Halogenated benzoates

Chlorobenzoates may be formed during the initial steps in the aerobic degradation of PCBs, and their further metabolism illustrates a number of pathways. There are several reactions that carry out dehalogenation including dioxygenation, hydrolysis, and reduction. 

FIGURE 9.12 Ring-cleavage pathways for the biodegradation of chlorocatechols formed from 3- and 4-chlorobenzoates. 

A two-component 2-halobenzoate 1,2-dioxygenase has been purified from Pseudomonas cepacia strain 2CBS that is able to metabolize 2-fluorobenzoate, 2-chlorobenzoate, 2-bromobenzoate, and 2-iodobenzoate to catechol by concomitant decarboxylation and loss of halide (Fetzner et al. 1992). The inducible 2-halobenzoate 1,2-dioxygenase consisted of 

FIGURE 9.13 Degradation of 2-bromobenzoate by Pseudomonas aeruginosa strain 142. (From Neilson, A.H. and Allard, A.-S., The Handbook of Environmental Chemistry, Vol. 3R, Springer Verlag, 2002, pp. 1-74. With permission.) 

Burkholderia sp. strain NK8 can be grown with benzoate, and both 3- and 4-chloro-benzoate. The genes encoding enzymes for the dioxygenation of benzoates (cbeABCD) and catechols (catA, catBC) have been cloned and analyzed (Francisco et al. 2001). Both 

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Burkholderia cepacia strain 2CBS is able to degrade ort/jo-halogenated benzoates by dioxygenation to catechol with the elimination of halide and decarboxylation. The enzyme contains a ferredoxin-and a Rieske-type [2Fe-2S] center. These could be distinguished on the basis of their EPR spectra, and the results were compared with those for other [2Fe-2S] clusters (Riedel et al. 1995). 

The application of di-arene dihydrodiols has been noted in Part 1 of this chapter, and it is sufficient to note here the application of a mutant of Alcaligenes eutrophus strain B9 that is blocked in the degradation of benzoate (and some halogenated benzoates). This produced the cA-l,2-dihydrodiol (Reiner and Hegeman 1971), and has been used as the source of ring B for the synthesis of a range of tetracyclines (Charest et al. 2005). A range of substituted c -dihydrodiols has been produced, and it has been shown that for 3-substituted benzoates both 3- and 5-substituted cA-dihydrodiols were formed (Reineke et al. 1978). 

Aerobic degradation of chlorinated arene hydrocarbons, including the important group PCBs, and chlorobenzoates that are produced from them as metabolites, is generally initiated by dihy-droxylation of the rings to dihydrodiols followed by dehydrogenation to catechols. Halide may be lost simultaneously and for 2-halogenated benzoates, both halide and carboxyl. Salient aspects are summarized, and attention drawn to selected aspects of enzyme inhibition. 

In snmmary, a range of pathways are available for the degradation of halogenated benzoates ... 

Dioxygenation with concomitant loss of carboxyl and halogen to prodnce catechols (2-, 2,3-, and 2,5-halogenated benzoates)... 

A contrasting pathway was found for the metabolism of 2-fluoro-4-nitrobenzoate by Nocardia erythropolis that does not involve loss of fluorine concomitant with decarboxylation (Cain et al. 1968). The pathway (Figure 9.30) therefore differs from what has subsequently emerged as the principal pathway for metabolism of nonhydroxylated 2-halogenated benzoates. 

Deweerd KA, DL Bedard (1999) Use of halogenated benzoates and other halogenated aromatic compounds to stimulate the microbial dechlorination of PCBs. Environ Sci Technol 33 2057-2063. 

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. 

Ge, W., Kessehnan, E., Tahnon, Y, Hart, D. J., and Zakin, J. L., Correlation of micellar nanostructure, drag reduction and rheological behavior of dilute CTAC-halogen benzoate counterion systems with different halogen substitutions and counterion to CATC ratio, J. Non-Newtonian Fluid Mech., 154, 1-12 (2008). 

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