Recent advances in azo dye degrading enzyme research

Abstract

Azo dyes, which are characterized by one or more azo bonds, are a predominant class of colorants used in tattooing, cosmetics, foods, and consumer products. These dyes are mainly metabolized by bacteria to colorless aromatic amines, some of which are carcinogenic, by azoreductases that catalyze a NAD(P)H-dependent reduction. The resulting amines are further degraded aerobically by bacteria. Some bacteria have the ability to degrade azo dyes both aerobically and anaerobically. Plant-degrading white rot fungi can break down azo dyes by utilizing a number of oxidases and peroxidases as well. In yeast, a ferric reductase system participates in the extracellular reduction of azo dyes. Recently, two types of azoreductases have been discovered in bacteria. The first class of azoreductases is monomeric flavin-free enzymes containing a putative NAD(P)H binding motif at their N-termini; the second class is polymeric flavin dependent enzymes which are studied more extensively. Azoreductases from bacteria represent novel families of enzymes with little similarity to other reductases. Dissociation and reconstitution of the flavin dependent azoreductases demonstrate that the non-covalent bound flavin prosthetic group is required for the enzymatic functions. In this review, structures and carcinogenicity of azo colorants, protein structure, enzymatic function, and substrate specificity, as well as application of the azo dyes and azoreductases will be discussed.

Fig. 1

Reductive degradation of azo dyes by azoreductases.

Fig. 2

Chemical structures of some azo pigments identified in tattoo inks.

Fig. 3

Azo linkage in the drug is cleaved by azoreductases resulting in releasing an anti-inflammatory agent, 5-aminosalicylic acid, and an antibiotic agent, sulfapyridine, in the colon (Adapted from [19]).

Fig. 4

Unrooted phylogenetic tree of NADPH-flavin azoreductases. Based on TBALSTN results, protein sequences showing similarities were aligned using Jutun Hein method. The unrooted tree was generated using MegAlign of DNASTAR software package. Genbank accession numbers are as follows: Archaeoglobus fulgidus, AE000972; Bacillus anthracis, AE017225; Bacillus cereus, AE017005; Bacillus licheniformis, CP000002; Bacillus subtilis, AB071366; Bacillus sp. OY1-2; Bartonella henselae, BX897699; Clostridium perfringens, BA000016; Gloeobacter violaceus, BA000045; Haloarcula marismortui AY596297; Halobacterium sp. NRC-1, AE005072; Lactobacillus plantarum, AL935261; Mesorhizobium loti, BA000045; Mycobacterium bovis, BX248344; Mycobacterium tuberculosis, AE000516; Nocardia farcinica, AP006618; Nostoc sp. PPC 7120, BA000019; Rhodobacter sphaeroides, AY150311; Staphylococcus aureus, AY545994; Staphylococcus epidermidis, AE016745; Streptomyces avermitilis, AP005049; Streptomyces coelicolor, AL939107; Sinorhizobium meliloti, AL595985; Xanthomonas oryzae AE013598 (Adapted from [61]).

Fig. 5

Mechanisms of the dissociation and re-association of azoreductase of Staphylococcus aureus expressed in Escherichia coli (Adapted from [61]).