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

Abstract

A new extracellular laccase produced by Streptomyces ipomoea CECT 3341 (SilA) was cloned, overexpressed and its physico-chemical characteristics assessed together with its capability to decolourise and detoxify an azo-type dye. Molecular analysis from the deduced sequence revealed that SilA represents a TAT type signal peptide at the N-terminus and only two cupredoxine domains; this is consistent with reports from two other Streptomyces laccases but contrasts with most laccases in which three cupredoxine domains have been described. Heterologous expression and purification of SilA revealed that the homo-dimeric nature of this enzyme as the only active form of the enzyme. Among the physico-chemical characteristics identified, its stability at high pH and temperature, together with its resistance to elevated concentrations of NaCl and to typical laccase inhibitors such as sodium azide confirmed the unique properties of this novel enzyme. The range of substrates that can be oxidized by SilA was found to be pH-dependent; at alkaline pH, SilA was able to oxidize a wide range of phenolic compounds including compounds derived from the lignin structure, including both syringyl and guayacil moieties. 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.