Laccases: structure, function, and potential application in water bioremediation

Abstract

The global rise in urbanization and industrial activity has led to the production and incorporation of foreign contaminant molecules into ecosystems, distorting them and impacting human and animal health. Physical, chemical, and biological strategies have been adopted to eliminate these contaminants from water bodies under anthropogenic stress. Biotechnological processes involving microorganisms and enzymes have been used for this purpose; specifically, laccases, which are broad spectrum biocatalysts, have been used to degrade several compounds, such as those that can be found in the effluents from industries and hospitals. Laccases have shown high potential in the biotransformation of diverse pollutants using crude enzyme extracts or free enzymes. However, their application in bioremediation and water treatment at a large scale is limited by the complex composition and high salt concentration and pH values of contaminated media that affect protein stability, recovery and recycling. These issues are also associated with operational problems and the necessity of large-scale production of laccase. Hence, more knowledge on the molecular characteristics of water bodies is required to identify and develop new laccases that can be used under complex conditions and to develop novel strategies and processes to achieve their efficient application in treating contaminated water. Recently, stability, efficiency, separation and reuse issues have been overcome by the immobilization of enzymes and development of novel biocatalytic materials. This review provides recent information on laccases from different sources, their structures and biochemical properties, mechanisms of action, and application in the bioremediation and biotransformation of contaminant molecules in water. Moreover, we discuss a series of improvements that have been attempted for better organic solvent tolerance, thermo-tolerance, and operational stability of laccases, as per process requirements.

Keywords: Bioremediation; Emerging contaminants; Laccases; Water bodies.

Fig. 1

Schematic illustration of the potential sources of water contaminants and their bioremediation by laccases. Emerging contaminants such as antibiotics, endocrine disruptors, dye-based pollutants and pharmaceutical drugs are often released into the environment causing harmful impacts and health problems to humans and other animals, water treatment with laccases and their biotechnological approaches generate less-toxic, inert or fully degraded compounds

Fig. 2

Phylogenetic tree constructed with some of the different organism sources of laccases, as well as some of their applications in bioremediation. According to their bacterial, insect, plant or fungal origin, they are colored with blue, red, green or orange, respectively. The alignments and phylogenetic relationships were done using the MEGA X suite

Fig. 3

Cartoon structures of the three-domain laccase from Bacillus subtilis (PDB 1GSK) and the homotrimeric two-domain laccase from Streptomyces coelicolor (PDB 3CG8). The domain assignations were made using the SWORD partition algorithm

Fig. 4

Representation of the different amino acids of the catalytic site that coordinates the catalytic coppers in Trametes versicolor laccase (PDB 1KYA). The amino acids of the histidine-cysteine pathway are in green

Fig. 5

Laccase structure conservation and function. a Structure of Trametes versicolor (PDB ID 1GYC), and Bacillus subtilis (PDB ID 1GSK) laccases compared to Cucurbita pepo (zucchini) ascorbate oxidase (PDB ID 1AOZ) from left to right. Domain 1 (D1) is at the front and right of the structure, domain 2 (D2) is behind and in the upper portion, domain 3 (D3) is at the left. Brown spheres symbolize the position of copper atoms, T1 above the trinuclear cluster. b The molecular surface shows protruding chemical groups, in red, and concave or cavity regions, in green. Some of these latter regions correspond to the ligand-binding site (LB) along with the dioxygen molecule entrance (O₂) and the water exit (H₂O) channels. Central and right images were created from that on the left by rotating it 30° over the horizontal axis, or 30° over the vertical axis, respectively

Fig. 6

Mechanism, kinetic model and structural elements involved in laccase functional properties and reaction. a Representation of the laccase mechanism of action in the active site of Trametes versicolor laccase (PDB 1KYA). In orange are represented those aminoacids involved in the binding, stabilization and orientation of the substrate, in grey and green those that are involved in the coordination of catalytic coppers and the electron transfer and in yellow those that transfer protons for the oxygen assisted reduction. b Laccase action on a lignin-model illustrating the domino effect [198]. c Complex two-site ping-pong bi-bi kinetic model proposed for the laccase reaction [184, 185]. d Structural and functional elements involved in different steps of the laccase reaction