Combination of thermal and biological treatments for bio-removal and detoxification of some recalcitrant synthetic dyes by betaine-induced thermostabilized laccase

Abstract

The presence of recalcitrant synthetic dyes in environment is a major concern due to their possible threat on living organisms and aquatic ecosystems. To investigate the potential of combined thermal and enzymatic decolorization, betaine was applied to protect the secondary and tertiary structures as well as the enzymatic activity of laccase under thermal stress. At 60 °C, betaine 1 M significantly enhanced the stability of laccase in addition to keeping its catalytic efficiency ( p < 0.05). Differential scanning calorimetry showed a 20 °C increase in T m of laccase in the presence of betaine. Once incubated at 60 °C, betaine exhibited superior protein-stabilizing properties than at 25 °C. Furthermore, fluorescence spectroscopy supported this concept that the overall folded structure of laccase became more rigid in the presence of betaine at 60 °C. Betaine-stabilized laccase was used for combined thermal and enzymatic decolorization of some recalcitrant organic dyes. This osmolyte could improve the enzymatic removal of most dyes at 60 °C. A detoxification study with respect to the inhibition of bacterial growth showed a decrease in microtoxicity of the laccase-treated dye solution in the presence of betaine at 60 °C. • Betaine protects secondary and tertiary structures of laccase under thermal stress. • Betaine enhances the thermal stability and catalytic efficiency of laccase. • Laccase shows a dramatic increase in α -helix content in the presence of betaine. • Betaine improved the enzymatic removal of some recalcitrant synthetic dyes at 60 °C. • Microtoxicity of treated dyes was remarkably decreased compared to nontreated dyes