Anaerobic pathways

Anaerobic pathway of vitamin Bl2 biosynthesis 98H(47)1051. Biosynthesis of thiamine 97AG(E)1032. 

The chemical engineering approach began with an analysis of the biochemistry of platelet metabolism. Like many cells, platelets consume glucose by two pathways, an oxidative pathway and an anaerobic pathway. The oxidative pathway produces carbon dioxide, which makes the solution containing the platelets more acidic (lower pH) and promotes anaerobic metabolism. This second metabolic pathway produces large amounts of lactic acid, further lowering pH. The drop in pH from both pathways kills the platelets. 

The formation of hydrogen is accompanied with VFAs or solvent production during an anaerobic digestion process. Therefore, the distribution of VFA concentrations and their finctions is a usefiil indicator for monitoring hydrogen production. Fig. 3 shows the variations in alcohol and VFAs. Most of the VFAs were analyzed as acetate and butyrate, and most of the alcohols were analyzed as ethanol. The propionate concentration was below the analytical limit. It indicates that the anaerobic pathway in the reactor is not propionic-type fermentation but but5nrate-type fermentation. Clostridium butyricum is considered to be the dominant... 

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. 

Successful field-scale applications of engineered bioremediation systems have been limited to the aerobic pathway, as opposed to the anaerobic pathway. The advantages of the aerobic pathway include the following ... 

There are a number of aerobes that are known to use MTBE as a sole carbon and energy source anaerobic pathways and the types of microorganisms involved are less well documented. 

Mitochondria from body wall muscle and probably the pharynx lack a functional TCA cycle and their novel anaerobic pathways rely on reduced organic acids as terminal electron acceptors, instead of oxygen (Saz, 1971 Ma et al, 1993 Duran et al, 1998). Malate and pyruvate are oxidized intramitochondrially by malic enzyme and the pyruvate dehydrogenase complex, respectively, and excess reducing power in the form of NADH drives Complex II and [3-oxidation in the direction opposite to that observed in aerobic organelles (Kita, 1992 Duran et al, 1993 Ma et al,... 

Several attempts have been made to investigate anaerobic pathways for desulfurization. Mixed cultures of sulfate-reducing bacteria were used to desulfurize model compounds, thiophenes [12], organosulfides [12,13], and petroleum preparations [14-16], Production of hydrogen sulfide and biphenyl from DBT has been demonstrated in other studies [17-22],... 

A microbial biocatalyst in context of BDS is defined as a microorganism expressing enzymes capable of removing sulfur selectively from organosulfur compounds. The definition of a biocatalyst, in general, also includes use of one or more enzymes, by themselves or in a cellular extract (used either in suspended form or carrier-supported form) for removal of sulfur. Additionally, biocatalyst can be a microbial consortium as well. Aerobic as well as anaerobic pathways for sulfur removal have been reported. The anaerobic routes, however, have been plagued with lack of reproducibility preventing further development. 

Experimental studies conducted by Galletti et al. and Matamoros et al. [89, 90] have shown that aerobic conditions are generally more efficient than anaerobic pathways in removal of the majority of emerging contaminants. 

We examine two groups of contaminants that exhibit biologically mediated transformation in the subsurface petroleum hydrocarbons and pesticides. Both aerobic and anaerobic pathways for their biodegradation are considered. 

Anaerobic pathway for biosynthesis of monounsaturated fatty acids in E. coli. Synthesis of monounsaturated fatty acids follows the pathway described previously for saturated fatty acids until the intermediate j8-hydroxydecanoyl-ACP is reached. At this point an apparent competition arises between the enzymes involved in saturated and unsaturated fatty acid synthesis. 

In contrast to the anaerobic pathway found in E. coli, the aerobic pathway in eukaryotic cells introduces double bonds after the saturated fatty acid has been synthesized. Stearoyl-CoA (18 0) is the major substrate for desaturation. Stearic acid is made by the fatty acid synthase as a minor product, the major product being palmitic acid, and is activated to its CoA derivative by acyl-CoA synthase. In eukaryotic cells an enzyme complex associated with the endoplasmic reticulum desaturates stearoyl-CoA to oleoyl-CoA (18 1A9). This remarkable reaction requires NADH and 02 and results in the formation of a double bond in the middle of an acyl chain with no activating groups nearby. The chemical mechanism for desaturation of long-chain acyl-CoAs remains unclear. 

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 above discussion, the energy yields of the different anaerobic pathways of metabolism are expressed in terms of moles of ATP per mole of substrate fermented. This is the traditional way to express the energetic efficiency of fermentations and for some purposes it is informative and useful. However, it is important to recall that in vivo these pathways are linked to ATP utilizing pathways (usually ATPases). At steady state, rates of ATP synthesis by these fermentations equal rates of ATP utilization. For classical glycolysis, for example... 

The thermodynamic yield of the various anaerobic pathways is related to their affinity for substrates. Sulfate-reducing bacteria outcompete methanogens by maintaining H2 concentrations below the threshold for the process to be thermodynamically feasible (Lovley et al., 1982), and Fe(III) reducers do the same to SOU reducers. In fact, the equilibrium concentration of H2 can be used to predict the dominant metabolic pathways in a given environment (Lovley and Goodwin, 1988 Lovley et al., 1994a). 

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