**3.1 Main enzymes used in the bioremediation of textile dyes and toxicity of degradation by-products**

Enzyme-mediated bioremediation has gained notoriety due to its versatility and efficiency in the degradation of persistent organic pollutants, thus being applied in industrial, biotechnological, and environmental processes [38]. These enzymes can be obtained from the extraction of intracellular and extracellular metabolites from cultures of certain species of bacteria, fungi, algae, and plants [39].

**Table 3** shows some studies related to the degradation of dyes by enzymes produced by microorganisms. As it is shown, many of the tested can decolorize the dyes, as well as provide a decrease in their toxicity, as in the case, for example, of horseradish peroxidase, which promotes the decrease in the toxicity of the methyl orange dye.



#### **Table 3.**

*Main results of studies on dye bioremediation by enzymes and degradation by-products.*

#### *Enzymatic Bioremediation of Dyes from Textile Industry Effluents DOI: http://dx.doi.org/10.5772/intechopen.103064*

As reported in the literature, dye-decolorizing microorganisms produce a variety of enzymes, including azoreductase, riboflavin reductase, laccase, peroxidases, NADH-DCIP reductase, tyrosinase, reductase, and aminopyrine N-demethylase, lignin peroxidase, and veratryl alcohol oxidase [39]. Among those enzymes, the main ones responsible for the discoloration of azo dyes are azoreductases, laccases, and peroxidases [35].

Azoreductases are considered the main degradation enzymes produced by bacteria [30]. Such enzymes can be of two types—*i*. membrane-bound or *ii*. intracellular, which are thermostable, hydrophilic, and, in general, capable of degrading azo dyes more efficiently [51]. Laccases are copper-dependent and use oxygen to degrade lignin and other aromatic compounds, including textile dyes [32]. Because of their higher redox potential, fungal laccases, when compared to plant or bacterial laccases, are used to treat various xenobiotics, including textile dyes, present in water and soils [40].

Peroxidases also play a role in the degradation of the azo dye and are oxidoreductases, which contain heme. Peroxidases are present in plants, microorganisms, and animals. The mechanism of action of such enzymes is similar to that of laccases, providing the degradation of the dye without the production of toxic by-products [30]. Peroxidases act especially on synthetic dyes, degrading their respective constituents through the oxidative polymerization of phenolic compounds to form insoluble polymers [52]. An association between oxide-reducing enzymes can significantly reduce the toxicity of dyes [39].

Enzymes are proteins easily affected by changes in pH, and small variations in the medium's pH can result in changes in the ionization phase of the active site and the distribution of charge in the protein structure, possibly affecting its affinity for the substrate [52]. Thus, one of the main challenges of enzymatic treatment is the deactivation of the biocatalyst caused, mainly, by the denaturation of the enzyme, due to the pH of the medium or extreme temperatures, which can alter the conformation of the enzyme's active site [53]. Despite the many advances in enzymatic engineering, enzymes are still expensive and/or labile and, as a result, the industrial application of enzymes often requires their immobilization in a matrix (support) [54].

It is essential to evaluate the toxicity of effluents containing dyes after they have undergone enzymatic biodegradation, as some degradation products are mutagenic and carcinogenic, which represents a threat to human and animal health [30]. Thus, phytotoxicity tests are widely used and, according to the literature, among the bioindicators considered suitable for the detection of environmental toxicity, *Artemia salina and Daphnia magna* are cited [43].

Ali et al. [55] performed phytotoxicity studies, whose results indicate that MG-Y-SH can convert the toxic azo dye RR120 into nontoxic metabolites. However, many studies reported in the literature lack further tests to evaluate the by-products of enzymatic dye degradation, as well as the effects of these by-products on the environment.
