Improving biocatalytic properties of an azoreductase via the N-terminal fusion of formate dehydrogenase

Azoreductases require NAD(P)H to reduce azo dyes but the costly price of NAD(P)H limits its application. Formate dehydrogenase (FDH) allows NAD(P)+ recycling and therefore, the fusion of these two biocatalysts seems promising. This study investigated the changes to the fusion protein involving azoreductase (AzoRo) of Rhodococcus opacus 1CP and FDH (FDHC23S and FDHC23SD195QY196H) of Candida boidinii in different positions with His-tag as the linker. The position affected enzyme activities as AzoRo activity decreased by 20-fold when it is in the N-terminus of the fusion protein. FDHC23S+AzoRo was the most active construct and was further characterized. Enzymatic activities of FDHC23S+AzoRo decreased compared to parental enzymes but showed improved substrate scope - accepting bulkier dyes. Moreover, pH has an influence on the stability and activity of the fusion protein because at pH 6 (pH that is suboptimal for FDH), the dye reduction decreased to more than 50% and this could be attributed to the impaired NADH supply for the AzoRo part.

Wasteful Azo Dyes as a Source of Biologically Active Building Blocks

In this work, an environment-friendly enzymatic strategy was developed for the valorisation of dye-containing wastewaters. We set up biocatalytic processes for the conversion of azo dyes representative of the main classes used in the textile industry into valuable aromatic compounds: aromatic amines, phenoxazinones, phenazines, and naphthoquinones. First, purified preparations of PpAzoR azoreductase efficiently reduced mordant, acid, reactive, and direct azo dyes into aromatic amines, and CotA-laccase oxidised these compounds into phenazines, phenoxazinones, and naphthoquinones. Second, whole cells containing the overproduced enzymes were utilised in the two-step enzymatic conversion of the model mordant black 9 dye into sodium 2-amino-3-oxo-3H-phenoxazine-8-sulphonate, allowing to overcome the drawbacks associated with the use of expensive purified enzymes, co-factors, or exquisite reaction conditions. Third, cells immobilised in sodium alginate allowed recycling the biocatalysts and achieving very good to excellent final phenoxazine product yields (up to 80%) in water and with less impurities in the final reaction mixtures. Finally, one-pot systems using recycled immobilised cells co-producing both enzymes resulted in the highest phenoxazinone yields (90%) through the sequential use of static and stirring conditions, controlling the oxygenation of reaction mixtures and the successive activity of azoreductase (anaerobic) and laccase (aerobic).

Multifunctional Programmable DNA Nanotrain for Activatable Hypoxia Imaging and Mitochondrion-Targeted Enhanced Photodynamic Therapy

Programmable DNA-based nanostructures (e.g., nanotrains, nanoflowers, and DNA dendrimers) provide new approaches for safe and effective biological imaging and tumor therapy. However, few studies have reported that DNA-based nanostructures respond to the hypoxic microenvironment for activatable imaging and organelle-targeted tumor therapy. Herein, we innovatively report an azoreductase-responsive, mitochondrion-targeted multifunctional programmable DNA nanotrain for activatable hypoxia imaging and enhanced efficacy of photodynamic therapy (PDT). Cyanine structural dye (Cy3) and black hole quencher 2 (BHQ2), which were employed as a fluorescent mitochondrion-targeted molecule and azoreductase-responsive element, respectively, covalently attached to the DNA hairpin monomers. The extended guanine (G)-rich sequence at the end of the DNA hairpin monomer served as a nanocarrier for the photosensitizer 5,10,15,20-tetrakis(4-N-methylpyridiniumyl) porphyrin (TMPyP4). Upon initiation between the DNA hairpin monomer and initiation probe, the fluorescence of Cy3 and the singlet oxygen (¹O₂) generation of TMPyP4 in the programmable nanotrain were effectively quenched by BHQ2 through the fluorescence resonance energy transfer (FRET) process. Once the programmable nanotrain entered cancer cells, the azo bond in BHQ2 will be reduced to amino groups by the high expression of azoreductase under hypoxia conditions; then, the fluorescence of Cy3 and the ¹O₂ generation of TMPyP4 will significantly be restored. Furthermore, due to the mitochondrion-targeting characteristic endowed by Cy3, the TMPyP4-loaded nanotrain would accumulate in the mitochondria of cancer cells and then demonstrate enhanced PDT efficacy under light irradiation. We expect that this programmable DNA nanotrain-based multifunctional nanoplatform could be effectively used for activatable imaging and high performance of PDT in hypoxia-related biomedical field.

Bio-Decolorization of Synthetic Dyes by a Halophilic Bacterium Salinivibrio sp

Synthetic dyes, extensively used in various industries, act as pollutants in the aquatic environment, and pose a significant threat to living beings. In the present study, we assessed the potential of a halophilic bacterium Salinivibrio kushneri HTSP isolated from a saltpan for decolorization and bioremediation of synthetic dyes. The genomic assessment of this strain revealed the presence of genes encoding the enzymes involved in decolorization mechanisms including FMN-dependent NADH azoreductase Clade III, which cleave the azo bond of the dye, and the enzymes involved in deamination and isomerization of intermediate compounds. The dye decolorization assay was performed using this bacterial strain on three water-soluble dyes in different concentrations: Coomassie brilliant blue (CBB) G-250 (500–3,000 mg/L), Safranin, and Congo red (50–800 mg/L). Within 48 h, more than 80% of decolorization was observed in all tested concentrations of CBB G-250 and Congo red dyes. The rate of decolorization was the highest for Congo red followed by CBB G-250 and then Safranin. Using UV-Visible spectrometer and Fourier Transform Infrared (FTIR) analysis, peaks were observed in the colored and decolorized solutions. The results indicated a breakdown of dyes upon decolorization, as some peaks were shifted and lost for different vibrations of aromatic rings, aliphatic groups (–CH₂, –CH₃) and functional groups (–NH, –SO₃H, and –SO₃⁻) in decolorized solutions. This study has shown the potential of S. kushneri HTSP to decolorize dyes in higher concentrations at a faster pace than previously reported bacterial strains. Thus, we propose that our isolated strain can be utilized as a potential dye decolorizer and biodegradative for wastewater treatment.

Purification, characterization, and crystal structure of YhdA-type azoreductase from Bacillus velezensis

Azoreductases are being extensively investigated for their ability to initiate degradation of recalcitrant azo dyes through reduction of azo bonds. There is great interest in studying their diversity, structure, and function to facilitate better understanding and effective application. Current study reports azoreductase enzyme from Bacillus velezensis, which showed 69.5% identity to the Bacillus subtilis azoreductase YhdA. The enzyme was homotetrameric and molecular weight of each subunit was 20 kDa. It decolorized azo dyes with different structures. The V_max for decolorization of congo red, methyl orange and methyl red was 14.7, 28.6, and 77.9 nmol/min/mg, respectively. The enzyme contained FMN as cofactor and used NADPH as the favored co-substrate. It was oxygen-insensitive, but the presence of reducing agents enhanced its activity, which is a new finding. The azoreductase expression in B. velezensis was found to be unaffected by addition of azo dyes, although azo dyes are known to induce azoreductase expression in few organisms. The enzyme was thermostable with melting temperature of 89.5°C and functioned in wide temperature range. Further, the enzyme was crystallized and its structure was solved. The structural basis of its functional attributes is discussed. In our knowledge, this is the first report on characterization of azoreductase enzyme from B. velezensis.

Performance of Burkholderia multivorans CCA53 for ethyl red degradation

The discharge of industrial dyes and their breakdown products are often environmentally harmful. Here, we describe a biodegradation method using Burkholderia multivorans CCA53, which exhibits a capacity to degrade azo dyes, particularly ethyl red. Under the optimized culture conditions, 100 μM ethyl red was degraded more than 99% after incubation for 8 h. Real-time PCR analysis of azoR1 and azoR2, encoding two azoreductases, revealed that transcription level of these genes is enhanced at early phase under the optimized conditions. For a more practical approach, hydrolysates were prepared from eucalyptus or Japanese cedar chips or rice straw, and rice straw hydrolysate was used as the best medium for ethyl red biodegradation. Under those conditions, ethyl red was also degraded with high efficiency (>91%). We have thus constructed a potentially economical method for the biodegradation of ethyl red.

Azoreductase-Responsive Metal-Organic Framework-Based Nanodrug for Enhanced Cancer Therapy via Breaking Hypoxia-induced Chemoresistance

The insufficient oxygen supply may cause hypoxia in a solid tumor, which can lead to drug resistance and unsatisfactory chemotherapy effect. To address this issue, a new nanodrug has been developed with azoreductase-responsive functional metal-organic frameworks (AMOFs), where chemotherapeutic drugs were encapsulated in the AMOFs and small interfering RNAs (siRNAs) were absorbed on the surface of AMOFs. The siRNA was designed to contain hypoxia-inducible factor (HIF)-1α against RX-0047, which can induce significant downregulation of HIF-1α protein. The azobenzene units within the frameworks of AMOFs could be reduced to amines by the highly expressed azoreductase under the oxygen-deficient environment, which results in azoreductase-responsive release of the encapsulated drugs and siRNAs under the hypoxic condition. Therefore, once the drug-loaded AMOF entered the hypoxic cancer cells, the azoreductase-responsive release of siRNA could decrease the efflux of chemotherapeutic drugs via inhibiting the expressions of HIF-1α, multidrug resistance gene 1, and P-glycoprotein. This nanodrug can thus efficiently break hypoxia-induced chemoresistance and result in high-efficient cancer therapy in hypoxic tumors. As far as we know, this is the first attempt to construct an AMOF-based nanodrug with hypoxic harvesting behaviors. This proof-of-concept research provides a simple strategy for the construction of hypoxic-responsive AMOFs and also offers a unique on-command drug delivery platform, which can effectively break hypoxia-induced chemoresistance.

Improving the thermal stability of azoreductase from Halomonas elongata by introducing a disulfide bond via site-directed mutagenesis

Azoreductases mainly reduce azo dyes, the largest class of colorants, to colorless aromatic amines. AzoH, a new azoreductase from the halophilic bacterium, Halomonas elongata, has been recently cloned and expressed in Escherichia coli. The aim of this study was to improve thermal stability of this enzyme by introducing new disulfide bonds. Since X-ray crystallography was not available, homology modeling and molecular dynamics was used to construct the enzyme three-dimensional structure. Potential disulfide bonds for increasing thermal stability were found using DIScover online software. Appropriate mutations (L49C/D108C) to form a disulfide bond were introduced by the Quik-Change method. Mutant protein expressed in E. coli showed increased thermal stability at 50 °C (increased half-life from 12.6 Min in AzoH to 26.66 Min in a mutated enzyme). The mutated enzyme could also tolerate 5% (w/v) NaCl and retained 30% of original activity after 24 H incubation, whereas the wild-type enzyme was completely inactivated. According to circular dichroism studies, the secondary structure was not altered by this mutation; however, a blue shift in intrinsic florescent graph revealed changes in the tertiary structure. This is the first study to improve thermal stability and salt tolerance of a halophilic azoreductase.

Identification of novel members of the bacterial azoreductase family in Pseudomonas aeruginosa

Azoreductases are a family of diverse enzymes found in many pathogenic bacteria as well as distant homologues being present in eukarya. In addition to having azoreductase activity, these enzymes are also suggested to have NAD(P)H quinone oxidoreductase (NQO) activity which leads to a proposed role in plant pathogenesis. Azoreductases have also been suggested to play a role in the mammalian pathogenesis of Pseudomonas aeruginosa. In view of the importance of P. aeruginosa as a pathogen, we therefore characterized recombinant enzymes following expression of a group of putative azoreductase genes from P. aeruginosa expressed in Escherichia coli. The enzymes include members of the arsenic-resistance protein H (ArsH), tryptophan repressor-binding protein A (WrbA), modulator of drug activity B (MdaB) and YieF families. The ArsH, MdaB and YieF family members all show azoreductase and NQO activities. In contrast, WrbA is the first enzyme to show NQO activity but does not reduce any of the 11 azo compounds tested under a wide range of conditions. These studies will allow further investigation of the possible role of these enzymes in the pathogenesis of P. aeruginosa.

The crystal structure of Pseudomonas putida azoreductase - the active site revisited

The enzymatic degradation of azo dyes begins with the reduction of the azo bond. In this article, we report the crystal structures of the native azoreductase from Pseudomonas putida MET94 (PpAzoR) (1.60 Å), of PpAzoR in complex with anthraquinone-2-sulfonate (1.50 Å), and of PpAzoR in complex with Reactive Black 5 dye (1.90 Å). These structures reveal the residues and subtle changes that accompany substrate binding and release. Such changes highlight the fine control of access to the catalytic site that is required by the ping-pong mechanism, and in turn the specificity offered by the enzyme towards different substrates. The topology surrounding the active site shows novel features of substrate recognition and binding that help to explain and differentiate the substrate specificity observed among different bacterial azoreductases.

Characteristics of major Escherichia coli reductases involved in aerobic nitro and azo reduction

AIMS

Escherichia coli is able to reduce azo compounds such as methyl red (MR) and nitro compounds such as 7-nitrocoumarin-3-carboxylic acid (7NCCA). The aim of this study was to clarify the specificity of the major E. coli reductases.

METHODS AND RESULTS

Enzymatic assays with pure enzymes obtained after cloning, overproduction and purification under native or denaturing conditions were performed on three enzymes: AzoR, NfsA and NfsB. Their dependence on putative cofactors such as flavin mononucleotide (FMN), NADH and NADPH was studied as well as the reductase capacity of E. coli mutants depleted for one, two or three of the corresponding genes.

CONCLUSIONS

AzoR was able to reduce both MR and 7NCCA, whereas NfsA and NfsB could only reduce the nitro compound. AzoR and NfsB were strictly FMN dependent in contrast to NfsA. At a low oxygen concentration, the three proteins were not mandatory for azo reduction and nitro reduction, but in optimal aerobic conditions, azoR was essential for MR reduction, and an nfsA/nfsB combination was important for 7NCCA reduction. Overexpression of azoR gene was able to compensate for the loss of nfsA and nfsB under aerobic conditions.

SIGNIFICANCE AND IMPACT OF STUDY

These data provide new insights into the substrate specificity of major E. coli nitroreductases and demonstrate that oxygen is an important parameter to take into account in studies of nitroreductase activity.

Toxicological significance of azo dye metabolism by human intestinal microbiota

Approximately 0.7 million tons of azo dyes are synthesized each year. Azo dyes are composed of one or more R₁-N=N-R₂ linkages. Studies have shown that both mammalian and microbial azoreductases cleave the azo bonds of the dyes to form compounds that are potentially genotoxic. The human gastrointestinal tract harbors a diverse microbiota comprised of at least several thousand species. Both water-soluble and water-insoluble azo dyes can be reduced by intestinal bacteria. Some of the metabolites produced by intestinal microbiota have been shown to be carcinogenic to humans although the parent azo dyes may not be classified as being carcinogenic. Azoreductase activity is commonly found in intestinal bacteria. Three types of azoreductases have been characterized in bacteria. They are flavin dependent NADH preferred azoreductase, flavin dependent NADPH preferred azoreductase, and flavin free NADPH preferred azoreductase. This review highlights how azo dyes are metabolized by intestinal bacteria, mechanisms of azo reduction, and the potential contribution in the carcinogenesis/mutagenesis of the reduction of the azo dyes by intestinal microbiota.

Toxicological significance of azo dye metabolism by human intestinal microbiota

Approximately 0.7 million tons of azo dyes are synthesized each year. Azo dyes are composed of one or more R₁-N=N-R₂ linkages. Studies have shown that both mammalian and microbial azoreductases cleave the azo bonds of the dyes to form compounds that are potentially genotoxic. The human gastrointestinal tract harbors a diverse microbiota comprised of at least several thousand species. Both water-soluble and water-insoluble azo dyes can be reduced by intestinal bacteria. Some of the metabolites produced by intestinal microbiota have been shown to be carcinogenic to humans although the parent azo dyes may not be classified as being carcinogenic. Azoreductase activity is commonly found in intestinal bacteria. Three types of azoreductases have been characterized in bacteria. They are flavin dependent NADH preferred azoreductase, flavin dependent NADPH preferred azoreductase, and flavin free NADPH preferred azoreductase. This review highlights how azo dyes are metabolized by intestinal bacteria, mechanisms of azo reduction, and the potential contribution in the carcinogenesis/mutagenesis of the reduction of the azo dyes by intestinal microbiota.

Recent advances in azo dye degrading enzyme research

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.

Recent advances in azo dye degrading enzyme research

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.