A sustainable approach for degradation and detoxification of malachite green by an engineered polyphenol oxidase at high temperature

Laccases are green biocatalysts that are advantageous in the bioremediation of the triphenylmethane dye malachite green (MG). Here, Cblac from Caldicellulosiruptor bescii was characterized to be a novel DUF152 laccase. Cblac mutant Mut8 was designed with enhanced thermostability, which was demonstrated by the Thermofluor assay, circular dichroism spectrum, and structural analyses. MG bioremediation by Mut8 at high temperatures was performed and examined for its potential to degrade and detoxify the high-temperature MG effluent. Approximately 100% of 100 mg/L of MG was degraded by 40 U/L of Mut8 within 2 h at 75 °C. Mut8 initiated the degradation via oxidative cleavage of the central C–C bond and N-demethylation of MG, and the intermediates were further demethylated, hydroxylated and oxidized. Moreover, the systematic toxicity assessment of the MG degradation products using bacteria, Zea mays, and zebrafish demonstrated that Mut8 thoroughly eliminated MG toxicity. Taken together, Mut8 could effectively decolorize and detoxify MG at high temperatures, showing great promise as an environmental friendly and effective approach to remediate MG effluent immediately released from the dyeing process, thus avoiding the cooling process and lowering the cost simultaneously.