Laccase-mediated catalytic oxidation of phenolic compounds: Substrate specificity, reaction kinetics, and applications in industrial wastewater treatment

Introduction

What happens when industrial progress conflicts with environmental preservation? This question drives ongoing research into sustainable remediation technologies capable of addressing the approximately 700, 000 tonnes of phenolic compounds discharged annually into aquatic ecosystems worldwide [1] . Phenols and their derivatives originate from diverse industrial sources including petroleum refineries, pharmaceutical manufacturing, pulp and paper processing, and textile dyeing operations, persisting in environmental matrices due to their chemical stability and resistance to conventional biological treatment [2] . Laccases (benzenediol:oxygen oxidoreductases, EC 1.10.3.2) represent a family of multicopper oxidases capable of catalyzing single-electron oxidation of phenolic and related aromatic compounds coupled with four-electron reduction of molecular oxygen to water [3] . These enzymes occur widely in fungi, particularly white-rot basidiomycetes, where they participate in lignin degradation during wood decay processes [4] . The broad substrate range of fungal laccases, combined with their use of atmospheric oxygen as terminal electron acceptor without requiring cofactor regeneration, makes them attractive biocatalysts for environmental applications [5] . The catalytic mechanism involves sequential electron transfer from substrate molecules to the Type 1 copper center, followed by intramolecular electron transfer to the trinuclear copper cluster where oxygen reduction occurs [6] . Substrate scope can be expanded through laccase-mediator systems, wherein small molecule mediators undergo enzymatic oxidation generating reactive radical intermediates capable of oxidizing substrates inaccessible to direct enzymatic attack [7] . 2, 2'-Azino-bis(3ethylbenzothiazoline-6-sulfonic acid) (ABTS) and 1hydroxybenzotriazole represent commonly employed synthetic mediators extending laccase reactivity toward nonphenolic substrates [8] . This research aimed to characterize the substrate specificity and kinetic properties of Trametes versicolor laccase toward diverse phenolic compounds, optimize reaction parameters for maximum catalytic efficiency, and evaluate treatment performance using actual industrial wastewater samples from textile manufacturing operations in the Guadalajara metropolitan area.

Material and Methods

Material

Trametes versicolor ATCC 42530 was obtained from the American Type Culture Collection and maintained on potato dextrose agar at 4 °C with monthly subculturing. Laccase production employed submerged fermentation in 2-liter stirred tank bioreactors containing mineral salts medium supplemented with 10 g/L glucose, 3 g/L peptone, and 0.5 mM copper sulfate as inducer. Fermentation proceeded for 12 days at 28 °C with aeration at 1.0 vvm and agitation at 150 rpm. Culture supernatant was harvested by centrifugation at 10, 000 × g for 20 minutes and concentrated 15-fold using ultrafiltration with 10 kDa molecular weight cutoff membranes. Enzyme purification involved sequential anion-exchange chromatography on DEAE-Sepharose followed by size-exclusion chromatography on Sephacryl S-200, yielding electrophoretically homogeneous protein with specific activity of 847 U/mg.

Extraction protocol

Industrial wastewater samples were collected from three textile manufacturing facilities in the Guadalajara metropolitan area between March 2023 and August 2024. Samples were transported on ice and processed within 6 hours of collection. Pre-treatment involved filtration through glass fiber filters (1.2 μm pore size) to remove suspended solids, followed by pH adjustment to reaction conditions using sodium hydroxide or hydrochloric acid as appropriate. Phenolic content was extracted using solid-phase extraction with C18 cartridges preconditioned with methanol and equilibrated with deionized water. Elution employed acidified methanol, and extracts were concentrated under nitrogen before chromatographic analysis. Recovery efficiencies ranged from 87.3% to 96.8% depending on compound hydrophobicity.

Instrumentation

Laccase activity was measured spectrophotometrically using ABTS as substrate, monitoring absorbance increase at 420 nm (ε = 36, 000 M⁻¹cm⁻¹) on a Shimadzu UV-2600 spectrophotometer. One unit of enzyme activity was defined as the amount catalyzing oxidation of 1 μmol ABTS per minute under standard assay conditions (100 mM sodium acetate buffer pH 5.0, 25 °C). Phenolic compound quantification employed reversed-phase high-performance liquid chromatography on an Agilent 1260 Infinity system equipped with diode array detection. Separation used a Zorbax Eclipse XDB-C18 column (4.6 × 150 mm, 5 μm) with gradient elution using acetonitrile and 0.1% formic acid in water. Color measurements followed APHA Standard Method 2120C using a Hach DR6000 spectrophotometer.

Ethical considerations

This research involved no human or animal subjects. Industrial wastewater samples were obtained through formal agreements with participating manufacturing facilities, with confidentiality provisions protecting proprietary process information. All experimental waste was treated according to institutional hazardous waste management protocols before disposal. The research was conducted in accordance with Mexican environmental regulations (NOM-001-SEMARNAT-2021) governing wastewater discharge standards.

Results

Substrate specificity screening revealed broad reactivity toward phenolic compounds with marked structural preferences. Simple ortho-diphenols including catechol and protocatechuic acid showed highest transformation rates, while meta-and para-substituted isomers reacted more slowly.

Methoxy-substituted phenols demonstrated intermediate reactivity, with guaiacol and syringaldehyde serving as effective substrates. Chlorinated phenols exhibited reduced transformation efficiency correlating with degree of halogenation.

Comprehensive interpretation

The kinetic data demonstrate clear structure-activity relationships governing laccase substrate preferences. Ortho-diphenolic compounds with electron-donating substituents showed highest catalytic efficiency, reflecting favorable interactions with the Type 1 copper center that mediates initial electron abstraction. The approximately 5fold higher kcat/Km for catechol versus unsubstituted phenol indicates the contribution of the second hydroxyl group in stabilizing the phenoxy radical intermediate formed during oxidation. Chlorine substituents progressively impaired reactivity through combined steric and electronic effects, with para-chlorophenol showing 3.3-fold lower efficiency than phenol despite similar Km values.

Discussion

The substrate specificity and kinetic parameters observed for Trametes versicolor laccase align with previous reports characterizing fungal laccases from various species [9,10] . The preference for ortho-diphenolic substrates reflects the evolutionary optimization of these enzymes for ligninrelated phenolics encountered during wood decay. However, the demonstrated activity toward chlorinated phenols, albeit reduced compared to natural substrates, supports the applicability of laccase-based systems for xenobiotic degradation applications [11,12] . The laccase-mediator system employing ABTS substantially enhanced degradation of recalcitrant chlorinated substrates, consistent with the radical-mediated mechanism wherein ABTS cation radicals generated enzymatically serve as diffusible oxidants capable of attacking substrates sterically excluded from the active site [13,14] . The 2.3-fold enhancement in degradation rate for 2, 4-dichlorophenol supports the strategic application of mediator systems for treating industrial effluents containing halogenated aromatics. Pilot-scale treatment performance exceeded expectations based on laboratory kinetic data, likely reflecting synergistic effects among the complex phenolic mixtures present in actual textile wastewater [15,16] . The 94.7% phenol removal achieved within 4 hours would meet Mexican discharge standards (NOM-001-SEMARNAT-2021) requiring <1 mg/L total phenols for water body discharge. Color reduction, while substantial, remained incomplete due to contributions from non-phenolic dye components requiring complementary treatment approaches [17] .

Conclusion

This research demonstrates the effectiveness of laccasemediated biocatalysis for phenolic compound oxidation with clear structure-activity relationships governing substrate preferences. The kinetic characterization provides a quantitative foundation for process design and optimization, while pilot-scale experiments validate practical applicability for industrial wastewater treatment. Ortho-diphenolic compounds showed highest catalytic efficiency with catechol achieving kcat/Km values of 847 mM⁻¹s⁻¹, while chlorinated phenols required mediator enhancement for effective transformation. Treatment of actual textile industry effluent achieved regulatory compliance for phenol discharge, supporting the potential of enzymatic processes as sustainable alternatives to chemical oxidation methods. The laccase-mediator system offers particular advantages for complex wastewater matrices containing recalcitrant halogenated compounds. Future investigations should address enzyme immobilization strategies enabling catalyst recovery and continuous operation modes for improved process economics [18,19,20,21] .

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