Detoxification of azo dyes by a novel pH-versatile, salt-resistant laccase from Streptomyces ipomoea

摘要

A newly identified extracellular laccase produced by Streptomyces ipomoea CECT 3341 (SilA) was cloned and overexpressed, and its physicochemical characteristics assessed together with its capability to decolorize and detoxify an azo_type dye. Molecular analysis of the deduced sequence revealed that SiIA contains a TAT-type signal peptide at the N-termi_nus and only two cupredoxine domains; this is consistent with reports describing two other Streptomyces laccases but con_trasts with most laccases, which contain three cupredoxine domains. The heterologous expression and purification of SilA revealed that the homodimer is the only active form of the enzyme. Its stability at high pH and temperature, together with its resistance to high concentrations of NaCI and to typical laccase inhibitors such as sodium azide confirmed the unique prop_erties of this novel laccase. The range of substrates that Si1A is able to oxidize was found to be pH-dependent; at alkaline pH, Si1A oxidized a wide range of phenolic compounds, including the syringyl and guayacil moieties derived from lignin. The oxidative potential of this enzyme to use phenolic compounds as natural redox mediators was shown through the coordinated action of SilA and acetosyringone (as mediator), which resulted in the complete detoxification of the azo-type dye Orange II.