Bioremediation of synthetic dyes: Dye decolorizing peroxidases (DyPs)

Synthetic dyes are a complex of aromatic molecules that possess coloration and are employed in many industries such as textile, leather, paper and pulp, plastics, food, pharmaceutical, cosmetics, etc. Based on the structural variation, different types of dyes such as azo, anthraquinone, reactive, triphenylmethane, acidic, basic, neutral, disperse, direct dyes, etc., are available and used in various industries. These dyes are released into the environment as effluents without proper treatment. These dyes are toxic, carcinogenic, and mutagenic, which affects life. They are resistant to conventional wastewater methods such as precipitation, filtration, and absorption. In the current scenario, enzymatic treatment is a most favorable approach to process these dyes because of its low energy cost and more ecofriendly nature. Peroxidases (EC1.11.1.x) are the heme-containing oxidoreductase enzymes that catalyze the H2O2-dependent oxidation of a variety of substrates such as dyes, small organic compounds, inorganic compounds, etc. Heme-containing peroxidases were initially classified into plant peroxidases and animal peroxidases. Subsequently, they were classified into various superfamilies on the basis of phylogenetic analysis: peroxidase-catalase, peroxidase-chlorite dismutase, peroxidase cyclooxygenase, di-heme peroxidase, and haloperoxidase. A new category of peroxidase was recently proposed, known as the family of dye-decolorizing (DyP-type) peroxidase. DyPs are the novel heme proteins that are structurally unrelated to other classical peroxidases. According to the PeroxiBase database, DyPs are categorized into four classes: A, B, C, and D. DyPs are receiving significant attention due to their stability in acidic conditions. Biochemical and mutational studies of DyPs have contributed to understanding the catalytic mechanism and role of amino acids in the enzyme catalysis. Furthermore, the structural and biophysical characterization of DyPs and comparative studies unveiled the substrate-binding sites, including the δ-site, the γ-site, and the surface-exposed redox-active residues; these sites are reported in other heme peroxidases also. DyPs have shown potential applications for the degradation of a wide range of dyes as well as lignin and β-carotene degradation. Therefore, DyPs are being used as a biocatalyst in industrial biotechnology. Further, protein engineering studies can be applied to increase the substrate-binding site and to enhance the degradation potential toward different substrates.