Nitric oxide is a potent inhibitor of the cbb(3)-type heme-copper oxidases

Cyanide Insensitive Oxidase Confers Hydrogen Sulfide and Nitric Oxide Tolerance to Pseudomonas aeruginosa Aerobic Respiration

Hydrogen sulfide (H2S) and nitric oxide (NO) are long-known inhibitors of terminal oxidases in the respiratory chain. Yet, they exert pivotal signaling roles in physiological processes, and in several bacterial pathogens have been reported to confer resistance against oxidative stress, host immune responses, and antibiotics. Pseudomonas aeruginosa, an opportunistic pathogen causing life-threatening infections that are difficult to eradicate, has a highly branched respiratory chain including four terminal oxidases of the haem-copper type (aa3, cbb3-1, cbb3-2, and bo3) and one oxidase of the bd-type (cyanide-insensitive oxidase, CIO). As Escherichia coli bd-type oxidases have been shown to be H2S-insensitive and to readily recover their activity from NO inhibition, here we tested the effect of H2S and NO on CIO by performing oxygraphic measurements on membrane preparations from P. aeruginosa PAO1 and isogenic mutants depleted of CIO only or all other terminal oxidases except CIO. We show that O2 consumption by CIO is unaltered even in the presence of high levels of H2S, and that CIO expression is enhanced and supports bacterial growth under such stressful conditions. In addition, we report that CIO is reversibly inhibited by NO, while activity recovery after NO exhaustion is full and fast, suggesting a protective role of CIO under NO stress conditions. As P. aeruginosa is exposed to H2S and NO during infection, the tolerance of CIO towards these stressors agrees with the proposed role of CIO in P. aeruginosa virulence.

Bioenergetics and Reactive Nitrogen Species in Bacteria

The production of reactive nitrogen species (RNS) by the innate immune system is part of the host’s defense against invading pathogenic bacteria. In this review, we summarize recent studies on the molecular basis of the effects of nitric oxide and peroxynitrite on microbial respiration and energy conservation. We discuss possible molecular mechanisms underlying RNS resistance in bacteria mediated by unique respiratory oxygen reductases, the mycobacterial bcc-aa₃ supercomplex, and bd-type cytochromes. A complete picture of the impact of RNS on microbial bioenergetics is not yet available. However, this research area is developing very rapidly, and the knowledge gained should help us develop new methods of treating infectious diseases.

Vibrio cholerae requires oxidative respiration through the bd-I and cbb3 oxidases for intestinal proliferation

Vibrio cholerae respires both aerobically and anaerobically and, while oxygen may be available to it during infection, other terminal electron acceptors are proposed for population expansion during infection. Unlike gastrointestinal pathogens that stimulate significant inflammation leading to elevated levels of oxygen or alternative terminal electron acceptors, V. cholerae infections are not understood to induce a notable inflammatory response. To ascertain the respiration requirements of V. cholerae during infection, we used Multiplex Genome Editing by Natural Transformation (MuGENT) to create V. cholerae strains lacking aerobic or anaerobic respiration. V. cholerae strains lacking aerobic respiration were attenuated in infant mice 10⁵-fold relative to wild type, while strains lacking anaerobic respiration had no colonization defect, contrary to earlier work suggesting a role for anaerobic respiration during infection. Using several approaches, including one we developed for this work termed Comparative Multiplex PCR Amplicon Sequencing (CoMPAS), we determined that the bd-I and cbb₃ oxidases are essential for small intestinal colonization of V. cholerae in the infant mouse. The bd-I oxidase was also determined as the primary oxidase during growth outside the host, making V. cholerae the only example of a Gram-negative bacterial pathogen in which a bd-type oxidase is the primary oxidase for energy acquisition inside and outside of a host.

The bacterium that causes cholera, Vibrio cholerae, can grow with or without oxygen. When growing without oxygen it may use other molecules that serve the same purpose as oxygen, acting as a terminal electron acceptor in an energy-generating process known as respiration. Given the largely anaerobic nature of the gastrointestinal tract, and the lack of significant inflammation during cholera infection, a process that can stimulate elevated levels of oxygen and other terminal electron acceptors, we sought to understand the respiratory mechanisms of V. cholerae during infection. We used a powerful genome-editing method to construct mutant strains of V. cholerae lacking some or all of the complement of proteins required for aerobic or anaerobic respiration. By analyzing these mutants in the laboratory and in intestinal colonization of infant mice, we determined that the ability to respire without oxygen is completely dispensable for V. cholerae to thrive during infection. We determined that two of the four oxygen-dependent respiration mechanisms are essential for V. cholerae to grow during infection, with the other two dispensable for wild type levels of colonization.

Activation of O2 and NO in heme-copper oxidases - mechanistic insights from computational modelling

Heme-copper oxidases are transmembrane enzymes involved in aerobic and anaerobic respiration. The largest subgroup contains the cytochrome c oxidases (CcO), which reduce molecular oxygen to water. A significant part of the free energy released in this exergonic process is conserved as an electrochemical gradient across the membrane, via two processes, electrogenic chemistry and proton pumping. A deviant subgroup is the cytochrome c dependent NO reductases (cNOR), which reduce nitric oxide to nitrous oxide and water. This is also an exergonic reaction, but in this case none of the released free energy is conserved. Computational studies applying hybrid density functional theory to cluster models of the bimetallic active sites in the heme-copper oxidases are reviewed. To obtain a reliable description of the reaction mechanisms, energy profiles of the entire catalytic cycles, including the reduction steps have to be constructed. This requires a careful combination of computational results with certain experimental data. Computational studies have elucidated mechanistic details of the chemical parts of the reactions, involving cleavage and formation of covalent bonds, which have not been obtainable from pure experimental investigations. Important insights regarding the mechanisms of energy conservation have also been gained. The computational studies show that the reduction potentials of the active site cofactors in the CcOs are large enough to afford electrogenic chemistry and proton pumping, i.e. efficient energy conservation. These results solve a conflict between different types of experimental data. A mechanism for the proton pumping, involving a specific and crucial role for the active site tyrosine, conserved in all CcOs, is suggested. For the cNORs, the calculations show that the low reduction potentials of the active site cofactors are optimized for fast elimination of the toxic NO molecules. At the same time, the low reduction potentials lead to endergonic reduction steps with high barriers. To prevent even higher barriers, which would lead to a too slow reaction, when the electrochemical gradient across the membrane is present, the chemistry must occur in a non-electrogenic manner. This explains why there is no energy conservation in cNOR.

Nitric Oxide Does Not Inhibit but Is Metabolized by the Cytochrome bcc-aa3 Supercomplex

Nitric oxide (NO) is a well-known active site ligand and inhibitor of respiratory terminal oxidases. Here, we investigated the interaction of NO with a purified chimeric bcc-aa₃ supercomplex composed of Mycobacterium tuberculosis cytochrome bcc and Mycobacterium smegmatisaa₃-type terminal oxidase. Strikingly, we found that the enzyme in turnover with O₂ and reductants is resistant to inhibition by the ligand, being able to metabolize NO at 25 °C with an apparent turnover number as high as ≈303 mol NO (mol enzyme)⁻¹ min⁻¹ at 30 µM NO. The rate of NO consumption proved to be proportional to that of O₂ consumption, with 2.65 ± 0.19 molecules of NO being consumed per O₂ molecule by the mycobacterial bcc-aa₃. The enzyme was found to metabolize the ligand even under anaerobic reducing conditions with a turnover number of 2.8 ± 0.5 mol NO (mol enzyme)⁻¹ min⁻¹ at 25 °C and 8.4 µM NO. These results suggest a protective role of mycobacterial bcc-aa₃ supercomplexes against NO stress.

Role of the Two Metals in the Active Sites of Heme Copper Oxidases-A Study of NO Reduction in cbb3 Cytochrome c Oxidase

The superfamily of heme copper oxidases reduces molecular oxygen or nitric oxide, and the active sites comprise a high-spin heme group (a₃ or b₃) and a non-heme metal (Cu_B or Fe_B). The cbb₃ C family of cytochrome c oxidases, with the high-spin heme b₃ and Cu_B in the active site, is a subfamily of the heme copper oxidases that can reduce both molecular oxygen, which is the main substrate, and nitric oxide. The mechanism for NO reduction in cbb₃ oxidase is studied here using hybrid density functional theory and compared to other cytochrome c oxidases (A and B families), with a high-spin heme a₃ and Cu_B in the active site, and to cytochrome c dependent NO reductase, with a high-spin heme b₃ and a non-heme Fe_B in the active site. It is found that the reaction mechanism and the detailed reaction energetics of the cbb₃ oxidases are not similar to those of cytochrome c dependent NO reductase, which has the same type of high-spin heme group but a different non-heme metal. This is in contrast to earlier expectations. Instead, the NO reduction mechanism in cbb₃ oxidases is very similar to that in the other cytochrome c oxidases, with the same non-heme metal, Cu_B, and is independent of the type of high-spin heme group. The conclusion is that the type of non-heme metal (Cu_B or Fe_B) in the active site of the heme copper oxidases is more important for the reaction mechanisms than the type of high-spin heme, at least for the NO reduction reaction. The reason is that the proton-coupled reduction potentials of the active site cofactors determine the energetics for the NO reduction reaction, and they depend to a larger extent on the non-heme metal. Observed differences in NO reduction reactivity among the various cytochrome c oxidases may be explained by differences outside the BNC, affecting the rate of proton transfer, rather than in the BNC itself.

Functional interactions between nitrite reductase and nitric oxide reductase from Paracoccus denitrificans

Denitrification is a microbial pathway that constitutes an important part of the nitrogen cycle on earth. Denitrifying organisms use nitrate as a terminal electron acceptor and reduce it stepwise to nitrogen gas, a process that produces the toxic nitric oxide (NO) molecule as an intermediate. In this work, we have investigated the possible functional interaction between the enzyme that produces NO; the cd₁ nitrite reductase (cd₁NiR) and the enzyme that reduces NO; the c-type nitric oxide reductase (cNOR), from the model soil bacterium P. denitrificans. Such an interaction was observed previously between purified components from P. aeruginosa and could help channeling the NO (directly from the site of formation to the side of reduction), in order to protect the cell from this toxic intermediate. We find that electron donation to cNOR is inhibited in the presence of cd₁NiR, presumably because cd₁NiR binds cNOR at the same location as the electron donor. We further find that the presence of cNOR influences the dimerization of cd₁NiR. Overall, although we find no evidence for a high-affinity, constant interaction between the two enzymes, our data supports transient interactions between cd₁NiR and cNOR that influence enzymatic properties of cNOR and oligomerization properties of cd₁NiR. We speculate that this could be of particular importance in vivo during metabolic switches between aerobic and denitrifying conditions.

Nitric oxide in plants: pro- or anti-senescence

Senescence is a regulated process of tissue degeneration that can affect any plant organ and consists of the degradation and remobilization of molecules to other growing tissues. Senescent organs display changes at the microscopic level as well as modifications to internal cellular structure and differential gene expression. A large number of factors influencing senescence have been described including age, nutrient supply, and environmental interactions. Internal factors such as phytohormones also affect the timing of leaf senescence. A link between the senescence process and the production of nitric oxide (NO) in senescing tissues has been known for many years. Remarkably, this link can be either a positive or a negative correlation depending upon the organ. NO can be both a signaling or a toxic molecule and is known to have multiple roles in plants; this review considers the duality of NO roles in the senescence process of two different plant organs, namely the leaves and root nodules.

The response of Sphingopyxis granuli strain TFA to the hostile anoxic condition

Sphingomonads comprises a group of interesting aerobic bacteria because of their ubiquity and metabolic capability of degrading many recalcitrant contaminants. The tetralin-degrader Sphingopyxis granuli strain TFA has been recently reported as able to anaerobically grow using nitrate as the alternative electron acceptor and so far is the only bacterium with this ability within the sphingomonads group. To understand how strain TFA thrives under anoxic conditions, a differential transcriptomic analysis while growing under aerobic or anoxic conditions was performed. This analysis has been validated and complemented with transcription kinetics of representative genes of different functional categories. Results show an extensive change of the expression pattern of this strain in the different conditions. Consistently, the most induced operon in anoxia codes for proteases, presumably required for extensive changes in the protein profile. Besides genes that respond to lack of oxygen in other bacteria, there are a number of genes that respond to stress or to damage of macromolecules, including genes of the SOS DNA-damage response, which suggest that anoxic conditions represent a hostile environment for this bacterium. Interestingly, growth under anoxic conditions also resulted in repression of all flagellar and type IV pilin genes, which suggested that this strain shaves its appendages off while growing in anaerobiosis.

Heme redox potentials hold the key to reactivity differences between nitric oxide reductase and heme-copper oxidase

Despite high structural homology between NO reductases (NORs) and heme-copper oxidases (HCOs), factors governing their reaction specificity remain to be understood. Using a myoglobin-based model of NOR (Fe_BMb) and tuning its heme redox potentials (E°') to cover the native NOR range, through manipulating hydrogen bonding to the proximal histidine ligand and replacing heme b with monoformyl (MF-) or diformyl (DF-) hemes, we herein demonstrate that the E°' holds the key to reactivity differences between NOR and HCO. Detailed electrochemical, kinetic, and vibrational spectroscopic studies, in tandem with density functional theory calculations, demonstrate a strong influence of heme E°' on NO reduction. Decreasing E°' from +148 to -130 mV significantly impacts electronic properties of the NOR mimics, resulting in 180- and 633-fold enhancements in NO association and heme-nitrosyl decay rates, respectively. Our results indicate that NORs exhibit finely tuned E°' that maximizes their enzymatic efficiency and helps achieve a balance between opposite factors: fast NO binding and decay of dinitrosyl species facilitated by low E°' and fast electron transfer facilitated by high E°'. Only when E°' is optimally tuned in Fe_BMb(MF-heme) for NO binding, heme-nitrosyl decay, and electron transfer does the protein achieve multiple (>35) turnovers, previously not achieved by synthetic or enzyme-based NOR models. This also explains a long-standing question in bioenergetics of selective cross-reactivity in HCOs. Only HCOs with heme E°' in a similar range as NORs (between -59 and 200 mV) exhibit NOR reactivity. Thus, our work demonstrates efficient tuning of E°' in various metalloproteins for their optimal functionality.