Improving the Indigo Carmine Decolorization Ability of a Bacillus amyloliquefaciens Laccase by Site-Directed Mutagenesis

Indigo carmine is a typical recalcitrant dye which is widely used in textile dyeing processes. Laccases are versatile oxidases showing strong ability to eliminate hazardous dyes from wastewater. However, most laccases require the participation of mediators for efficient decolorization of indigo carmine. Here we describe the improvement of the decolorization ability of a bacterial laccase through site-directed mutagenesis. A D501G variant of Bacillus amyloliquefaciens laccase was constructed and overexpressed in Escherichia coli. The laccase activity in the culture supernatant achieved 3374 U center dot L-1 for the mutant. Compared with the wild-type enzyme, the D501G exhibited better stability and catalytic efficiency. It could decolorize more than 92% of indigo carmine without additional mediators in 5 h at pH 9.0, which was 3.5 times higher than the wild-type laccase. Isatin sulfonic acid was confirmed to be the main product of indigo carmine degradation by UV-vis and LC-MS analyses.