Quinone and iron porphyrin mediated reduction of nitroaromatic compounds in homogeneous aqueous solution

119

Quinones can function as redox mediators in the unspecific anaerobic reduction of azo compounds by various bacterial species. These quinones are enzymatically reduced by the bacteria and the resulting hydroquinones then reduce in a purely chemical redox reaction the azo compounds outside of the cells. Recently, it has been demonstrated that the addition of lawsone (2-hydroxy-1,4-naphthoquinone) to anaerobically incubated cells of Escherichia coli resulted in a pronounced increase in the reduction rates of different sulfonated and polymeric azo compounds. In the present study it was attempted to identify the enzyme system(s) responsible for the reduction of lawsone by E. coli and thus for the lawsone-dependent anaerobic azo reductase activity. An NADH-dependent lawsone reductase activity was found in the cytosolic fraction of the cells. The enzyme was purified by column chromatography and the amino-terminal amino acid sequence of the protein was determined. The sequence obtained was identical to the sequence of an oxygen-insensitive nitroreductase (NfsB) described earlier from this organism. Subsequent biochemical tests with the purified lawsone reductase activity confirmed that the lawsone reductase activity detected was identical with NfsB. In addition it was proven that also a second oxygen-insensitive nitroreductase of E. coli (NfsA) is able to reduce lawsone and thus to function under adequate conditions as quinone-dependent azo reductase.Azo dyes are widely used as dyes for textiles, food, cosmetics, and various other purposes (51). They are usually very stable under aerobic conditions and therefore resist biodegradation in conventional aerobic sewage treatment systems. In contrast, under anaerobic conditions various bacterial strains are able to reduce azo dyes to amines and thus decolorize the dyes (10, 39, 43). These anaerobic reactions are usually extremely unspecific with regard to the organisms involved and the dyes converted. In these processes, low-molecular-weight redox mediators (e.g., quinones) are often involved which are enzymatically reduced by the bacteria. The reduced mediator compounds (e.g., hydroquinones) can then reduce the azo compounds outside of the cells in a purely chemically and therefore extremely unspecific redox reaction (17,23).It was recently shown that among different quinones tested, especially anthraquinone-2-sulfonate (AQS) and lawsone (2-hydroxy-1,4-naphthoquinone) considerably increased the anaerobic reduction rates of azo dyes by Sphingomonas xenophaga BN6, Escherichia coli, and various other bacterial strains from different bacterial groups. For S. xenophaga BN6, AQS was identified as the most effective redox mediator from all quinones investigated and it was suggested that the membrane bound NADH:ubiquinone oxidoreductase of the respiratory chain was responsible for this activity (23,35). In contrast to S. xenophaga BN6, cells of E. coli demonstrated much higher reduction rates with lawsone compared to AQS (35). These results suggested that the reduction of lawsone by...

mentioning

confidence: 78%

Oxygen-Insensitive Nitroreductases NfsA and NfsB of Escherichia coli Function under Anaerobic Conditions as Lawsone-Dependent Azo Reductases

Rau

2003

119

“…HS and quinone analogs such as AQDS, juglone, and lawsone also reduce inorganic and organic molecules (32,46,47). AQDS was reported to reduce U(VI) in solution or adsorbed to the surface of oxide solids (16,26).…”

Section: Discussionmentioning

confidence: 99%

“…AQDS was reported to reduce U(VI) in solution or adsorbed to the surface of oxide solids (16,26). Juglone and lawsone reduced trinitrotoluene in aqueous suspension (47). Although high-molecular-weight HS are insoluble and can merely adsorb organic contaminants, low-molecular-weight HS are soluble and will likely promote these reactions in situ.…”

Section: Discussionmentioning

confidence: 99%

Microbially Mediated Biodegradation of Hexahydro-1,3,5-Trinitro-1,3,5- Triazine by Extracellular Electron Shuttling Compounds

Kwon

Finneran

2006

The potential for humic substances to stimulate the reduction of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) was investigated. This study describes a novel approach for the remediation of RDX-contaminated environments using microbially mediated electron shuttling. Incubations without cells demonstrated that reduced AQDS transfers electrons directly to RDX, which was reduced without significant accumulation of the nitroso intermediates. Three times as much reduced AQDS (molar basis) was needed to completely reduce RDX. The rate and extent of RDX reduction differed greatly among electron shuttle/acceptor amendments for resting cell suspensions of Geobacter metallireducens and G. sulfurreducens with acetate as the sole electron donor. AQDS and purified humic substances stimulated the fastest rate of RDX reduction. The nitroso metabolites did not significantly accumulate in the presence of AQDS or humic substances. RDX reduction in the presence of poorly crystalline Fe(III) was relatively slow and metabolites transiently accumulated. However, adding humic substances or AQDS to Fe(III)-containing incubations increased the reduction rates. Cells of G. metallireducens alone reduced RDX; however, the rate of RDX reduction was slow relative to AQDSamended incubations. These data suggest that extracellular electron shuttle-mediated RDX transformation is not organism specific but rather is catalyzed by multiple Fe(III)-and humic-reducing species. Electron shuttle-mediated RDX reduction may eventually become a rapid and effective cleanup strategy in both Fe(III)-rich and Fe(III)-poor environments.

“…Because the reaction of 1,2-NQ with amines or other nucleophiles is rather unspecific, it may be expected that several other compounds with mediator activity are also formed in the system studied here. Similar reactions may also be relevant for other biological systems, because aromatic 1,2-dihydroxy compounds are formed as intermediates during the bacterial degradation of almost all aromatic compounds and hydroquinones have been shown to be able to reduce a wide range of natural and man-made substances, such as nitroaromatic compounds and ferric iron (13,16,32,33,44,49,55).…”

Section: Discussionmentioning

confidence: 99%

Identification of Quinoide Redox Mediators That Are Formed during the Degradation of Naphthalene-2-Sulfonate by Sphingomonas xenophaga BN6

Keck

Rau

Reemtsma

et al. 2002

During aerobic degradation of naphthalene-2-sulfonate (2NS), Sphingomonas xenophaga strain BN6 produces redox mediators which significantly increase the ability of the strain to reduce azo dyes under anaerobic conditions. It was previously suggested that 1,2-dihydroxynaphthalene (1,2-DHN), which is an intermediate in the degradative pathway of 2NS, is the precursor of these redox mediators. In order to analyze the importance of the formation of 1,2-DHN, the dihydroxynaphthalene dioxygenase gene (nsaC) was disrupted by gene replacement. The resulting strain, strain AKE1, did not degrade 2NS to salicylate. After aerobic preincubation with 2NS, strain AKE1 exhibited much higher reduction capacities for azo dyes under anaerobic conditions than the wild-type strain exhibited. Several compounds were present in the culture supernatants which enhanced the ability of S. xenophaga BN6 to reduce azo dyes under anaerobic conditions. Two major redox mediators were purified from the culture supernatants, and they were identified by high-performance liquid chromatography-mass spectrometry and comparison with chemically synthesized standards as 4-amino-1,2-naphthoquinone and 4-ethanolamino-1,2-naphthoquinone.Wastewaters from textile industries are often highly colored due to residual dyestuff from the dyeing processes. It has been estimated that up to 60% of the total dyestuff used in the dyeing processes may be found in the wastewaters (3). Only small amounts of these dyes are removed by conventional aerobic biological wastewater treatment systems (37, 46). On the other hand, it is well known that azo dyes are reduced anaerobically by different microorganisms, which usually results in the generation of colorless aromatic amines (2,5,7,12,14,51,56). These amines are in most cases recalcitrant under anaerobic conditions (6). Therefore, it has been repeatedly suggested that two-stage anaerobic-aerobic treatment systems should be used to reduce the azo dyes anaerobically and to mineralize the amines formed in a subsequent aerobic stage (1,36,38,45,51,54). The first successful example of mineralization of a sulfonated azo dye by an anaerobic-aerobic treatment process involved a 6-aminonaphthalene-2-sulfonate-degrading mixed bacterial culture. This culture consisted of the naphthalenesulfonate-degrading strain Sphingomonas xenophaga BN6 in a mutualistic coculture with other bacterial strains (21,34,50). Recently, it was found that strain BN6 uses a new mechanism for conversion of azo dyes. During aerobic degradation of naphthalene-2-sulfonate (2NS), redox mediators are released, which mediate the reduction of azo dyes under anaerobic conditions (27). It was proposed that these redox mediators shuttle electrons from the cells to the azo dyes, which results in purely chemical, extremely nonspecific, extracellular reductive cleavage of the azo bond. Aerobic preincubation of strain BN6 with 2NS increased subsequent anaerobic reduction of the azo dyes almost 20-fold. It was suggested that 1,2-dihydroxynaphthalene (1,2-DHN), which is the ...