PCB-catabolism

The next step of PCB-catabolism is catalyzed by a 2,3-dihydrodiol dehydrogenase and regenerates NADH, and leads to the formation of 2,3-dihydroxybiphenyl. With respect to the reaction medianism of the fist two steps of PCB degradation, there is a great similarity to other catabolic pathways of aromatics, like benzene, toluene, benzoate, naphthalene, etc. 

Hernandez BS, JJ Arensdorf, DD Focht (1995) Catabolic characteristic of biphenyl-ntilizing isolates which cometabolize PCBs. Biodegradation 6 75-82. 

The aromatic degradative pathways receiving the most attention include those for toluene, benzoate, PCBs, and naphthalene. Five completely independent routes of toluene catabolism have been well characterized for aerobic bacteria (Figure 11.1). To date, all five pathways have been described in Pseudomonas and Burkholderia, despite the fact that at least two have been shown to be encoded by broad-host-range degradative plasmids. 

The ubiquitous occurrence of bacteria, which can utilize biphenyl as a sole source of carbon and energy, has been reported. Several species of Gram-negative and Gram-positive bacteria have been shown to degrade biphenyl and PCBs. The main pathway for biphenyl catabolism is outlined in Figure 3 [58-66]. 

Figure 3. Catabolic pathway for degradation of biphenyl and PCB by bacteria. I, biphenyl II, 2,3-dihydroxy-4-phenylhexa-2,4-diene (2,3-dihydrodiol) HI, 2,3-dihydro] biphenyl IV, 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid (meto-cleavage compound) V, benzoic add.

In parallel with molecular nonculture methods, the well-established methods of enrichment culture are more frequently being applied under anaerobic and other nonstandard conditions in an effort to obtain novel microbial types. This approach also suggests that biodegradative capabilities are more widespread in the microbial world than has been appreciated by some. For example, halophiles have been identified which metabolize nitroarenes, and members of the Heliobacterium group are known that catabolize polychlorinated biphenyls (PCBs) and chlorophenols (O Table 15.2). These and other recent observations are expanding the taxonomic range of bacteria that catabolize environmental pollutants. Further experiments are likely to expand this further. 

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