The distribution of redox equivalents in the anaerobic respiratory chain of Paracoccus denitrificans

Long-term exposure to nano-TiO2 interferes with microbial metabolism and electron behavior to influence wastewater nitrogen removal and associated N2O emission

The extensive use of nano-TiO₂ has caused concerns regarding their potential environmental risks. However, the stress responses and self-recovery potential of nitrogen removal and greenhouse gas N₂O emissions after long-term nano-TiO₂ exposure have seldom been addressed yet. This study explored the long-term effects of nano-TiO₂ on biological nitrogen transformations in a sequencing batch reactor at four levels (1, 10, 25, and 50 mg/L), and the reactor's self-recovery potential was assessed. The results showed that nano-TiO₂ exhibited a dose-dependent inhibitory effect on the removal efficiencies of ammonia nitrogen and total nitrogen, whereas N₂O emissions unexpectedly increased. The promoted N₂O emissions were probably due to the inhibition of denitrification processes, including the reduction of the denitrifying-related N₂O reductase activity and the abundance of the denitrifying bacteria Flavobacterium. The inhibition of carbon source metabolism, the inefficient electron transfer efficiency, and the electronic competition between the denitrifying enzymes would be in charge of the deterioration of denitrification performance. After the withdrawal of nano-TiO₂ from the influent, the nitrogen transformation efficiencies and the N₂O emissions of activated sludge recovered entirely within 30 days, possibly attributed to the insensitive bacteria survival and the microbial community diversity. Overall, this study will promote the current understanding of the stress responses and the self-recovery potential of BNR systems to nanoparticle exposure.

Involvement of the cbb3-Type Terminal Oxidase in Growth Competition of Bacteria, Biofilm Formation, and in Switching between Denitrification and Aerobic Respiration

Paracoccus denitrificans has a branched electron transport chain with three terminal oxidases transferring electrons to molecular oxygen, namely aa₃-type and cbb₃-type cytochrome c oxidases and ba₃-type ubiquinol oxidase. In the present study, we focused on strains expressing only one of these enzymes. The competition experiments showed that possession of cbb₃-type oxidase confers significant fitness advantage during oxygen-limited growth and supports the biofilm lifestyle. The aa₃-type oxidase was shown to allow rapid aerobic growth at a high oxygen supply. Activity of the denitrification pathway that had been expressed in cells grown anaerobically with nitrate was fully inhibitable by oxygen only in wild-type and cbb₃ strains, while in strains aa₃ and ba₃ dinitrogen production from nitrate and oxygen consumption occurred simultaneously. Together, the results highlight the importance of the cbb₃-type oxidase for the denitrification phenotype and suggest a way of obtaining novel bacterial strains capable of aerobic denitrification.

Biochar as electron donor for reduction of N2O by Paracoccus denitrificans

Biochar (BC) has been shown to influence microbial denitrification and mitigate soil N2O emissions. However, it is unclear if BC is able to directly stimulate the microbial reduction of N2O to N2. We hypothesized that the ability of BC to lower N2O emissions could be related not only to its ability to store electrons, but to donate them to bacteria that enzymatically reduce N2O. Therefore, we carried out anoxic incubations with Paracoccus denitrificans, known amounts of N2O, and nine contrasting BCs, in the absence of any other electron donor or acceptor. We found a strong and direct correlation between the extent and rates of N2O reduction with BC's EDC/EEC (electron donating capacity/electron exchange capacity). Apart from the redox capacity, other BC properties were found to regulate the BC's ability to increase N2O reduction by P. denitrificans. For this specific BC series, we found that a high H/C and ash content, low surface area and poor lignin feedstocks favored N2O reduction. This provides valuable information for producing tailored BCs with the potential to assist and promote the reduction of N2O in the pursuit of reducing this greenhouse gas emissions.

Modeling Denitrification as an Electric Circuit Accurately Captures Electron Competition between Individual Reductive Steps: The Activated Sludge Model-Electron Competition Model

Heterotrophic denitrification consists of the four-step sequential reduction of nitrate to dinitrogen gas over nitrite, nitric oxide, and nitrous oxide. Oxidation processes, commonly of organic compounds, provide the electrons needed for the sequential reaction steps. The intracellular electron distribution is a competitive process among the four reduction steps. In this study, a model describing organic carbon oxidation and four-step denitrification through electron competition is proposed [Activated Sludge Model-Electron Competition (ASM-EC)]. The model describes denitrification rates as an analogy to how current intensity varies through a parallel set of resistors in electric circuits. The ASM-EC model was calibrated with data from batch experiments with heterotrophic denitrifying communities, where reduction of mixtures of nitrogen oxides was monitored, while different carbon sources were supplied in excess. The carbon sources included methanol, ethanol, acetate, and their ternary mixture. The electron distribution preference and electron uptake rates varied between the carbon sources and were captured by the model structure for most of the experiments. The ASM-EC model uses fewer parameters compared to existing state-of-the-art denitrification models and performed equally well in the tested scenarios. We advocate the use of this model for denitrification in the activated sludge model, which can easily be integrated in existing model structures, because it provides a parsimonious description of electron competition during denitrification.

Substrate Diffusion within Biofilms Significantly Influencing the Electron Competition during Denitrification

A common and long-existing operational issue of wastewater denitrification is the unexpected accumulation of nitrite (NO₂^-) that could suppress the activity of various microorganisms involved in biological wastewater treatment process and nitrous oxide (N₂O) that could emit as a potent greenhouse gas. Recently, it has been confirmed that the accumulation of these denitrification intermediates in biological wastewater treatment process is greatly influenced by the electron competition between the four denitrification steps. However, little is known about this in biofilm systems. In this work, we applied a mathematical model that links carbon oxidation and nitrogen reduction processes through a pool of electron carriers, to assess electron competition in denitrifying biofilms. Simulations were performed comprehensively at seven combinations of electron acceptor addition scheme (i.e., simultaneous addition of one, two or three among nitrate (NO₃^-), NO₂^-, and N₂O) to compare the effect of electron competition on NO₃^-, NO₂^- and N₂O reduction. Overall, the effects of substrate loading, biofilm thickness and effective diffusion coefficients on electron competition are not always intuitive. Model simulations show that electron competition was intensified due to the substrate load limitation (from 120 to 20 mg COD/L) and increasing biofilm thicknesses (from 0.1 to 1.6 mm) in most cases, where electrons were prioritized to nitrite reductase because of the insufficient electron donor availability in the biofilm. In contrast, increasing effective diffusion coefficients did not pose a significant effect on electron competition and only increased electrons distributed to nitrite reductase when both NO₂^- and N₂O are added.

Staphylococcus epidermidis: metabolic adaptation and biofilm formation in response to different oxygen concentrations

Staphylococcus epidermidis has become a major health hazard. It is necessary to study its metabolism and hopefully uncover therapeutic targets. Cultivating S. epidermidis at increasing oxygen concentration [O2] enhanced growth, while inhibiting biofilm formation. Respiratory oxidoreductases were differentially expressed, probably to prevent reactive oxygen species formation. Under aerobiosis, S. epidermidis expressed high oxidoreductase activities, including glycerol-3-phosphate dehydrogenase, pyruvate dehydrogenase, ethanol dehydrogenase and succinate dehydrogenase, as well as cytochromes bo and aa3; while little tendency to form biofilms was observed. Under microaerobiosis, pyruvate dehydrogenase and ethanol dehydrogenase decreased while glycerol-3-phosphate dehydrogenase and succinate dehydrogenase nearly disappeared; cytochrome bo was present; anaerobic nitrate reductase activity was observed; biofilm formation increased slightly. Under anaerobiosis, biofilms grew; low ethanol dehydrogenase, pyruvate dehydrogenase and cytochrome bo were still present; nitrate dehydrogenase was the main terminal electron acceptor. KCN inhibited the aerobic respiratory chain and increased biofilm formation. In contrast, methylamine inhibited both nitrate reductase and biofilm formation. The correlation between the expression and/or activity or redox enzymes and biofilm-formation activities suggests that these are possible therapeutic targets to erradicate S. epidermidis.

Nitrate levels modulate the abundance of Paracoccus sp. in a biofilm community

Conditions required to enhance a particular species efficient in degradative capabilities is very useful in wastewater treatment processes. Paracoccus sp. is known to efficiently reduce nitrogen oxides (NOx) due to the branched denitrification pathway. Individual-based simulations showed that the relative fitness of Paracoccus sp. to Pseudomonas sp. increased significantly with nitrate levels above 5 mM. Spatial structure of the biofilm showed substantially less nitrite levels in the areas of Paracoccus sp. dominance. The simulation was validated in a laboratory reactor harboring biofilm community by fluorescent in situ hybridization, which showed that increasing nitrate levels enhanced the abundance of Paracoccus sp. Different levels of NOx did not display any significant effect on biofilm formation of Paracoccus sp., unlike several other bacteria. This study shows that the attribute of Paracoccus sp. to tolerate and efficiently reduce NOx is conferring a fitness payoff to the organism at high concentrations of nitrate in a multispecies biofilm community.

Evaluating two concepts for the modelling of intermediates accumulation during biological denitrification in wastewater treatment

The accumulation of the denitrification intermediates in wastewater treatment systems is highly undesirable, since both nitrite and nitric oxide (NO) are known to be toxic to bacteria, and nitrous oxide (N2O) is a potent greenhouse gas and an ozone depleting substance. To date, two distinct concepts for the modelling of denitrification have been proposed, which are represented by the Activated Sludge Model for Nitrogen (ASMN) and the Activated Sludge Model with Indirect Coupling of Electrons (ASM-ICE), respectively. The two models are fundamentally different in describing the electron allocation among different steps of denitrification. In this study, the two models were examined and compared in their ability to predict the accumulation of denitrification intermediates reported in four different experimental datasets in literature. The N-oxide accumulation predicted by the ASM-ICE model was in good agreement with values measured in all four cases, while the ASMN model was only able to reproduce one of the four cases. The better performance of the ASM-ICE model is due to that it adopts an "indirect coupling" modelling concept through electron carriers to link the carbon oxidation and the nitrogen reduction processes, which describes the electron competition well. The ASMN model, on the other hand, is inherently limited by its structural deficiency in assuming that carbon oxidation is always able to meet the electron demand by all denitrification steps, therefore discounting electron competition among these steps. ASM-ICE therefore offers a better tool for predicting and understanding intermediates accumulation in biological denitrification.

Effect of pH on N₂O reduction and accumulation during denitrification by methanol utilizing denitrifiers

Acidic pH has previously been found to increase nitrous oxide (N₂O) accumulation during heterotrophic denitrification in biological wastewater treatment. However, the mechanism of this phenomenon still needs to be clarified. By using an enriched methanol utilizing denitrifying culture as an example, this paper presents a comprehensive study on the effect of pH (6.0-9.0) on N₂O reduction kinetics with N₂O as the sole electron acceptor, as well as the effect of pH on N₂O accumulation with N₂O as an intermediate of nitrate reduction. The pH dependency of nitrate and nitrite reduction was also investigated. The maximum biomass-specificN₂O reduction rate is higher than the corresponding maximum nitrate and nitrite reduction rates in the entire pH range studied. However, the maximum biomass-specific N₂O reduction rate is much more sensitive to pH variation outside of the optimal range (pH 7.5 to pH 8.0) than the maximum biomass-specific nitrate and nitrite reduction rates. The half-saturation coefficient of the N₂O reductase increased from 0.10 mg N₂O-N/L to 0.92 mg N₂O-N/L as pH increased from pH 6.0 to 9.0. At pH 6.0, approximately 20% and 40% of the denitrified nitrate accumulated as N₂O in the presence and absence of methanol (as an exogenous carbon source), respectively. However, at pH 6.5, these fractions decreased to 0% and 30%, respectively. No N₂O accumulation occurred at pH 7.0 to 9.0 independent of the availability of methanol. These results suggest that the competition for electrons among different nitrogen oxides reductases likely plays a role in N₂O accumulation at low pH conditions.

Effect of nitrate on biogenic sulfide production

The addition of 59 mM nitrate inhibited biogenic sulfide production in dilute sewage sludge (10% [vol/vol]) amended with 20 mM sulfate and either acetate, glucose, or hydrogen as electron donors. Similar results were found when pond sediment or oil field brines served as the inoculum. Sulfide production was inhibited for periods of at least 6 months and was accompanied by the oxidation of resazurin from its colorless reduced state to its pink oxidized state. Lower amounts of nitrate (6 or 20 mM) and increased amounts of sewage sludge resulted in only transient inhibition of sulfide production. The addition of 156 mM sulfate to bottles with 59 mM nitrate and 10% (vol/vol) sewage sludge or pond sediment resulted in sulfide production. Nitrate, nitrite, and nitrous oxide were detected during periods where sulfide production was inhibited, whereas nitrate, nitrite, and nitrous oxide were below detectable levels at the time sulfide production began. The oxidation of resazurin was attributed to an increase in nitrous oxide which persisted in concentration of about 1.0 mM for up to 5 months. The numbers of sulfate-reducing organisms decreased from 10 CFU ml sludge to less than detectable levels after prolonged incubation of oxidized bottles. The addition of 10 mM glucose to oxidized bottles after 14.5 weeks of incubation resulted in rereduction of the resazurin and subsequent sulfide production. The prolonged inhibition of sulfide production was attributed to an increase in oxidation-reduction potential due to biogenic production of nitrous oxide, which appeared to have a cytotoxic effect on sulfate-reducing populations.

Mass Spectrometric Studies of the Effect of pH on the Accumulation of Intermediates in Denitrification by Paracoccus denitrificans

We have used a quadrupole mass spectrometer with a gas-permeable membrane inlet for continuous measurements of the production of N(2)O and N(2) from nitrate or nitrite by cell suspensions of Paracoccus denitrificans. The use of nitrate and nitrite labeled with N was shown to simplify the interpretation of the results when these gases were measured. This approach was used to study the effect of pH on the production of denitrification intermediates from nitrate and nitrite under anoxic conditions. The kinetic patterns observed were quite different at acidic and alkaline pH values. At pH 5.5, first nitrate was converted to nitrite, then nitrite was converted to N(2)O, and finally N(2)O was converted to N(2). At pH 8.5, nitrate was converted directly to N(2), and the intermediates accumulated to only low steady-state concentrations. The sequential usage of nitrate, nitrite, and nitrous oxide observed at pH 5.5 was simulated by using a kinetic model of a branched electron transport chain in which alternative terminal reductases compete for a common reductant.

Influence of Volatile Fatty Acids on Nitrite Accumulation by a Pseudomonas stutzeri Strain Isolated from a Denitrifying Fluidized Bed Reactor

Intermediate nitrite accumulation during denitrification by Pseudomonas stutzeri isolated from a denitrifying fluidized bed reactor was examined in the presence of different volatile fatty acids. Nitrite accumulated when acetate or propionate served as the carbon and electron source but did not accumulate in the presence of butyrate, valerate, or caproate. Nitrite accumulation in the presence of acetate was caused by differences in the rates of nitrate and nitrite reduction and, in addition, by competition between nitrate and nitrite reduction pathways for electrons. Incubation of the cells with butyrate resulted in a slower nitrate reduction rate and a faster nitrite reduction rate than incubation with acetate. Whereas nitrate inhibited the nitrite reduction rate in the presence of acetate, no such inhibition was found in butyrate-supplemented cells. Cytochromes b and c were found to mediate electron transport during nitrate reduction by the cells. Cytochrome c was reduced via a different pathway when nitrite-reducing cells were incubated with acetate than when they were incubated with butyrate. Furthermore, addition of antimycin A to nitrite-reducing cells resulted in partial inhibition of electron transport to cytochrome c in acetate-supplemented cells but not in butyrate-supplemented cells. On the basis of these findings, we propose that differences in intermediate nitrite accumulation are caused by differences in electron flow to nitrate and nitrite reductases during oxidation of either acetate or butyrate.

Nitric oxide oscillations in Paracoccus denitrificans: the effects of environmental factors and of segregating nitrite reductase and nitric oxide reductase into separate cells

Nitric oxide is a denitrification intermediate which is produced from nitrite and then further converted via nitrous oxide to nitrogen. Here, the effect of low concentrations of the protonophore carbonylcyanide m-chlorophenylhydrazone on the time courses for dissolved gases was examined. While NO was found to oscillate, N(2)O only increased gradually as the reduction of nitrite progressed. The frequency and shape of protonophore-induced NO oscillations were influenced by temperature and the concentration of electron donor N,N,N',N'-tetramethyl-p-phenylene diamine (TMPD) in a manner compatible with the observed differential effects on the two involved enzyme activities. We demonstrated the existence of a pH interval, where [NO] oscillates even without uncoupler addition. Occurrence of nitric oxide oscillations in mixtures of a nitrite reductase mutant with a nitric oxide reductase mutant suggests that they cannot be due to a competition of the enzymes for redox equivalents from one common respiratory chain.

NarK is a nitrite-extrusion system involved in anaerobic nitrate respiration by Escherichia coli

Escherichia coli can use nitrate as a terminal electron acceptor for anaerobic respiration. A polytopic membrane protein, termed NarK, has been implicated in nitrate uptake and nitrite excretion and is thought to function as a nitrate/nitrite antiporter. The longest-lived radioactive isotope of nitrogen, 13N-nitrate (half-life = 9.96 min) and the nitrite-sensitive fluorophore N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide have now been used to define the function of NarK. At low concentrations of nitrate, NarK mediates the electrogenic excretion of nitrite rather than nitrate/nitrite exchange. This process prevents intracellular accumulation of toxic levels of nitrite and allows further detoxification in the periplasm through the action of nitrite reductase.

Oscillations of nitric oxide concentration in the perturbed denitrification pathway of Paracoccus denitrificans

The metabolism of nitric oxide in Paracoccus denitrificans has been studied using a Clark-type electrode. The uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP) and the SH reagent N-ethylmaleimide, both of which released nitric oxide from cells respiring nitrite, were found to be efficient inhibitors of nitric oxide reductase activity. Control experiments with another uncoupler, pentachlorophenol, showed that the inhibitory effect of CCCP was not the result of a decrease in membrane potential. The denitrification pathway in cells with partly inhibited nitric oxide reductase, or in a reconstituted system containing purified nitric reductase and membrane vesicles, exhibited marked sustained oscillations of nitric oxide concentration. The occurrence of the oscillations was strictly dependent on the initial concentration of nitrite. The observed oscillatory kinetics is considered to reflect two regulatory signals destabilizing the denitrification pathway, namely the inhibition of nitric oxide reductase by nitric oxide and/or by nitrite.

Evidence for the role of soluble cytochrome c in the dissimilatory reduction of nitrite and nitrous oxide by cells of Paracoccus denitrificans

The role of periplasmic cytochrome c in the denitrification pathway has been investigated using a wild-type and/or a cytochrome c deficient strain of Paracoccus denitrificans. The reconstitution experiments with the isolated proteins showed that bacterial cytochrome c-550 restored the electron transport from the cytoplasmic membrane to soluble nitrite reductase (cytochrome cd1). In response to decreased aeration lasting 3 h, the HUUG25 strain synthesized nitrous-oxide reductase significantly starved of electrons from the respiratory chain and only very small amounts of soluble cytochrome c. The membrane-bound part of the respiratory chain catalyzing the reduction of soluble cytochrome c resembled an autologous region in wild-type cells kinetically and by its sensitivity to antimycin. In the periplasmic fraction obtained from anaerobically grown wild-type cells N2O caused the reoxidation of endogenous cytochrome(s) c previously reduced by N,N,N',N' tetramethyl-p-phenylenediamine plus ascorbate. All these results indicate the involvement of soluble cytochrome(s) c as the electron donor(s) for the reduction of NO2- and N2O in the periplasmic space of cells.

Metabolic regulation including anaerobic metabolism in Paracoccus denitrificans

Under anaerobic circumstances in the presence of nitrate Paracoccus denitrificans is able to denitrify. The properties of the reductases involved in nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide reductase are described. For that purpose not only the properties of the enzymes of P. denitrificans are considered but also those from Escherichia coli, Pseudomonas aeruginosa, and Pseudomonas stutzeri. Nitrate reductase consists of three subunits: the alpha subunit contains the molybdenum cofactor, the beta subunit contains the iron sulfur clusters, and the gamma subunit is a special cytochrome b. Nitrate is reduced at the cytoplasmic side of the membrane and evidence for the presence of a nitrate-nitrite antiporter is presented. Electron flow is from ubiquinol via the specific cytochrome b to the nitrate reductase. Nitrite reductase (which is identical to cytochrome cd1) and nitrous oxide reductase are periplasmic proteins. Nitric oxide reductase is a membrane-bound enzyme. The bc1 complex is involved in electron flow to these reductases and the whole reaction takes place at the periplasmic side of the membrane. It is now firmly established that NO is an obligatory intermediate between nitrite and nitrous oxide. Nitrous oxide reductase is a multi-copper protein. A large number of genes is involved in the acquisition of molybdenum and copper, the formation of the molybdenum cofactor, and the insertion of the metals. It is estimated that at least 40 genes are involved in the process of denitrification. The control of the expression of these genes in P. denitrificans is totally unknown. As an example of such complex regulatory systems the function of the fnr, narX, and narL gene products in the expression of nitrate reductase in E. coli is described. The control of the effects of oxygen on the reduction of nitrate, nitrite, and nitrous oxide are discussed. Oxygen inhibits reduction of nitrate by prevention of nitrate uptake in the cell. In the case of nitrite and nitrous oxide a competition between reductases and oxidases for a limited supply of electrons from primary dehydrogenases seems to play an important role. Under some circumstances NO formed from nitrite may inhibit oxidases, resulting in a redistribution of electron flow from oxygen to nitrite. P. denitrificans contains three main oxidases: cytochrome aa3, cytochrome o, and cytochrome co. Cytochrome o is proton translocating and receives its electrons from ubiquinol. Some properties of cytochrome co, which receives its electrons from cytochrome c, are reported.(ABSTRACT TRUNCATED AT 400 WORDS)

Determination of nitrate by conversion to nitrite using Paracoccus denitrificans

A new method of determination of nitrate was developed, utilizing the nitrate reductase activity of Paracoccus denitrificans in which a further reduction of nitrate is blocked either by a mutation affecting formation of cytochromes c or by inhibition of the electron flow to nitrite reductase by mucidin. After deproteinization of the sample with zinc acetate the nitrite produced is determined colorimetrically.

Removal of nitrate from water by cells of Paracoccus denitrificans in a membrane flow reactor

A new type of flow bioreactor designed to remove nitrate from water was developed. Denitrification activity of native Paracoccus denitrificans cells was used, the cells being separated from the refined medium by a semipermeable membrane. Relationships between the degree of nitrate conversion and the denitrification rate, on the one hand, and the volume flow rate and the amount of biomass, on the other, together with the results concerning denitrification during closed-circuit recirculation of the medium are discussed.

Formation of a potent respiratory inhibitor at nitrite reduction by nitrite reductase isolated from the bacterium Paracoccus denitrificans

A new method of dissimilatory nitrite reductase (cytochrome cd1) isolation from the periplasmic fraction of anaerobically grown cells of the bacterium Paracoccus denitrificans was developed, using ionex and gel permeation chromatography with FPLC system (Pharmacia, Sweden). In experiments with isolated enzyme it was shown that through a nitrite reduction, catalysed by this enzyme, a substance (presumably nitric oxide) was formed which at submicromolar concentrations inhibited terminal cytochrome oxidase of the respiratory chain of the same bacterium. These results help to explain formerly observed sensitivity of bacterial oxidase activity to NO2- and the mechanism of switching the electron flow from O2 to nitrogen terminal acceptors.

Separate binding sites for antimycin and mucidin in the respiratory chain of the bacterium Paracoccus denitrificans and their occurrence in other denitrificans bacteria

By means of the method of fluorimetric titration it has been shown that mucidin does not affect the attachment of antimycin to membranes from anaerobically grown Paracoccus denitrificans. The fluorimetric titration with antimycin can be used in the determination of the amount of the cytochrome bc1 complex in the membrane. In cells inhibited with antimycin, the oxidation of cytochromes c was accompanied by the reduction of cytochrome b; in the presence of mucidin this effect did not take place. The results, which indicated a difference in binding sites, were interpreted in terms of the Q-cycle [Mitchell (1976) J. Theor. Biol. 62, 327-367; Trumpower (1981) Biochim. Biophys. Acta 639, 129-155]. Comparable sensitivity towards antimycin and mucidin was shown by other typical denitrifying bacteria: Pseudomonas stutzeri and Alcaligenes xylosoidans, subspecies denitrificans.

Control of respiration rate in non-growing cells of Paracoccus denitrificans

By means of fluorimetric measurement and by direct determination of intracellular NAD+ and NADH contents, it was proved that the respiration rate of Paracoccus denitrificans cells utilizing glucose is limited by processes preceding NADH oxidation in the respiratory chain, so that the membrane NADH dehydrogenase is not saturated by its substrate. In the separated membrane fraction on saturation with exogenous NADH the main limiting factor is represented by NADH: ubiquinone oxidoreductase.

The effect of uncoupler on the distribution of the electron flow between the terminal acceptors oxygen and nitrite in the cells of Paracoccus denitrificans

The preferential utilization of oxygen, the terminal acceptor, in anaerobically grown cells of Paracoccus denitrificans was abolished in the presence of uncoupler (3 microM carbonyl cyanide m-chlorophenylhydrazone) which brought about a switch to the reduction of nitrite. It has been proved by measuring the redox state of cytochromes that this effect is due to the inhibition of the electron flow to oxygen caused by nitrite, which attains the site of its inhibitory action when the membrane potential is lowered.

The function of cytoplasmic membrane of Paracoccus denitrificans in controlling the rate of reduction of terminal acceptors

The rate of reduction of terminal acceptors (nitrate, nitrite, and oxygen) in anaerobically grown cells of Paracoccus denitrificans increased on permeabilization of cytoplasmic membrane. It was proved that under aerobic conditions the increase of the rate of nitrate reduction was caused by: (i) the abolishment of the permeability barrier for nitrate, (ii) the enhancement of the influx of redox equivalents to the respiratory chain due to the stimulation of succinate dehydrogenase reaction, and (iii) the inhibition of electron flow to oxygen by endogenously formed nitrite. Nitrite inhibits oxygen reduction by its interaction with the terminal part of the respiratory chain (I50 = 15 microM) localized at the inner aspect of the cytoplasmic membrane. The distribution of nitrite between intact cells and the suspension medium follows the Nernst equation for monovalent anion. The possible physiological consequences of the low intracellular nitrite concentration are discussed.