Degradation, anaerobic pathway

The fate of toluene and o-xylene in an aquifer contaminated with BTEX was examined by injecting toluene-dj and o-xylene-djo followed by quantification of the label in benzyl succinate and 2-methylbenzene succinate (Reusser et al. 2002) that are established metabolites on the anaerobic pathway for the degradation of toluene and o-xylene. 

The aerobic degradation of furan-2-carboxylate has already been summarized in Part 1 of this chapter and emphasis is placed here on anaerobic pathways. 

In contrast, Meckenstock et al. [280] reported larger isotopic enrichments in residual toluene, 3-6%o and up to 10%o during anaerobic and aerobic biodegradation experiments, respectively. These results indicated that isotopic fractionation effects may be different for different compounds, terminal electron-accepting processes (TEAP), degradative metabolic pathways, or microbial populations. 

Fig. 20.1. Generalized scheme of the main pathways of aerobic and anaerobic carbohydrate degradation in parasitic flatworms. The aerobic pathway is indicated by open arrows, whereas the anaerobic pathway (malate dismutation) is indicated by solid arrows. Abbreviations AcCoA, acetyl-CoA ASCT, acetateisuccinate CoA-transferase C, cytochrome c CI-CIV, complexes I—IV of the respiratory chain CITR, citrate FRD, fumarate reductase FUM, fumarate MAL, malate Methylmal-CoA, methylmalonyl-CoA OXAC, oxaloacetate PEP, phosphoenolpyruvate PROP, propionate Prop-CoA, propionyl-CoA PYR, pyruvate RQ, rhodoquinone SDH, succinate dehydrogenase SUCC, succinate Succ CoA, succinyl CoA UQ, ubiquinone.

Some eukaryotes can survive hypoxia by using simple fermentations in which the electrons from glycolysis are transferred to pyruvate or a derivative of it. Many variations of this type of fermentation exist, resulting in end products such as lactate or ethanol (Fig. 5.1). The formation of lactate produces 2 mol of ATP per glucose degraded, is found in all phyla, and it is the sole anaerobic pathway of evolutionarily more advanced species like arthropoda and vertebrates (Livingstone 1991). 

In the RT3D simulation, advective/dispersive transport of each contaminant is assumed. Sorption is modeled as a linear equilibrium process and biodegradation is modeled as a first-order process. Due to the assumed degradation reaction pathways (Fig. 2) transport of the different compounds is coupled. In the study, four reaction zones were delineated, based on observed geochemistry data. Each zone (two anaerobic zones, one transition zone, and one aerobic zone) has a different value for the biodegradation first-order rate constant for each contaminant. For example, since PCE is assumed to degrade only under... 

Phenol is biodegradable by both aerobic and anaerobic pathways. Little will accumulate in plants or animals and complete aerobic bacterial degradation will produce carbon dioxide. Still phenol is considered a potent insecticide, herbicide, and fungicide. The LC50 for aquatic organisms ranges from 12 to 68 mgl ... 

Finally, there is also an anaerobic pathway for fatty acid degradation in E. coli that is shared by other gram-negative bacteria [19]. This system allows the utilization of fatty acids in the anaerobic intestinal environment. 

Aromatic compounds can also be degraded anaerobically, via reductive pathways, which have been reviewed. The best-characterized example is the degradation of benzoic acid in Rhodopseudomonas palustris and Thauera aromatica via the pathway shown in Figure 8. Benzoyl-CoA is formed, and then a reductase enzyme is able to reduce the aromatic ring to a cyclohexadiene. Following two consecutive additions of water, and oxidation to a /3-keto ester, hydrolytic cleavage gives a linear 7-carbon CoA thioester, which can then be broken down via fatty acid /3-oxidation. 

It is true that the study of the distribution of enzymes in the cell has only just begun, nevertheless they have broi ht to our notice several important facts concerning cellular metabolism. From these studies we see that the cytoplasm is the prindpal i on for the performance of anaerobic degradations which provide the mitochondrial machine with fatty adds, amino adds, and pyruvic add. Since most of the glycolysis occurs in it, the cytoplasm is manufacturing energy-rich bonds by an anaerobic pathway. In addition the cytoplasm, or more particularly the microsomes, appears to be the home of protein synthesis. 

Thauera sp. strain DNT-1 is able to degrade toluene under aerobic conditions mediated by a dioxygenase, and under denitrifying conditions in the absence of oxygen by the anaerobic benzylsuccinate pathway (Shinoda et al. 2004). Whereas the tod genes were induced under aerobic conditions, the bss genes were induced under both aerobic and anaerobic conditions. 

Conventional use has been made of the radioisotope C, and details need hardly be given here. Illustrative examples include the elucidation of pathways for the anaerobic degradation of amino acids (Chapter 7, Part 1) and purines (Chapter 10, Part 1). Some applications have used C with high-resolution Fourier transform NMR in whole-cell suspensions, and this is equally applicable to molecules containing the natural or the synthetic P nuclei. As noted later, major advances in NMR have made it possible to use natural levels of C. 

The complex pathway for the anaerobic degradation of propionate by Smithella propionica and Methanospirillum hungatei involves reaction of two molecules of propionate followed by rearrangement to 3-ketohexanoate. The details were elucidated using C-propionate labeled at Cj, C2, C3, or at both Cj and C3 (de Bok et al. 2001). 

The degradation of alkynes has been the subject of sporadic interest during many years, and the pathway has been clearly delineated. It is quite distinct from those used for alkanes and alkenes, and is a reflection of the enhanced nucleophilic character of the alkyne C C bond. The initial step is hydration of the triple bond followed by ketonization of the initially formed enol. This reaction operates during the degradation of acetylene itself (de Bont and Peck 1980), acetylene carboxylic acids (Yamada and Jakoby 1959), and more complex alkynes (Figure 7.18) (Van den Tweel and de Bont 1985). It is also appropriate to note that the degradation of acetylene by anaerobic bacteria proceeds by the same pathway (Schink 1985b). 

In contrast, dissimilation of acetate may take place by reversal of the pathway used by organisms snch as Clostridium thermoaceticum for the synthesis of acetate from COj. In the degradation of acetate, the pathway involves a dismutation in which the methyl group is successively oxidized via methyl THF to COj while the carbonyl group is oxidized via bound carbon monoxide. Snch THF-mediated reactions are of great importance in the anaerobic degradation of pnrines, which is discussed in Chapter 10, Part 1. 

Anaerobic degradation of cycloalkanes has seldom been reported. The pathway used for the degradation of ethylcyclopentane by a sulfate-reducing enrichment is analogous to the fumarate pathway used for -alkanes (Part 1 of this chapter) with the formation of 3-ethylcyclopentanecar-boxylate followed by ring fission to 3-ethylpentan-l,5-dioate (Rios-Hamandez et al. 2003). 

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