Cometabolic pathway

Electron donors—In direct biodegradation pathways, the contaminant acts as the electron donor or substrate. However, during cometabolic degradation, a different electron donor is metabolized, resulting in the consequential oxidation of the contaminant. In some contaminated plumes, other electron donors, such as other constituents of gasoline, may also be present. In cases where they are not, and cometabolic degradation pathways are desired, electron donors may be added. 

Aerobic bacteria complete most of the petroleum bioremediation applications, particularly those above the groundwater table. Aerobes are those bacteria that require an oxygen source as their TEA. Conversely, anaerobic species require the absence of oxygen (anoxic conditions) for their respiration. In situ anaerobic bioremediation is typically only conducted in the saturated zone because of the difficulty in maintaining a strict anaerobic environment. In some instances, facultative anaerobes are utilized because they can alter the respiration to be metabolically active under both anaerobic and aerobic conditions. As such, the type of TEA available will dictate the metabolism and subsequent degradation mode. The most common TEAs used for bioremediation are listed in Table 2. Careful selection of microbe-TEA combinations can enable a specific degradation pathway to facilitate cometabolism and prevent undesired degradation by-products. 

C has been used as a tracer in the study of the degradation of [1-13C]-acenaphthene both in pure cultures that were degrading naphthalene and phenanthrene by cometabolism, and in a mixed culture that was enriched with creosote (Selifonov et al. 1998). The degradation pathway that is initiated by benzylic monooxygenation could be determined by isolation of intermediate metabolites, and the method proved applicable to the situation where only limited biotransformation of the substrates takes place by partial oxidation. 

The early literature on monoterpene biotransformation was highly influenced by the approach used in steroid biotransformations and mainly focused on terpenoids accumulated by fungal strains which do not mineralize the substrate but partly oxidize it by fortuitous cometabolism. These studies often resulted in the accumulation of a mixture of different products in low yields and at low concentrations [1]. Several bacteria which completely mineralize monoterpenes have been described more recently. It has become obvious from the later studies that multiple pathways are involved in the degradation of monoterpenes in many of these microorganisms, and consequently it has been difficult to obtain mutants allowing the accumulation of partially oxidized products. 

Herbicides, 2,4-D and 2,4,5-T are structurally related, the latter having an extra chlorine atom at position 5. Unlike 2,4-D, 2,4,5-T is poorly biodegradable, and persists for long periods, hence constituting a pollution problem [155]. Cometabolism of 2,4,5-T by Brevibacterium sp. resulted in the formation of the product, tentatively identified as 3,5-dichlorocatechol. Bacterial cometabohsm of 2,4,5-T was also described by Rosenberg and Alexander [156], who proposed a degradation pathway of 2,4,5-T in soil [157]. Reductive dechlorination of 2,4,5-T by anaerobic microorganisms was described by Suflita et al. [130]. 

Figure 23.2.2. Anaerobic degradation of carbon tetrachloride. An example of anaerobic dehalogenation, using carbon tetrachloride as the model compound. In many cases, these reactions occur under cometabolic conditions meaning that an alternative growth substrate must be present to serve as an electron donor to drive the reduction reactions whereby carbon tetrachloride is used as the electron acceptor. Three known pathways for microbial degradation of carbon tetrachloride have been identified [U.E. Krone, R.K. Thauer, H.P. Hogenkamp, and K. Steinbach, Biochemistry, 3d 0), 2713 (1991) C.H. Lee, T.A. Lewis, A. Paszczynski, andR.L. Crawford Biochem Biophys Res Commun, 261(3), 562 (1999)]. These pathways are not enzymatically driven but rely on corrinoid and corrinoid-like molecules to catalyze these reactions.

The third most common biodegradation pathway for chlorinated hydrocarbons is cometabolism via enzymatic reactions occurring fortuitously with oxidation of compounds such as methane or toluene. These three pathways are, of course, complicated by the fact that 1) each can only occur under specific chemical conditions 2) the dominant pathway changes as the source of electron donor and/or acceptor (both anthropogenic and indigenous) is reduced and 3) There are several different isomers, and other compounds of chlorinated hydrocarbons, that can enter these pathways from biotic or abiotic processes. In other words, it is a complicated process. 

Researchers noticed that the biodegradation of TCE was inhibited by acetylene, a specific inhibitor of MMO. This observation supported the hypothesis that methanotrophs were responsible for TCE degradation. The study conducted by Little et al. (1988) confirmed this hypothesis and identified a possible course of TCE decomposition. The pathway of the cometabolic degradation of TCE is presented in Figure 5.7. McCarty Semprini (1994) noticed... 

Incomplete reduction of a nitro group yielding a hydroxylaminoaromatic compound and the rearrangement of the latter into an aminophenol was first described in the yeast Rhodosporidium sp. as part of cometabolic transformation of 4-chloronitrobenzene (4). Characteristically, reductive pathways have been postulated for the mineralization of a number of nitroaromatics (de Bont, this volume). In many cases, however, aminoaromatic structures could not be identified as intermediates. Therefore, the above novel catabolic pathway with a hydroxylaminoaromatic compound as a key metabolite could be of more general importance in aerobic mineralization of nitroaromatic compounds. 

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