Pathways anaerobic respiration

In the absence of 02 the E. coli FNR protein induces proteins of the anaerobic respiration pathways. Nitrate also has its own two-component system that senses nitrate availability and activates transcription of enzymes catalyzing nitrate respiration.162 An expanded two-component system induces sporulation in Bacillus subtilis in response to poor growth conditions.163164 The crystal structure of one of two response regulators (SpoOF) has a structure closely related to that of CheY and the nitrate response regulator NarL. 

In addition to the reduction potentials of different terminal electron acceptors utilized in the anaerobic ETC, presence of other compounds has also found to be essential for the efficiency of anaerobic respiration. One important factor is metal ion homeostasis. This is because many enzymes in the anaerobic respiratory pathways are metal containing enzymes. It was reported that expression of operons containing enzymes in the anaerobic respiration pathway is highly reduced upon iron limitation under anaerobic conditions while the aerobic respiration pathways are only modestly affected in E. coli (Cotter et al. 1992). Moreover, induction of DMSO respiration pathways is reported to be dependent on the molybdate uptake (McNichoIas et al. 1998). 

When SO42- is depleted and methanogenesis is the dominant form of anaerobic respiration, it can proceed through either of the following two pathways fermentation or C02 reduction. 

Fig. 9. The coupling of electron and proton flow in succinate iquinone oxidoreduc-tases in aerobic (a,c) and anaerobic respiration (b,d), respectively. Positive and negative sides of the membrane are as described for Fig. 1. (a) and (b) Electroneutral reactions as catalyzed by C-type SQR enzymes (a) and D-type E. coli QFR (b). (c) Utilization of a transmembrane electrochemical potential Ap as possibly catalyzed by A-type and B-type enzymes, (d) Electroneutral fumarate reduction by B-type QFR enzymes with a proposed compensatory E-pathway. ...

The outcome of competition between microorganisms for electron donors can be predicted from thermodynamic theory (Section 8.08.1.2 Table 1 Zehnder and Stumm, 1988), and these predictions are generally consistent with empirical data. Temporal succession of the microbial metabolic pathways that dominate respiration occurs upon the flooding of an oxidized soil or sediment (Figure 1). Not surprisingly, most examples of temporal succession in anaerobic respiration processes have come from wetland soils, which are subject to cycles of flooding and exposure (Turner and Patrick, 1968 Ponnamperuma, 1972 Achtnich et al., 1995a Yao et al., 1999). However, the same pattern is observed in sediments and even upland soils (Peters and Conrad, 1996). 

Elevation of the ethanol content occurs during the maturation stage of fruit development when acidity declines, total solids increase, and the flavor compounds accumulate to characteristically ripe levels. Since ethanol is a normal product of anaerobic respiration in plants, its accumulation signals a change in the pathway of energy metabolism. This summary of research on the control of ethanol accumulation in citrus fruit describes an approach to identify the role of energy metabolism in the bioregulation of maturation. 

Denitrifying bacteria are frequently encountered as anaerobic degraders of diverse xenobiotics. They are facultatively anaerobic and frequently also degrade xenobiotics aerobically. However, the pathways as well as the specific patterns are different under aerobic and denitrifying conditions. Nitrate is reduced to N2 via nitrate, NO, and N2O. Iron (III) or manganese (IV) reduction is a geochemically important alternative pathway of anaerobic respiration and has been shown repeatedly to be linked... 

Newman et al. (56), and Rochette et al. (68) suggest that the reduction of arsenate by dissolved sulfide is very slow at circumneutral pH values. However, at pH values less than 5, the reduction rates of arsenate due to sulfide may be significant in natural systems, where half-lives as short as 21 hr have been reported (68) for this abiotic pathway (Table 3). Rochette et al. (68) also revealed the potential importance of intermediate As-O-S species in electron transfer reactions between sulfide and arsenate, such as H2 As OsS H2As 02S , and H2 As OS2. It is not known whether these chemical species may also serve as important redox active species for microbial metabolism. These authors have also compared the rates of As(V) reduction in the presence of sulfide versus those rates expected via dissimilatory reduction by an arsenate-respiring organism (strain SES-3) (54) and for those measured in lake sediments (69) at pH values less than 5, reduction rates due to dissolved sulfide can become more significant than reduction rates due to anaerobic respiration where As(V) is used as the terminal electron acceptor (Fig. 8). 

PriiB, B.M., Campbell, J.W., Van Dyk, T.K., Zhu, C., Kogan, Y. and Matsumura, P. (2003). FUiD/FlhC is a regulator of anaerobic respiration and the Entner-Doudoroff pathway through induction of the methyl-accepting chemotaxis protein Aer. J. Bacterial. 185, 534- 43. 

There are two major pathways in respiration. Oxic respiration (called aerobic respiration in the older literature) requires molecular oxygen, whereas anoxic respiration (anaerobic respiration) occurs in the absence of molecular oxygen. Oxic respiration uses the Krebs cycle to obtain energy from the reaction given above for cellular respiration ... 

Dissimilatory nitrite reductases (NiR) play a pivotal role in the anaerobic respiration cascade of denitrifying bacteria, archaea and fungi by catalysing the first committed step of the pathway.The net reaction of NiR yields the conversion of nitrite (N02 ) into gaseous nitric oxide (NO) and water (H2O) (eqn (3.12) ref. 207) ... 

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