Thermostable Bacterial Laccase: Catalytic Properties and Its Application in Biotransformation of Emerging Pollutants

Laccases have been predominantly reported in fungi, and primarily, fungal laccases are currently exploited in industrial applications. However, extremophilic bacterial laccases possess immense potential, as they can withstand extreme temperatures, pH, and salt concentrations. In addition, unlike fungal laccases, the production of bacterial laccases is cost-effective. Therefore, bacterial laccases are gaining significant attention for their large-scale applications. Previously, we reported a novel thermostable laccase (LacT) from Brevibacillus agri. Herein, we have confirmed that LacT shares a high sequence similarity with CotA laccase from Bacillus amyloliquefaciens. Peptide mass fingerprinting of LacT was conducted via matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF/MS-MS). Inductively coupled plasma-optical emission spectroscopic (ICP-OES) analysis revealed the presence of ∼3.95 copper ions per protein molecule. Moreover, the secondary and tertiary structure of LacT was studied using circular dichroism (CD) and fluorescence spectroscopy. The absence of notable shifts in CD and fluorescence spectra with an increase in temperature established that LacT remains intact even at elevated temperatures. Analysis of the thermal denaturation profile of LacT by thermogravimetric analysis (TGA) also confirmed its temperature stability. Thereafter, we exploited LacT in its application for the bioremediation of phenolic endocrine disruptors, namely, triclosan, 4,4'-dihydroxybiphenyl, and dienestrol. LacT oxidizes 4,4'-dihydroxybiphenyl and triclosan but no LacT activity was detected with dienestrol. The rate of biotransformation of 4,4'-dihydroxybiphenyl and triclosan increased in the presence of CuSO₄ and a redox mediator, ABTS. Transformation of dienestrol was observed only with LacT in the presence of ABTS. This study establishes the application of LacT for the bioremediation of phenolic compounds.