail, and further explored the underlying mechanism using X-ray photoelectron spectroscopy (XPS) and theoretical calculations. We demonstrated for the first time that HRP could convert Mn(II) into Mn(III) and then generate MnO2 colloid, which significantly inhibited the removal of MeP but had little effect on the removal of AAP. Furthermore, theoretical calculations were applied to reveal the potential mechanism on the different impacts of Mn(II). Additionally, we also found that the presence of Mn(II) altered the product distribution of MeP by inhibiting the polymerization pathway during the HRP-catalyzed MeP transformation. This study will be beneficial for understanding the removal of MeP and AAP mediated by enzymes.
Section snippets
Materials
All reagents were ACS grade or better as described in the Supporting information. Methylparaben (MeP, purity >99%) and HRP (type 2, units 250 U mg−1) were purchased from Sigma-Aldrich Corporation (Shanghai, China). Acetaminophen (AAP, purity >99.5%) and other reagents were obtained from Aladdin Chemistry Co., Ltd. (Shanghai, China).
Reaction procedures
Experiments were conducted in 10 mL glass test tubes, which were incubated on a rotary shaker at 25.0 °C. The reaction solutions were all prepared in 10 mM phosphate
Removal of MeP and AAP mediated by HRP
The removal of MeP and AAP mediated by HRP were conducted with varying initial enzyme activities. As shown in Fig. 1A and B, the removal of the two pollutants increased with increasing HRP dosage, demonstrating that HRP mediated effective transformation of MeP and AAP. Moreover, the changes of MeP and AAP concentrations followed first order kinetics at each investigated enzyme dosages, indicating that HRP activity was relatively constant during the reaction. Notably, significant difference was
Conclusions
In summary, this study demonstrated that HRP mediated effective removal of MeP and AAP, but the conversion rate of AAP was about 284 times that of MeP. The presence of dissolved Mn(II) with a relative low concentration (0.2 μM) could significantly inhibit the HRP-catalyzed transformation of MeP, but had little influence on the removal of AAP. Moreover, our study proved that HRP could convert Mn(II) into Mn(III), and theoretical calculations demonstrated that when Mn(II) coexisted with HRP and
CRediT authorship contribution statement
Zhimin Gong: Conceptualization, Resources, Investigation, Methodology, Writing-original draft.
Gaobo Wang: Theoretical calculation.
Huanhuan Shi: Writing - review & editing.
Shuai Shao: Writing - review & editing.
Mengjie Wang: Validation.
Kun Lu: Conceptualization, Funding acquisition, Writing - review & editing.
Shixiang Gao: Conceptualization, Funding acquisition, Supervision, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (21906080, 21577059) and the Natural Science Foundation of Jiangsu Province (BK20190318).
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