Nitrite respiration, anaerobic

The nitrite formed is either excreted directly or reduced by non-ATP-yielding reactions to ammonia. The enzyme machinery for both processes, nitrate/nitrite respiration and denitrification, is formed only under anaerobic conditions or conditions of low oxygen tension. In fact, the activities of the enzymes involved in dissimila-tory nitrate reduction are strongly inhibited by oxygen. Thus, denitrification and nitrate/nitrite respiration take place only when oxygen is absent or available in insufficient amounts. 

Surprisingly, its biological redox partners remain largely unknown. It has been implicated in anaerobic nitrite respiration and it has been shown that azurin can donate electrons to nitrite reductase, a function that is proposed to be carried out by another cupredoxin, pseudoazurin (see Section IV, E). On the other hand, azurin is not an inducible protein and denitrifying bacteria express azurin constitutively under aerobic conditions. 

Bacterial assimilatory nitrate reductases have similar properties.86/86a In addition, many bacteria, including E. coli, are able to use nitrate ions as an oxidant for nitrate respiration under anaerobic conditions (Chapter 18). Tire dissimilatory nitrate reductases involved also contain molybdenum as well as Fe-S centers.85 Tire E. coli enzyme receives electrons from reduced quinones in the plasma membrane, passing them through cytochrome b, Fe-S centers, and molybdopterin to nitrate. The three-subunit aPy enzyme contains cytochrome b in one subunit, an Fe3S4 center as well as three Fe4S4 clusters in another, and the molybdenum cofactor in the third.87 Nitrate reduction to nitrite is also on the pathway of denitrification, which can lead to release of nitrogen as NO, NzO, and N2 by the action of dissimi-latory nitrite reductases. These enzymes873 have been discussed in Chapters 16 and 18. 

As noted in Section 62.1.9.6, reduction of nitrate may occur by assimilatory or dissimilatory pathways. In the former case, the nitrate produced is reduced further to ammonia, which is incorporated into the cell. In the latter case, nitrate is reduced anaerobically to nitrite, serving as an electron acceptor in the respiration of facultative or a few obligate anaerobic bacteria. The example of Escherichia coli has been considered in Section 62.1.13.4.3. This process is usually terminated at nitrite, which accumulates around the cells, but may proceed further1511 as nitrite-linked respiration in the process of denitrification. 

Organisms with anaerobic mitochondria can be divided into two different types those which perform anaerobic respiration and use an alternative electron acceptor present in the environment, such as nitrate or nitrite, and those which perform fermentation reactions using an endogenously produced, organic electron acceptor, such as fumarate (Martin et al. 2001 Tielens et al. 2002). An example of the first type is the nitrate respiration that occurs in several ciliates (Finlay et al. 1983), and fungi (Kobayashi et al. 1996 Takaya et al. 2003), which use nitrate and/or nitrite as the terminal electron acceptor of their mitochondrial electron-transport chain, producing nitrous oxide as... 

All plants depend on nitrate reductase to accomplish the seemingly trivial reaction of nitrate reduction to nitrite, often the first step of nitrogen assimilation into compounds required for growth (5, 22). Many bacteria use molybdenum or tungsten enzymes in anaerobic respiration where the terminal electron acceptor is a reducible molecule other than oxygen, such as nitrate (2, 50), polysulfide (51), trimethylamine oxide (33, 52) or dimethyl sulfoxide (DMSO) (2, 29, 30). 

Figure 28. Hypothetical anaerobic nitrogen cycle based on the following thermodynamically permissible reactions (1) ammonium oxidation to dinitrogen by carbon dioxide,. sulfate or ferric iron (no evidence at present, possibly kinetically limited) (2) dinitrogen fixation by various organic and inorganic reductants (known) (3) ammonium oxidation by nitrite or nitrate producing dinitrogen (known) (4) denitrification (known) (5) nitrite or nitrate respiration (known) (6) ferric iron oxidation of ammonium to nitrite or nitrate (no evidence at present) (7) nitrate assimilation (known) (8) ammonium assimilation and di.s,similation (known) (Fenchel etai, 1998).

Dissimilatory Reduction of Nitrate to Ammonium by Microbial Cultures. We studied nitrate reduction to ammonia by an obligate anaerobe, Clostridium, which cannot gain energy from this reduction by electron transport phosphorylation, and by a number of Enterobacteri-aceae (known to be nitrate respirers) that can gain energy via the nitrate to nitrite step. All these organisms converted NOg" to as the... 

Certain bacteria can utilize nitrate nitrogen as the sole nitrogen source for the synthesis of all nitrogen containing compounds of the cell (Payne, 1973). This nitrate assimilation can occur under both aerobic and anaerobic conditions. In other instances (Payne, 1973) nitrate serves as a terminal hydrogen acceptor under anaerobic conditions and this process is called nitrate respiration. In both cases the product of nitrate reduction is nitrite. The nitrate reductases from bacteria have been differentiated by Pichinoty and Piechaud (1968) into nitrate reductase A which is membrane bound and can reduce chlorate in addition to nitrate as a substrate and nitrate reductase B which is... 

Other experimental data indicate that the oxygen is affecting nitrite accumulation by increasing nitrite reduction. When conditions are optimized for entry of rotenone (mitochondrial site I inhibitor) into leaf secticHis the leaves under aerobic conditicHis accumulated one-half as much nitrite as those without rotenone under anaerobic conditions (Hageman et al., 1980b Reed and Hageman, 1977). The rotenone treatment reduced respiration by... 

It is well established that most of the known anaerobic prokaryotes perform oxidative phosphorylation without O2. Depending on the species and the metabolic conditions, these bacteria may use a large variety of inorganic (e.g., nitrate, nitrite, sulfate, thiosulfate, elemental sulfm, polysulfide sulfur) or organic compounds (e.g., fumarate, dimethylsulfoxide, trimethylamine-A -oxide, vinyl- or arylchlorides) as terminal electron acceptor instead of oxygen. The redox reactions with these acceptors are catalyzed by membrane-integrated electron transport chains and are coupled to the generation of an electrochemical proton potential (Ap) across the membrane. Oxidative phosphorylation in the absence of O2 is also termed anaerobic respiration . Oxidative phosphorylation with elemental sulfur is called sulfm respiration . Oxidative phosphorylation with polysulfide sulfur is called polysulfide respiration . 

Certain species of Enterobacteriaceae, especially 0. proteus, could utilise nitrates as electron acceptors for anaerobic respiration, resulting in reduction of nitrate into nitrite. Nitrite further could possibly react with secondary amines present in the wort, forming (V-nitrosoamine (Figure 8.2). A -nitrosomines are carcinogenic in nature (Smith, 1994). Hence, a considerable amount of apparent total N-nitroso compounds (ATNCs) represents a possible risk to health, and consequently their concentration is strictly monitored and limited to 20 gg/1 (Maiguerite and Walker, 2002). Because of the risk of W-nitroso compounds, the Enterobacteriaceae species related to brewery environments are monitored. 

Many bacterial genera contain denitrifying species Achromobacter, Alcaligenes (Alcaligenes odorans denitrifies nitrite). Bacillus, Chromobacterium, Coryne-bacterium, Halobacterium, Hyphomicrobium, Morax-ella, Paracoccus, Pseudomonas, Spirillum, Thiobacil-lus and Xanthomonas. In some species of Pseudomonas and Corynebacterium, N O is the final denitrification product. All these bacteria are aerobes that are able to respire (denitrify) nitrate under anaerobic conditions. The only true anaerobe able to carry out denitrification is Propionibacterium. There is no evidence for other intermediates in the above denitrification pathway, but in the formation of nitrous oxide (NjO) an NN bond must be formed, and there may exist transient enzyme-bound intermediates that have not yet been identified. The enzymology of denitrification from nitrite is poorly understood. It seems likely that each stage is linked to electron transport via a cytochrome system, but sites of ATP synthesis have not been unequivocally located. 

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) ... 

The production of ATP is achieved by glucose, pyruvate, and NADH oxidation in the presence of oxygen (aerobic respiration). In the absence or in the presence of limited amounts of oxygen, the glycolytic products will be metabolized by anaerobic fermentation using alternative substrates such as nitrite. ... 

A number of different enzymes can carry out the reduction of nitrite to either ammonium or nitric oxide and/or nitrous oxide. The latter types are involved with the denitrification process (Payne, 1973) and will not be considered here. Among the enzymes that catalyze the six-electron reduction of nitrite to ammonia, several different types are recognized. These are (I) assimilatory NiRs that function in biosynthetic nitrate assimilation of higher plants, algae, and fungi, (2) ammonia-forming dissimilatory NiRs involved in anaerobic nitrate respiration of diverse bacteria, and (3) assimilatory and dissimilatory sulfite reductases... 

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