Atypical laccases from the white-rot fungus Pleurotus ostreatus and their application for the treatment of industrial coloured effluents

Abstract

White-rot fungi are the most efficient decomposers of lignocellulose because of their capability to synthesize the relevant hydrolytic (cellulases and hemicellulases) and oxidative (laccases, lignin-peroxidases and Mn-peroxidases) extracellular enzymes required to degrade the major components of substrates (cellulose, hemicellulose, and lignin) into low-molecular-weight compounds that can be assimilated in fungal nutrition [1]. Recently, extensive research on these fungi has been conducted with the aim of isolating new organisms able to secrete new enzymes with capability to be used in industrial applications, such as bioremediation of polluted soils and industrial waste-waters, biobleaching and biopulping in pulp and paper industries, textile and food industries, etc. Fungal laccases (benzenediol: oxygen oxidoreductases; EC1.10.3.2) are ligninolytic enzymes that have been isolated from various fungi [2]. They belong to the class of the blue oxidases containing 4 copper atoms/molecule distributed in three different copper binding sites [3, 4]. The type-1 site is responsible for the intense blue colour of the enzyme due to a maximum absorbance at 605 nm; the type-2 site does not exhibit signals in the visible absorbance spectrum; and the type-3 site incorporates two copper centres and is responsible for a band near 330 nm. All these copper ions are involved in the catalytic mechanism. Laccases reduce oxygen to water and simultaneously perform a one electron oxidation of aromatic substrates (polyphenols, methoxysubstituted monophenols, aromatic amines, etc.). These enzymes are present in multiple isoforms, depending on the fungal species and environmental growth conditions [5, 6].