Laccase-Catalyzed Oxidation of Mixed Aqueous Phenolic Substrates at Low Concentrations

Inhibition mechanisms of Fe2+/Fe3+ and Mn2+ on fungal laccase-enabled bisphenol a polyreaction

Bisphenol A (BPA) is regarded as an endocrine disruptor associated with negative health effects in animals and humans. Laccase from white-rot fungus can enable BPA oxidation and auto-polymerization to circumvent its biotoxicity, but the work concerning the effect mechanisms of divalent and trivalent metal ions (MIs) on BPA polyreaction have rarely been reported. Herein, Trametes versicolor laccase-started BPA conversion within 1 h followed pseudo-first order kinetics, and the rate constant (k_prcs) and half-life were respectively 0.61 h^-1 and 1.14 h. The presence of Ca²⁺, Mg²⁺, Cu²⁺, Pb²⁺, Cd²⁺, Zn²⁺ and Al³⁺ exhibited insignificant impact on BPA removal, whereas Fe²⁺, Fe³⁺ and Mn²⁺ had a strong inhibiting effect. Compared with MI-free, the k_prcs values of BPA respectively lowered 34.4%, 44.3% and 98.4% in the presence of Fe²⁺, Fe³⁺ and Mn²⁺. Enzymatic activity and differential absorption spectrum disclosed that the inhibitory actions were accomplished by two different mechanisms. One is Fe²⁺ was preferentially oxidized into Fe³⁺ that restrained laccase activity at the initial stage of reaction, and subsequently, the formed Fe³⁺ complex bound with laccase T1-Cu site and thus impeded the single-electron transfer system. The other is Mn²⁺ was instantly oxidized by laccase to generate Mn³⁺-citrate complex, which completely consumed the dissolved O₂ in solution and consequently terminated BPA removal. Considering environmental bioremediation, T. versicolor laccase-enabled auto-polymerization is a simple and convenient candidate to eliminate BPA in enzymatic wastewater treatment, however the effects of Fe²⁺/Fe³⁺ and Mn²⁺ on BPA decontamination should be cautiously assessed.

Occurrence, environmental fate, ecological issues, and redefining of endocrine disruptive estrogens in water resources

The growing persistence of estrogenic pollutants in water resources is a worrying concern because of their endocrine disrupting activities and potentially hazardous consequences on the environmental matrices, ecology, and human health, even at low concentration. The long-term persistence of steroidal estrogens leads to their bioaccumulation in aquatic organisms that can further reach to humans via food chain route. Considering the toxicity of steroidal estrogens, it is important to mitigate these environmentally related hazardous contaminants. So far, several treatment methods, like adsorption, oxidation, irradiation, and electrochemical techniques have been proposed to eliminate estrogens from aqueous ecosystems. Nevertheless, high operational costs, insufficient removal, generation of toxic sludge, and the necessity of skilled maintenance and operating workers are the major hindrances associated with large scale applications. Bioremediation of steroidal estrogens using enzyme-based biocatalytic system has recently emerged as a promising alternative to remove and bio-transform estrogens from aqueous systems. However, the current literature lacks a critique focusing specifically and comprehensively on steroidal estrogens. The presented review is a critical assessment of the existing literature on steroid-based endocrine disruptive estrogens. A detailed description about the occurrence and eco-fate of steroidal estrogens is given with representative examples. The later half of the review stresses on the redefining (removal) of endocrine disruptive estrogens in water resources with particular reference to enzyme-based approaches.

Fungal laccase-triggered 17β-estradiol humification kinetics and mechanisms in the presence of humic precursors

Naturally-occurring phenolic acids (PAs) act as humic precursors that participate in the conversion behaviors and coupling pathways of steroidal estrogens (SEs) during laccase-triggered humification processes (L-THPs). Herein, the influences and mechanisms of PAs on Trametes versicolor laccase-evoked 17β-estradiol (E2) conversion kinetics and humification routes were explored. Fungal laccase was fleet in converting > 99% of E2, and the calculated pseudo-first-order velocity constant and half-time values were respectively 0.039 min^-1 and 17.906 min. PAs containing an O-dihydroxy moiety such as gallic acid and caffeic acid evidently hampered E2 humification owning to the yielded highly reactive O-quinones reversed E2 radicals by hydrogen transfer mechanism, implying that the inhibition effect was enormously dependent upon the number and position of the phenolic -OH present in humic precursors. Oligomers and polymers with carbon-carbon/oxygen links were tentatively found as E2 main humified species resulting from laccase-evoked successive oxidative-coupling. Note that PAs participating in the humification also encountered oxydehydrogenation, self-polymerization, and cross-binding to E2. Interestingly, the -COOH and -OCH₃ groups of PAs could be deprived in radical-caused self/co-polymerization. The generation of humified products not only circumvented the environmental risks of parent compounds but accelerated global carbon sequestration. To our knowledge, this is the first in-depth revelation of the humification pathways and related mechanisms of SEs with humic precursors in aquatic ecosystems by L-THPs.

Ecofriendly laccases treatment to challenge micropollutants issue in municipal wastewaters

In this study, a multidisciplinary approach investigated the enzymatic degradation of micropollutants in real, not modified, municipal wastewaters of a plant located in Italy. Stir Bar Sorptive Extraction combined to Gas Chromatography-Mass Spectrometric detection (SBSE-GC-MS) was applied to profile targeted pollutants in wastewaters collected after the primary sedimentation (W1) and the final effluent (W2). Fifteen compounds were detected at ng/L - μg/L, including pesticides, personal care products (PCPs) and drugs. The most abundant micropollutants were bis(2-ethylhexyl) phthalate, diethyl phthalate and ketoprofen. Laccases of Trametes pubescens MUT 2400 were very active against all the target micropollutants: except few cases, their concentration was reduced more than 60%. Chemical analysis and environmental risk do not always come together. To verify whether the treated wastewaters can represent a stressor for the aquatic ecosystem, toxicity was also evaluated. Raphidocelis subcapitata and Lepidium sativum tests showed a clear ecotoxicity reduction, even though they did not evenly respond. Two in vitro tests (E-screen test and MELN assay) were used to evaluate the estrogenic activity. Treatments already operating in the plant (e.g. activated sludge) partially reduced the estradiol equivalent concentration, and it was almost negligible after the laccases treatment. The results of this study suggest that laccases of T. pubescens are promising biocatalysts for the micropollutants transformation in wastewaters and surface waters.