Bacterial degradation of monocyclic aromatic amines

Aromatic amines are an important group of industrial chemicals, which are widely used for manufacturing of dyes, pesticides, drugs, pigments, and other industrial products. These compounds have been considered highly toxic to human beings due to their carcinogenic nature. Three groups of aromatic amines have been recognized: monocyclic, polycyclic, and heterocyclic aromatic amines. Bacterial degradation of several monocyclic aromatic amines has been studied in a variety of bacteria, which utilizes monocyclic aromatic amines as their sole source of carbon and energy. Several degradation pathways have been proposed and the related enzymes and genes have also been characterized. Many reviews have been reviewed toxicity of monocyclic aromatic amines; however, there is lack of review on biodegradation of monocyclic aromatic amines. The aim of this review is to summarize bacterial degradation of monocyclic aromatic amines. This review will increase our current understanding of biochemical and molecular basis of bacterial degradation of monocyclic aromatic amines.


Introduction
Aromatic amines are derivatives of aromatic hydrocarbons containing an amino group,  or an amine group (-NH), or a nitrogen (-N) atom in their structures. There are three types of aromatic amines: monocyclic, polycyclic, and heterocyclic, which have been observed in tobacco smoke, diesel exhaust, dyes, pesticides, pharmaceuticals, and polyurethane foams (Stellman, 1998;. Many aromatic amines are recognized as known or suspect human carcinogens, and mutagenicity of aromatic amines has been demonstrated in many test systems, including Big Blue transgenic mice (Layton et al., 1995;Suter et al., 1996;Stellman, 1998;Chung, 2000;Pira et al., 2010). Furthermore, they are potent inducer of the formation of methemoglobinemia in animals and humans (Ohta et al., 1983). Occupational exposure to aromatic amines causes an increased risk of bladder cancer in workers even 30 years after exposure (Pira et al., 2010). Several pesticides including diuron, metobromuron, linuron, isoproturon, chlorotoluron, acetochlor, bentazon, butachlor, metolachlor, amitraz, and vinclozolin may release several monocyclic aromatic amines in soil because of their microbial transformation (Dupret et al., 2011). Cigarette smoke releases several carcinogenic aromatic amines including p-toluidine, 2-naphthylamine, and 4-aminobiphenyl into the ambient air (Stabbert et al., 2003). Bladder cancer is strongly associated with cigarette smoking, probably due to exposure to aromatic amines in tobacco smoke (Pfeifer et al., 2002). Heterocyclic aromatic amines are generally produced in meats or fish when grilled or cooked at high temperatures (Steck et al., 2007). An epidemiological study showed that people who lifelong consume grilled meats and fish have a risk of postmenopausal breast cancer (Steck et al., 2007). Furthermore, higher exposures to heterocyclic aromatic amines may cause presence of DNA adducts, which are associated with carcinogenesis (Turesky, 2007).
Several reviews have been published dealing with toxicity of aromatic amines (Chung et al., 1997;Skipper et al., 2010;Besaratinia and Tommasi, 2013). Despite the fact, monocyclic aromatic amines are distributed throughout the environment including soil and groundwater, there is no review dealing with bacterial degradation of monocyclic aromatic amines. The aim of this review is to summarize bacterial degradation of monocyclic aromatic amines.

Bacterial Degradation of Monocyclic Aromatic Amines
Many bacteria have been isolated and characterized with their ability to mineralize or transform various monocyclic aromatic amines. Table 1 summarizes the role of various monocyclic aromatic amine-degrading bacteria. Bacterial degradation of monocyclic aromatic amines proceeds generally with release of ammonia. Ammonium ions may release either after the ring cleavage (Takenaka et al., 1997) or prior to the ring cleavage (Chang et al., 2003). Several mechanisms have been proposed for mineralization of monocyclic aromatic amines. Bacterial aerobic degradation of monocyclic aromatic amines may be initiated via one of the following mechanisms: (i) A dioxygenase may catalyze ring cleavage of aromatic amine (Takenaka et al., 1997), (ii) Dioxygenation of aromatic amine (Chang et al., 2003), (iii) Deamination of aromatic amine (Qu and Spain, 2011), (iv) Hydroxylation of aromatic amine , (v) Co-ligase mediated activation of aromatic amines to coenzyme A (CoA) thioesters (Schühle et al., 2001), and (vi) Dehalogenation of chlorinated aromatic amine (Hongsawat and Vangnai, 2011). In this section, the bacterial degradation of well-studied monocyclic aromatic amines including aniline, aminophenols, chloroaminophenols, anthranilate, 5-nitroanthranilate, 4-amino-3-hydroxybenzoate, methylanilines, and chloroanilines are discussed.
The initial conversion of aniline to catechol is a multistep reaction catalyzed by three enzymes, a glutamine synthetase (GS)-like enzyme, glutamine amidotransferase like enzyme, and an aniline dioxygenase (a large and small subunits of an oxygenase component and a ferredoxin-reductase component; Fujii et al., 1997;Fukumori and Saint, 1997;Murakami et al., 1998;Liang et al., 2005). In the first step, GS like enzyme catalyzed ATP-dependent ligation of L-glutamate to aniline to form gamma-glutamylanilide ( Figure 1A; Takeo et al., 2013). The next step, catalyzed by aniline dioxygenase involves conversion of gamma-glutamylanilide into catechol (Takeo et al., 2013). High concentrations of gamma-glutamylanilide are cytotoxic, but the action of another enzyme, glutamine amidotransferase, prevents its accumulation by converting it to aniline (Takeo et al., 2013). Five genes encoding these three enzymes involved in the conversion of aniline to catechol have been identified in a number of bacteria including P. putida UCC22 (Fukumori and Saint, 1997), Acinetobacter sp. YAA (Fujii et al., 1997), Frateuria sp. ANA-18 , Delftia acidovorans 7N (Urata et al., 2004), Delftia tsuruhatensis AD9 (Liang et al., 2005) and Delftia sp. AN3 (Zhang et al., 2008). These genes are located on either plasmid or chromosomal DNA. The plasmids of P. putida UCC22 (pTDN1,) and Acinetobacter sp. YAA (pYA1) contain aniline oxidation genes (tdnQTA1A2B or atdA1A2A3A4; Fujii et al., 1997;Fukumori and Saint, 1997). Murakami et al. (2003) expressed all five genes from Frateuria sp. ANA-18 in Escherichia coli and the recombinant bacteria exhibited the aniline oxidation activities. They demonstrated that deletion of tdnA1A2 or tdnQ genes resulted in loss of aniline oxidation activity. Apart of a tdn gene cluster, Frateuria sp. contain two catechol catabolic gene clusters cat1 and cat2 . The gene cluster cat1 may involve in the orthocleavage pathway of aniline degradation . Takeo et al. (2013) reported characterization of the atdA1 gene (encoding the enzyme similar to GS) from Acinetobacter sp. YAA and confirmed that the AtdA1 catalyzes conversion of aniline to gamma-glutamylanilide.
Anaerobic degradation of aniline was studied in sulfatereducing bacterium Desulfobacterium aniline (Schnell and Schink, 1991). Initially, aniline is carboxylated to 4-aminobenzoic acid that is transformed to 4-aminobenzoyl-CoA ( Figure 1B). The 4-aminobenzoyl-CoA undergoes reductive deamination to form benzoyl-CoA which enters the normal benzoate pathway, to form three acetyl-CoA. Few bacteria are able to degrade aniline under either aerobic or anaerobic conditions; Delftia sp. HY99 is one example of this capability (Kahng et al., 2000). Strain HY99 mineralized aniline acerbically via catechol and transformed aniline to 4-aminobenzoic acid under anaerobic conditions (Kahng et al., 2000).

Bacterial Degradation of Chloroaminophenols
Chloroaminophenols (chlorinated derivatives of aminophenols) are widely used in dye synthesis. In this subsection, pathways for bacterial degradation of 2-chloro-4-aminophenol (2C4AP) and 4-chloro-2-aminophenol (4C2AP) are described. The degradation pathway of 2C4AP was studied in an Arthrobacter sp. SPG that utilized 2C4AP as its sole source of carbon and energy (Arora et al., 2014a). The initial step of the 2C4AP degradation is deaminase-catalyzed hydrolytic deamination of 2C4AP into chlorohydroquinone (CHQ; Arora et al., 2014a). The next step, catalyzed by a CHQ-dehalogenase involves reductive dehalogenation of CHQ to hydroquinone (HQ). Further degradation of HQ proceeds via ring cleavage, catalyzed by HQ-dioxygenase ( Figure 3A).
Most of the bacteria degrade anthranilate via the catechol pathway in which anthranilate-1,2-dioxygenase catalyzes conversion of anthranilate to catechol, which is degraded further via the ortho-or meta-cleavage pathway (Chang et al., 2003) ( Figure 4A). The enzyme anthranilate-1,2-dioxygenase has been characterized from a number of bacteria (Eby et al., 2001;Chang et al., 2003). In Burkholderia cepacia DBO1, it is a threecomponent Rieske-type [2Fe-2S] dioxygenase with a reductase, a ferredoxin, and a two-subunit oxygenase (Chang et al., 2003). In Acinetobacter sp. ADP1 (Eby et al., 2001), P. aeruginosa PAO1 (Costaglioli et al., 2012) and P. putida P111, it is a two component complex composed of an oxygenase and a reductase. Kim et al. (2015) cloned and expressed the genes involved in the anthranilate degradation pathway from Pseudomonas sp. PAMC 2593. Two gene clusters have been identified in this strain; the antABC encodes the enzyme anthranilate dioxygenase that converts anthranilate to catechol whereas the catBCA encodes a catechol dioxygenase that cleaves to catechol to cis, cis-muconic acid (Kim et al., 2015).
The 3-hydroxyanthranilate pathway of anthranilate degradation involves an anthranilate hydroxylase-catalyzed conversion of anthranilate to 3-hydroxyanthranilate that is further degraded via the meta-cleavage (Liu et al., 2010) ( Figure 4C). The genes involved in this pathway have been identified and characterized in Geobacillus thermodenitrificans NG80-2 (Liu et al., 2010). The gene encoding anthranilate hydroxylase has been cloned, and expressed in E. coli and the purified protein was FAD-dependent hydroxylase. Two additional enzymes, riboflavin kinase/FMN adenylyltransferase and an FAD reductase, provide FAD for the anthranilate hydroxylase and genes encoding these enzymes were located in the same cluster in which gene encoding hydroxylase was located (Liu et al., 2010).

Bacterial Degradation of 5-Nitroanthranilate
5-Nitroanthranilate is a natural nitroaniline that is produced by the soil bacterium Streptomyces scabiei (the predominant causal agent of common scab of potato in North America; Qu and Spain, 2010). The degradation of this compound was studied in Bradyrhizobium sp. JS329 that utilizes it as its sole source of carbon, nitrogen and energy (Qu and Spain, 2010). The degradation pathway is initiated with hydrolytic deamination of 5-nitroanthranilate to 5-nitrosalicylic acid by 5-nitroanthranilate deaminase. Second step involves 5-nitrosalicylic dioxygenasecatalyzed ring cleavage of 5-nitrosalicylic acid without prior removal of nitro group (Qu and Spain, 2011). The nitro group is eliminated either during the ring fission or immediately following it and the product undergoes spontaneous lactonization. In the next step, lactone is hydrolyzed to maleylpyruvate by a 2-oxo-3-(5-oxofuran-2-ylidene) propanoate lactonase ( Figure 5A). The maleylpyruvate is further degraded via 3-fumarylpyruvate (Qu and Spain, 2011).

Bacterial Degradation of Methylanilines and Their Derivatives
In this subsection, the pathways for the bacterial degradation of methylanilines and their derivatives are summarized. P. testosterone can use 4-methylaniline (p-toluidine) as its sole source of carbon and energy and degraded it via 4-methylcatechol and 2-hydroxy-5-methyl-cis,cis-muconate semialdehyde (Raabe et al., 1984). The initial oxidation of p-toluidine resulted in formation of 4-methylcatechol that ringcleaved to 2-hydroxy-5-methyl-cis,cis-muconate semialdehyde by a meta-pyrocatechase (Raabe et al., 1984). Anaerobic degradation of 4-methylaniline was studied in anaerobic sulfate-reducing bacterium, Desulfobacula toluolica Tol2 that transformed it into p-aminophenylacetic acid and phenylacetic acid as dead end products (Raber et al., 1998).

Conclusion
Bacterial degradation of aniline and its chloro and methyl derivatives generally occurs via formation of corresponding catechols that degrade further via either the ortho-cleavage or the meta-cleavage pathway. The mechanism of catechol formation in aniline degradation has recently been postulated and the related genes and enzymes have been well-characterized. Future works on proteomics may increase our understanding towards bacterial degradation of aniline.
Bacterial degradation pathways for aminophenols and chloroaminophenols have also been studied. The degradation of these compounds generally initiated via either the ring cleavage or the hydrolytic deamination. The genes and enzymes involved in the aminophenol degradation have also been characterized whereas the genomics of the degradation pathways of chloroaminophenols have yet not studied.
Diverse mechanisms of the anthranilate degradation have been reported and four aerobic metabolic pathways including the catechol pathway, the gentisate pathway, the 3hydroxyanthranilate pathway, and the 2-aminobenzoyl-CoA pathway have been proposed. Amongst, the catechol pathway is the most common route for anthranilate degradation.
Little is known about bacterial degradation of other monocyclic aromatic amines. More bacteria should be isolated by the enrichment method using monocyclic aromatic amines as substrates and the biochemical and molecular characterization of biodegradation of monocyclic aromatic amines should be carried out in these bacteria.

Author Contributions
PA collected all the relevant publications, arranged the general structure of the review, drafted the text, and produced figures.