Over the last few decades many attempts have been made to use biocatalysts for the biotransformation of emerging contaminants in environmental matrices. Laccase, a multicopper oxidoreductase enzyme, has shown great potential in oxidizing a large number of phenolic and non-phenolic emerging contaminants. However, laccases and more broadly enzymes in their free form are biocatalysts whose applications in solution have many drawbacks rendering them currently unsuitable for large scale use. To circumvent these limitations, the enzyme can be immobilized onto carriers or entrapped within capsules; these two immobilization techniques have the disadvantage of generating a large mass of non-catalytic product. Insolubilization of the free enzymes as cross-linked enzymes (CLEAs) is found to yield a greater volume ratio of biocatalyst while improving the characteristics of the biocatalyst. Ultimately, novel techniques of enzymes insolubilization and stabilization are feasible with the combination of cross-linked enzyme aggregates (combi-CLEAs) and enzyme polymer engineered structures (EPESs) for the elimination of emerging micropollutants in wastewater. In this review, fundamental features of laccases are provided in order to elucidate their catalytic mechanism, followed by different chemical aspects of the immobilization and insolubilization techniques applicable to laccases. Finally, kinetic and reactor design effects for enzymes in relation with the potential applications of laccases as combi-CLEAs and EPESs for the biotransformation of micropollutants in wastewater treatment are discussed.
Our current global environmental challenges include the reduction of harmful chemicals and their derivatives. Bioremediation has been a key strategy to control the massive presence of chemicals in the environment. Enzymes including the phenoloxidases, laccases and tyrosinases, are increasingly being investigated as "green products" in the removal of many chemical contaminants in waters and soils. Both phenoloxidases are widespread in nature and attractive biocatalysts due to their ability to use readily available molecular oxygen as sole cofactor for their catalytic elimination of a large number of chemicals. Taking advantage of their catalytic potentials, remarkable advances have been made in the engineering of laccases to produce suitable biocatalysts in environmental applications. Studies about novel strategies of laccase immobilization and insolubilization for the treatment of chemical contaminants were provided. Likewise, tyrosinases are gaining increasing interest in environmental applications due to their catalytic similarities with laccases although they remain far less investigated to date. This disparity was addressed in this review along with the molecular features and catalytic mechanism of tyrosinases relevant in environmental applications. A perspective on the future use of laccases and tyrosinases in bioremediation was discussed.
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