The Anaerobic Degradation of Benzoate

Benzoyl-CoA reductase carries out the two-electron reduction of the aromatic ring dnring the anaerobic degradation of benzoate by Thauera aromatica. This involves two-electron transfer from ferredoxin, and a combination of EPR and Mossbaner spectroscopy showed the presence of three different clusters, while inactivation by oxygen was associated with partial conversion of [4Fe-4S] clnsters to [3Fe-4S] clnsters (Boll et al. 2000). 

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Hartel U, E Eckel, J Koch, G Fuchs, D, Linder, W Buckel (1993) Purification of glutaryl-CoA dehydrogenase from Pseudomonas sp., an enzyme involved in the anaerobic degradation of benzoate. Arch Microbiol 159 174-181. 

Unusual reactions have been encountered in the aerobic degradations carried out by Azoarcus evan-sii and Geobacillus stearothermophilus (Zaar et al. 2001). The anaerobic degradation of benzoate by Azoarcus evansii (Ebenau-Jehle et al. 2003) and Thauera aromatica (Domer and Boll 2002), and of 3-hydroxybenzoate by Th. aromatica (Laempe et al. 2001) is discussed later. 

Decarboxylation is a key step in the degradation of glutarate to butyrate and isobutyrate by dehydrogenation to glutaconate followed by decarboxylation to crotonate (Matties and Schink 1992). This pathway has been proposed for the degradation of pimelate that is a possible intermediate in the anaerobic degradation of benzoate (Hartel et al. 1993 Gallus and Schink 1994). 

The degradation of o-, m-, and p-phthalates under denitrifying conditions has been examined (Nozawa and Maruyama 1988), and after an initial decarboxylation to benzoate followed the pathway for the anaerobic degradation of benzoate that has been noted above formation of the CoA ester, reduction to cyclohex-l-ene carboxylate, hydroxylation to 2-hydroxycyclohexane car-boxylate, and ring fission to pimelic acid that was further degraded by P-oxi-dation. 

Geissler, J.F., C.S. Harwood, and J. Gibson. 1988. Purification and properties of benzoate-coenzyme A ligase, a Rhodopseudomonas palustris enzyme involved in the anaerobic degradation of benzoate. /. Bacteriol. 170 1709-1714. 

It has become clear that benzoate occupies a central position in the anaerobic degradation of both phenols and alkylated arenes such as toluene and xylenes, and that carboxylation, hydroxylation, and reductive dehydroxylation are important reactions for phenols that are discussed in Part 4 of this chapter. The simplest examples include alkylated benzenes, products from the carboxylation of napthalene and phenanthrene (Zhang and Young 1997), the decarboxylation of o-, m-, and p-phthalate under denitrifying conditions (Nozawa and Maruyama 1988), and the metabolism of phenols and anilines by carboxylation. Further illustrative examples include the following ... 

It seems clear that benzoate occupies a central position in the anaerobic degradation of both phenols and alkaryl hydrocarbons, and that carboxylation, hydroxylation, and reductive dehydroxylation are important—and less expected—reactions. 

Zhang, X. and J. Wiegel. 1992. The anaerobic degradation of 3-chloro4-hydroxyben-zoate in freshwater sediment proceeds via either chlorophenol or hydroxyben-zoate to phenol and subsequently to benzoate. Appl. Environ. Microbiol. 58 3580-3585. 

Degradation of Benzoates—Benzoate occupies a central role in the metabolism of a range of aromatic compounds, and the pathway for its anaerobic degradation under denitrifying or phototrophic conditions involves three cardinal reactions (further references in Harwood et al. 1999) (Figure 6.94a,b) the designation of the enzymes corresponds to those for the genes discussed below. 

Other reactions—Alternatives may be envisaged based on the anaerobic hydroxylation of toluene to 4-hydroxytoluene followed by oxidation of the methyl group and dehydroxylation to benzoate (Rudolphi et al. 1991). Although the role of this in the degradation has not been clearly established, two possibilities can be suggested (a) carboxylation and dehydroxylation to... 

Cr(VI) to Cr(III) anaerobically at the expense of the oxidation of organic compounds is uncommon. The oxidation of benzoate to C02 using Cr(VI) as oxidant has been accomplished in cultures that were initially enriched using nitrate as an additional electron acceptor (Shen et al. 1996). In the absence of nitrate, there was good agreement between the removal of benzoate and the reduction of Cr(VI), and this could be an attractive procedure in view of the low toxicity and ready degradability of benzoate if this were used in excess. 

Stable metabolic associations generally between pairs of anaerobic bacteria have been termed syntrophs, and these are effective in degrading a number of aliphatic carboxylic acids or benzoate under anaerobic conditions. These reactions have been discussed in reviews (Schink 1991, 1997 Lowe et al. 1993) that provide lucid accounts of the role of syntrophs in the degradation of complex organic matter. Two examples are given here to illustrate the experimental intricacy of the problems besetting the study of syntrophic metabolism under anaerobic conditions ... 

The degradation of Cg-benzene was studied in anaerobic enrichment cultures when phenol, benzoate, and toluene were detected, and the kinetics of their formation studied (Ulrich et al. 2005). 

Anaerobic CP degradation involves sequential reductive dehalogenations with MCPs or DCPs as final metabolites, or degradation may proceed to complete dechlorination to phenol, further transformation to benzoate and, ultimately, conversion to methane and carbon dioxide. Reductive dechlorination of PCP, for example, results in the formation of meta- or para-CPs as the end-products (e.g., Woods et al., 1989 Madsen Aamand, 1992), or anaerobic degradation may continue to complete mineralization (Boyd Shelton, 1984 Mohn Kennedy, 1992 Wu et al., 1993). In reductive dechlorinations, CPs serve as electron acceptors and need a suitable electron donor. 

Adaptation has also been observed in anaerobic systems. Linkfield et al. (9) reported acclimation periods of 20 to more than 170 days for a variety of halogenated benzoates in anaerobic lake sediments. The periods required for adaptation (acclimation) were characteristically associated with chemical structure, and not with environmentally influenced variables such as population or nutrient status. It was concluded that the acclimation periods preceeding degradation of these compounds was due to induction of necessary enzymes. These results illustrate the fact that enyme induction and regulation remains a potential cause of adaptive responses in microorganisms. 

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