Insights into redox mediator–enhanced laccase degradation of tetracycline and sulfamethoxazole: MD simulations and experimental validation

Laccase–mediator systems (LMS) can be used to degrade antibiotics due to their electron–transfer mechanism. However, the impact of mediators on conformational changes in laccase structure remains unclear. In this study, we aimed to elucidate the mechanisms underlying the degradation of tetracycline and sulfamethoxazole in the presence of different mediators using molecular dynamics simulations. The presence of mediators influences the partial conformation and compactness of laccase. Among the selected mediators, caffeic acid (CA) induced a β–sheet to loop transition within the active binding pocket of laccase, and enhanced the binding stability of laccase–antibiotic complexes. Residue–level energy decomposition revealed that CA shifted the binding model from electrostatic to hydrophobic interactions in the laccase–tetracycline system, whereas it enhanced π–π stacking and hydrophobic effects in the laccase–sulfamethoxazole system. In vitro degradation experiments confirmed that LMS CA could degrade > 90% of tetracycline and sulfamethoxazole. When degrading mixed antibiotics, the removal reached 88.17% for tetracycline in 3 h and 98.71% for sulfamethoxazole in 1 h. Protocatechuic acid, a structural analog of CA, exhibited the same effect as CA in promoting antibiotic degradation. Specifically, tetracycline degradation reached 95.10% at 2 h, and sulfamethoxazole degradation reached 94.89% at 1 h. The degradation pathways were elucidated via LC–MS. Toxicological assays confirmed a significant reduction in the toxicity of antibiotics. Mixed sulfamethoxazole and tetracycline could be degraded by LMS CA in simulated contaminated freshwater. Overall, elucidating the mechanism via in silico study benefits LMS design and highly efficient removal of antibiotics.