Crystal structure of quinol-dependent nitric oxide reductase from Geobacillus stearothermophilus

Gene cloning, expression and performance validation of nitric oxide dismutase

Nitrous oxide (N2O) is a significant contributor to global warming and possesses an ozone-depleting impact nearly 298 times that of CO2. To reduce N2O emissions, the newly-discovered nod gene which can directly convert NO into N2 and O2 was successfully cloned from the anaerobic denitrification sludge. The recombinant plasmid containing the nod gene was built, and the expression of nod gene in Escherichia coli was determined, leading to the construction of recombinant engineering bacteria. Results showed that the recombinant engineering bacteria E. coli BL21 (DE3)-pET28a-nod could autonomously degrade NO, with a degradation rate of 72 % within 48 h, and could produce 2479.72 ppm of N2 and 75.12 mL of O2. The cumulative O2 production of the sludge sample and recombinant E. coli within 8 h was 1.75 mL and 8.45 mL, respectively. The cumulative O2 production of recombinant E. coli was at least 4.82 times higher than that of the sludge sample. The investigation proposed a new biodegradation pathway for nitrogen pollution.

Diversity and evolution of nitric oxide reduction in bacteria and archaea

Nitrous oxide is a potent greenhouse gas whose production is catalyzed by nitric oxide reductase (NOR) members of the heme-copper oxidoreductase (HCO) enzyme superfamily. We identified several previously uncharacterized HCO families, four of which (eNOR, sNOR, gNOR, and nNOR) appear to perform NO reduction. These families have novel active-site structures and several have conserved proton channels, suggesting that they might be able to couple NO reduction to energy conservation. We isolated and biochemically characterized a member of the eNOR family from the bacterium Rhodothermus marinus and found that it performs NO reduction. These recently identified NORs exhibited broad phylogenetic and environmental distributions, greatly expanding the diversity of microbes in nature capable of NO reduction. Phylogenetic analyses further demonstrated that NORs evolved multiple times independently from oxygen reductases, supporting the view that complete denitrification evolved after aerobic respiration.

Global patterns in gene content of soil microbiomes emerge from microbial interactions

Microbial metabolism sustains life on Earth. Sequencing surveys of communities in hosts, oceans, and soils have revealed ubiquitous patterns linking the microbes present, the genes they possess, and local environmental conditions. One prominent explanation for these patterns is environmental filtering: local conditions select strains with particular traits. However, filtering assumes ecological interactions do not influence patterns, despite the fact that interactions can and do play an important role in structuring communities. Here, we demonstrate the insufficiency of the environmental filtering hypothesis for explaining global patterns in topsoil microbiomes. Using denitrification as a model system, we find that the abundances of two characteristic genotypes trade-off with pH; nar gene abundances increase while nap abundances decrease with declining pH. Contradicting the filtering hypothesis, we show that strains possessing the Nar genotype are enriched in low pH conditions but fail to grow alone. Instead, the dominance of Nar genotypes at low pH arises from an ecological interaction with Nap genotypes that alleviates nitrite toxicity. Our study provides a roadmap for dissecting how global associations between environmental variables and gene abundances arise from environmentally modulated community interactions.

Denitrifying Woodchip Bioreactors: A Microbial Solution for Nitrate in Agricultural Wastewater-A Review

Nitrate (NO3-) is highly water-soluble and considered to be the main nitrogen pollutants leached from agricultural soils. Its presence in aquatic ecosystems is reported to cause various environmental and public health problems. Bioreactors containing microbes capable of transforming NO3- have been proposed as a means to remediate contaminated waters. Woodchip bioreactors (WBRs) are continuous flow, reactor systems located below or above ground. Below ground systems are comprised of a trench filled with woodchips, or other support matrices. The nitrate present in agricultural drainage wastewater passing through the bioreactor is converted to harmless dinitrogen gas (N2) via the action of several bacteria species. The WBR has been suggested as one of the most cost-effective NO3--removing strategy among several edge-of-field practices, and has been shown to successfully remove NO3- in several field studies. NO3- removal in the WBR primarily occurs via the activity of denitrifying microorganisms via enzymatic reactions sequentially reducing NO3- to N2. While previous woodchip bioreactor studies have focused extensively on its engineering and hydrological aspects, relatively fewer studies have dealt with the microorganisms playing key roles in the technology. This review discusses NO3- pollution cases originating from intensive farming practices and N-cycling microbial metabolisms which is one biological solution to remove NO3- from agricultural wastewater. Moreover, here we review the current knowledge on the physicochemical and operational factors affecting microbial metabolisms resulting in removal of NO3- in WBR, and perspectives to enhance WBR performance in the future.

Reconnaissance of Oxygenic Denitrifiers in Agriculturally Impacted Soils

Row crop production in the agricultural Midwest pollutes waterways with nitrate, and exacerbates climate change through increased emissions of nitrous oxide and methane. Oxygenic denitrification processes in agricultural soils mitigate nitrate and nitrous oxide pollution by short-circuiting the canonical pathway to avoid nitrous oxide formation. Furthermore, many oxygenic denitrifiers employ a nitric oxide dismutase (nod) to create molecular oxygen that is used by methane monooxygenase to oxidize methane in otherwise anoxic soils. The direct investigation of nod genes that could facilitate oxygenic denitrification processes in agricultural sites is limited, with no prior studies investigating nod genes at tile drainage sites. Thus, we performed a reconnaissance of nod genes at variably saturated surface sites, and within a variably to fully saturated soil core in Iowa to expand the known distribution of oxygenic denitrifiers. We identified new nod gene sequences from agricultural soil and freshwater sediments in addition to identifying nitric oxide reductase (qNor) related sequences. Surface and variably saturated core samples displayed a nod to 16S rRNA gene relative abundance of 0.004% to 0.1% and fully saturated core samples had relative nod gene abundance of 1.2%. The relative abundance of the phylum Methylomirabilota increased from 0.6% and 1% in the variably saturated core samples to 3.8% and 5.3% in the fully saturated core samples. The more than 10-fold increase in relative nod abundance and almost 9-fold increase in relative Methylomirabilota abundance in fully saturated soils suggests that potential oxygenic denitrifiers play a greater nitrogen cycling role under these conditions. IMPORTANCE The direct investigation of nod genes in agricultural sites is limited, with no prior studies investigating nod genes at tile drains. An improved understanding of nod gene diversity and distribution is significant to the field of bioremediation and ecosystem services. The expansion of the nod gene database will advance oxygenic denitrification as a potential strategy for sustainable nitrate and nitrous oxide mitigation, specifically for agricultural sites.

A 2.2 Å cryoEM structure of a quinol-dependent NO Reductase shows close similarity to respiratory oxidases

Quinol-dependent nitric oxide reductases (qNORs) are considered members of the respiratory heme-copper oxidase superfamily, are unique to bacteria, and are commonly found in pathogenic bacteria where they play a role in combating the host immune response. qNORs are also essential enzymes in the denitrification pathway, catalysing the reduction of nitric oxide to nitrous oxide. Here, we determine a 2.2 Å cryoEM structure of qNOR from Alcaligenes xylosoxidans, an opportunistic pathogen and a denitrifying bacterium of importance in the nitrogen cycle. This high-resolution structure provides insight into electron, substrate, and proton pathways, and provides evidence that the quinol binding site not only contains the conserved His and Asp residues but also possesses a critical Arg (Arg720) observed in cytochrome bo3, a respiratory quinol oxidase.

Identifying antibiotics based on structural differences in the conserved allostery from mitochondrial heme-copper oxidases

Antimicrobial resistance (AMR) is a global health problem. Despite the enormous efforts made in the last decade, threats from some species, including drug-resistant Neisseria gonorrhoeae, continue to rise and would become untreatable. The development of antibiotics with a different mechanism of action is seriously required. Here, we identified an allosteric inhibitory site buried inside eukaryotic mitochondrial heme-copper oxidases (HCOs), the essential respiratory enzymes for life. The steric conformation around the binding pocket of HCOs is highly conserved among bacteria and eukaryotes, yet the latter has an extra helix. This structural difference in the conserved allostery enabled us to rationally identify bacterial HCO-specific inhibitors: an antibiotic compound against ceftriaxone-resistant Neisseria gonorrhoeae. Molecular dynamics combined with resonance Raman spectroscopy and stopped-flow spectroscopy revealed an allosteric obstruction in the substrate accessing channel as a mechanism of inhibition. Our approach opens fresh avenues in modulating protein functions and broadens our options to overcome AMR.

Evolution of quinol oxidation within the heme‑copper oxidoreductase superfamily

The heme‑copper oxidoreductase (HCO) superfamily is a large superfamily of terminal respiratory enzymes that are widely distributed across the three domains of life. The superfamily includes biochemically diverse oxygen reductases and nitric oxide reductases that are pivotal in the pathways of aerobic respiration and denitrification. The adaptation of HCOs to use quinol as the electron donor instead of cytochrome c has significant implication for the respiratory flexibility and energetic efficiency of the respiratory chains that include them. In this work, we explore the adaptation of this scaffold to two different electron donors, cytochromes c and quinols, with extensive sequence analysis of these enzymes from publicly available datasets. Our work shows that quinol oxidation evolved independently within the HCO superfamily at least seven times. Enzymes from only two of these independently evolved clades have been biochemically well-characterized. Combining structural modeling with sequence analysis, we identify putative quinol binding sites in each of the novel quinol oxidases. Our analysis of experimental and modeling data suggests that the quinol binding site appears to have evolved at the same structural position within the scaffold more than once.

New Horizons in Structural Biology of Membrane Proteins: Experimental Evaluation of the Role of Conformational Dynamics and Intrinsic Flexibility

A plethora of membrane proteins are found along the cell surface and on the convoluted labyrinth of membranes surrounding organelles. Since the advent of various structural biology techniques, a sub-population of these proteins has become accessible to investigation at near-atomic resolutions. The predominant bona fide methods for structure solution, X-ray crystallography and cryo-EM, provide high resolution in three-dimensional space at the cost of neglecting protein motions through time. Though structures provide various rigid snapshots, only an amorphous mechanistic understanding can be inferred from interpolations between these different static states. In this review, we discuss various techniques that have been utilized in observing dynamic conformational intermediaries that remain elusive from rigid structures. More specifically we discuss the application of structural techniques such as NMR, cryo-EM and X-ray crystallography in studying protein dynamics along with complementation by conformational trapping by specific binders such as antibodies. We finally showcase the strength of various biophysical techniques including FRET, EPR and computational approaches using a multitude of succinct examples from GPCRs, transporters and ion channels.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Distribution of Denitrification among Haloarchaea: A Comprehensive Study

Microorganisms from the Halobacteria class, also known as haloarchaea, inhabit a wide range of ecosystems of which the main characteristic is the presence of high salt concentration. These environments together with their microbial communities are not well characterized, but some of the common features that they share are high sun radiation and low availability of oxygen. To overcome these stressful conditions, and more particularly to deal with oxygen limitation, some microorganisms drive alternative respiratory pathways such as denitrification. In this paper, denitrification in haloarchaea has been studied from a phylogenetic point of view. It has been demonstrated that the presence of denitrification enzymes is a quite common characteristic in Halobacteria class, being nitrite reductase and nitric oxide reductase the enzymes with higher co-occurrence, maybe due to their possible role not only in denitrification, but also in detoxification. Moreover, copper-nitrite reductase (NirK) is the only class of respiratory nitrite reductase detected in these microorganisms up to date. The distribution of this alternative respiratory pathway and their enzymes among the families of haloarchaea has also been discussed and related with the environment in which they constitute the major populations. Complete denitrification phenotype is more common in some families like Haloarculaceae and Haloferacaceae, whilst less common in families such as Natrialbaceae and Halorubraceae.

Bacterial nitric oxide metabolism: Recent insights in rhizobia

Nitric oxide (NO) is a reactive gaseous molecule that has several functions in biological systems depending on its concentration. At low concentrations, NO acts as a signaling molecule, while at high concentrations, it becomes very toxic due to its ability to react with multiple cellular targets. Soil bacteria, commonly known as rhizobia, have the capacity to establish a N₂-fixing symbiosis with legumes inducing the formation of nodules in their roots. Several reports have shown NO production in the nodules where this gas acts either as a signaling molecule which regulates gene expression, or as a potent inhibitor of nitrogenase and other plant and bacteria enzymes. A better understanding of the sinks and sources of NO in rhizobia is essential to protect symbiotic nitrogen fixation from nitrosative stress. In nodules, both the plant and the microsymbiont contribute to the production of NO. From the bacterial perspective, the main source of NO reported in rhizobia is the denitrification pathway that varies significantly depending on the species. In addition to denitrification, nitrate assimilation is emerging as a new source of NO in rhizobia. To control NO accumulation in the nodules, in addition to plant haemoglobins, bacteroids also contribute to NO detoxification through the expression of a NorBC-type nitric oxide reductase as well as rhizobial haemoglobins. In the present review, updated knowledge about the NO metabolism in legume-associated endosymbiotic bacteria is summarized.

Architecture of bacterial respiratory chains

Bacteria power their energy metabolism using membrane-bound respiratory enzymes that capture chemical energy and transduce it by pumping protons or Na⁺ ions across their cell membranes. Recent breakthroughs in molecular bioenergetics have elucidated the architecture and function of many bacterial respiratory enzymes, although key mechanistic principles remain debated. In this Review, we present an overview of the structure, function and bioenergetic principles of modular bacterial respiratory chains and discuss their differences from the eukaryotic counterparts. We also discuss bacterial supercomplexes, which provide central energy transduction systems in several bacteria, including important pathogens, and which could open up possible avenues for treatment of disease.

Molecular understanding of heteronuclear active sites in heme-copper oxidases, nitric oxide reductases, and sulfite reductases through biomimetic modelling

Heme-copper oxidases (HCO), nitric oxide reductases (NOR), and sulfite reductases (SiR) catalyze the multi-electron and multi-proton reductions of O2, NO, and SO32-, respectively. Each of these reactions is important to drive cellular energy production through respiratory metabolism and HCO, NOR, and SiR evolved to contain heteronuclear active sites containing heme/copper, heme/nonheme iron, and heme-[4Fe-4S] centers, respectively. The complexity of the structures and reactions of these native enzymes, along with their large sizes and/or membrane associations, make it challenging to fully understand the crucial structural features responsible for the catalytic properties of these active sites. In this review, we summarize progress that has been made to better understand these heteronuclear metalloenzymes at the molecular level though study of the native enzymes along with insights gained from biomimetic models comprising either small molecules or proteins. Further understanding the reaction selectivity of these enzymes is discussed through comparisons of their similar heteronuclear active sites, and we offer outlook for further investigations.

Mechanisms of Energy Transduction by Charge Translocating Membrane Proteins

Life relies on the constant exchange of different forms of energy, i.e., on energy transduction. Therefore, organisms have evolved in a way to be able to harvest the energy made available by external sources (such as light or chemical compounds) and convert these into biological useable energy forms, such as the transmembrane difference of electrochemical potential (Δμ̃). Membrane proteins contribute to the establishment of Δμ̃ by coupling exergonic catalytic reactions to the translocation of charges (electrons/ions) across the membrane. Irrespectively of the energy source and consequent type of reaction, all charge-translocating proteins follow two molecular coupling mechanisms: direct- or indirect-coupling, depending on whether the translocated charge is involved in the driving reaction. In this review, we explore these two coupling mechanisms by thoroughly examining the different types of charge-translocating membrane proteins. For each protein, we analyze the respective reaction thermodynamics, electron transfer/catalytic processes, charge-translocating pathways, and ion/substrate stoichiometries.

Profiling structural diversity and activity of 2-alkyl-4(1H)-quinolone N-oxides of Pseudomonas and Burkholderia

We synthesized all major saturated and unsaturated 2-alkyl-4(1H)-quinolone N-oxides of Pseudomonas and Burkholderia, quantified their native production levels and characterized their antibiotic activities against competing Staphylococcus aureus. We demonstrate that quinolone core methylation and position of unsaturation in the alkyl-chain dictate antibiotic potency which supports the proposed mechanism of action.

Quantification of Ni-N-O Bond Angles and NO Activation by X-ray Emission Spectroscopy

A series of β-diketiminate Ni-NO complexes with a range of NO binding modes and oxidation states were studied by X-ray emission spectroscopy (XES). The results demonstrate that XES can directly probe and distinguish end-on vs side-on NO coordination modes as well as one-electron NO reduction. Density functional theory (DFT) calculations show that the transition from the NO 2s2s σ* orbital has higher intensity for end-on NO coordination than for side-on NO coordination, whereas the 2s2s σ orbital has lower intensity. XES calculations in which the Ni-N-O bond angle was fixed over the range from 80° to 176° suggest that differences in NO coordination angles of ∼10° could be experimentally distinguished. Calculations of Cu nitrite reductase (NiR) demonstrate the utility of XES for characterizing NO intermediates in metalloenzymes. This work shows the capability of XES to distinguish NO coordination modes and oxidation states at Ni and highlights applications in quantifying small molecule activation in enzymes.

A critical review of aerobic denitrification: Insights into the intracellular electron transfer

Aerobic denitrification is a novel biological nitrogen removal technology, which has been widely investigated as an alternative to the conventional denitrification and for its unique advantages. To fully comprehend aerobic denitrification, it is essential to clarify the regulatory mechanisms of intracellular electron transfer during aerobic denitrification. However, reports on intracellular electron transfer during aerobic denitrification are rather limited. Thus, the purpose of this review is to discuss the molecular mechanism of aerobic denitrification from the perspective of electron transfer, by summarizing the advancements in current research on electron transfer based on conventional denitrification. Firstly, the implication of aerobic denitrification is briefly discussed, and the status of current research on aerobic denitrification is summarized. Then, the occurring foundation and significance of aerobic denitrification are discussed based on a brief review of the key components involved in the electron transfer of denitrifying enzymes. Moreover, a strategy for enhancing the efficiency of aerobic denitrification is proposed on the basis of the regulatory mechanisms of denitrification enzymes. Finally, scientific outlooks are given for further investigation on aerobic denitrification in the future. This review could help clarify the mechanism of aerobic denitrification from the perspective of electron transfer.

Heterologous expression and functional study of nitric oxide reductase catalytic reduction peptide from Achromobacter denitrificans strain TB

Biological denitrification is a promising and green technology for air pollution control. To investigate the nitric oxide reductase (NOR) that dominates NO reduction efficiency in biological purification, the heterologous prokaryotic expression system of the norB gene, which encodes the core peptide of the catalytic reduction structure in the NOR from Achromobacter denitrificans strain TB, was constructed in Escherichia coli BL21 (DE3). Results showed that the 1218 bp-long norB gene was expressed at the highest level under 1.0 mM IPTG for 5 h at 30 °C, and the relative expression abundance of norB in recombinant E. coli was increased by 16.6 times compared with that of the wild-type TB. However, the NO reduction efficiency and NOR activity of strain TB was 2.7 and 1.83 times higher than those of recombinant E. coli, respectively. On the basis of genomic reassembly and protein structure modeling, the core peptide of the NOR catalytic reduction structure from Achromobacter sp. TB can independently exert NO reduction. The low NO degradation efficiency of recombinant E. coli may be due to the lack of a NorC-like structure that increases the enzyme activity of the NorB protein. The results of this study can be used as basis for further research on the structure and function of NOR.

Biological and Bioinspired Inorganic N-N Bond-Forming Reactions

The metallobiochemistry underlying the formation of the inorganic N-N-bond-containing molecules nitrous oxide (N2O), dinitrogen (N2), and hydrazine (N2H4) is essential to the lifestyles of diverse organisms. Similar reactions hold promise as means to use N-based fuels as alternative carbon-free energy sources. This review discusses research efforts to understand the mechanisms underlying biological N-N bond formation in primary metabolism and how the associated reactions are tied to energy transduction and organismal survival. These efforts comprise studies of both natural and engineered metalloenzymes as well as synthetic model complexes.

The active form of quinol-dependent nitric oxide reductase from Neisseria meningitidis is a dimer

Neisseria meningitidis is carried by nearly a billion humans, causing developmental impairment and over 100 000 deaths a year. A quinol-dependent nitric oxide reductase (qNOR) plays a critical role in the survival of the bacterium in the human host. X-ray crystallographic analyses of qNOR, including that from N. meningitidis (NmqNOR) reported here at 3.15 Å resolution, show monomeric assemblies, despite the more active dimeric sample being used for crystallization. Cryo-electron microscopic analysis of the same chromatographic fraction of NmqNOR, however, revealed a dimeric assembly at 3.06 Å resolution. It is shown that zinc (which is used in crystallization) binding near the dimer-stabilizing TMII region contributes to the disruption of the dimer. A similar destabilization is observed in the monomeric (∼85 kDa) cryo-EM structure of a mutant (Glu494Ala) qNOR from the opportunistic pathogen Alcaligenes (Achromobacter) xylosoxidans, which primarily migrates as a monomer. The monomer-dimer transition of qNORs seen in the cryo-EM and crystallographic structures has wider implications for structural studies of multimeric membrane proteins. X-ray crystallographic and cryo-EM structural analyses have been performed on the same chromatographic fraction of NmqNOR to high resolution. This represents one of the first examples in which the two approaches have been used to reveal a monomeric assembly in crystallo and a dimeric assembly in vitrified cryo-EM grids. A number of factors have been identified that may trigger the destabilization of helices that are necessary to preserve the integrity of the dimer. These include zinc binding near the entry of the putative proton-transfer channel and the preservation of the conformational integrity of the active site. The mutation near the active site results in disruption of the active site, causing an additional destabilization of helices (TMIX and TMX) that flank the proton-transfer channel helices, creating an inert monomeric enzyme.

Copper(I) Complex Mediated Nitric Oxide Reductive Coupling: Ligand Hydrogen Bonding Derived Proton Transfer Promotes N2O(g) Release

A cuprous chelate bearing a secondary sphere hydrogen bonding functionality, [(PV-tmpa)Cu^I]⁺, transforms ^•NO_(g) to N₂O_(g) in high-yields in methanol. Ligand derived proton transfer facilitates N-O bond cleavage of a putative hyponitrite intermediate releasing N₂O_(g), underscoring the crucial balance between H-bonding capabilities and acidities in (bio)chemical ^•NO_(g) coupling systems.

Monomeric structure of an active form of bovine cytochrome c oxidase

X-ray crystallographic analyses of mitochondrial cytochrome c oxidase (CcO) have been based on its dimeric form. Recent cryo-electron microscopy structures revealed that CcO exists in its monomeric form in the respiratory supercomplex. This study, using amphipol-stabilized CcO, shows that the activity of monomer is higher than that of the dimer. The crystal structure of monomer determined here shows that the local structure of one of the proton transfer pathways differs from that in the dimer. The crystal structure also shows that cardiolipins are located at the interface region in the supercomplex. Taken together, these results suggest that CcO in the monomeric state, dimeric state, and supercomplex state depending on cardiolipins are involved in regulation of respiratory electron transport.

Cytochrome c oxidase (CcO), a membrane enzyme in the respiratory chain, catalyzes oxygen reduction by coupling electron and proton transfer through the enzyme with a proton pump across the membrane. In all crystals reported to date, bovine CcO exists as a dimer with the same intermonomer contacts, whereas CcOs and related enzymes from prokaryotes exist as monomers. Recent structural analyses of the mitochondrial respiratory supercomplex revealed that CcO monomer associates with complex I and complex III, indicating that the monomeric state is functionally important. In this study, we prepared monomeric and dimeric bovine CcO, stabilized using amphipol, and showed that the monomer had high activity. In addition, using a newly synthesized detergent, we determined the oxidized and reduced structures of monomer with resolutions of 1.85 and 1.95 Å, respectively. Structural comparison of the monomer and dimer revealed that a hydrogen bond network of water molecules is formed at the entry surface of the proton transfer pathway, termed the K-pathway, in monomeric CcO, whereas this network is altered in dimeric CcO. Based on these results, we propose that the monomer is the activated form, whereas the dimer can be regarded as a physiological standby form in the mitochondrial membrane. We also determined phospholipid structures based on electron density together with the anomalous scattering effect of phosphorus atoms. Two cardiolipins are found at the interface region of the supercomplex. We discuss formation of the monomeric CcO, dimeric CcO, and supercomplex, as well as their role in regulation of CcO activity.

Dimeric structures of quinol-dependent nitric oxide reductases (qNORs) revealed by cryo-electron microscopy

Quinol-dependent nitric oxide reductases (qNORs) are membrane-integrated, iron-containing enzymes of the denitrification pathway, which catalyze the reduction of nitric oxide (NO) to the major ozone destroying gas nitrous oxide (N₂O). Cryo-electron microscopy structures of active qNOR from Alcaligenes xylosoxidans and an activity-enhancing mutant have been determined to be at local resolutions of 3.7 and 3.2 Å, respectively. They unexpectedly reveal a dimeric conformation (also confirmed for qNOR from Neisseria meningitidis) and define the active-site configuration, with a clear water channel from the cytoplasm. Structure-based mutagenesis has identified key residues involved in proton transport and substrate delivery to the active site of qNORs. The proton supply direction differs from cytochrome c-dependent NOR (cNOR), where water molecules from the cytoplasm serve as a proton source similar to those from cytochrome c oxidase.

The Response of nor and nos Contributes to Staphylococcus aureus Virulence and Metabolism

Staphylococcus aureus causes a wide spectrum of disease, with the site and severity of infection dependent on virulence traits encoded within genetically distinct clonal complexes (CCs) and bacterial responses to host innate immunity. The production of nitric oxide (NO) by activated phagocytes is a major host response to which S. aureus metabolically adapts through multiple strategies that are conserved in all CCs, including an S. aureus nitric oxide synthase (Nos). Previous genome analysis of CC30, a lineage associated with chronic endocardial and osteoarticular infections, revealed a putative NO reductase (Nor) not found in other CCs that potentially contributes to NO resistance and clinical outcome. Here, we demonstrate that Nor has true nitric oxide reductase activity, with nor expression enhanced by NO stress and anaerobic growth. Furthermore, we demonstrate that nor is regulated by MgrA and SrrAB, which modulate S. aureus virulence and hypoxic response. Transcriptome analysis of the S. aureus UAMS-1, UAMS-1 Δnor, and UAMS-1 Δnos strains under NO stress and anaerobic growth demonstrates that Nor contributes to nucleotide metabolism and Nos to glycolysis. We demonstrate that Nor and Nos contribute to enhanced survival in the presence of human human polymorphonuclear cells and have organ-specific seeding in a tail vein infection model. Nor contributes to abscess formation in an osteological implant model. We also demonstrate that Nor has a role in S. aureus metabolism and virulence. The regulation overlap between Nor and Nos points to an intriguing link between regulation of intracellular NO, metabolic adaptation, and persistence in the CC30 lineage.IMPORTANCE Staphylococcus aureus can cause disease at most body sites, and illness spans asymptomatic infection to death. The variety of clinical presentations is due to the diversity of strains, which are grouped into distinct clonal complexes (CCs) based on genetic differences. The ability of S. aureus CC30 to cause chronic infections relies on its ability to evade the oxidative/nitrosative defenses of the immune system and survive under different environmental conditions, including differences in oxygen and nitric oxide concentrations. The significance of this work is the exploration of unique genes involved in resisting NO stress and anoxia. A better understanding of the functions that control the response of S. aureus CC30 to NO and oxygen will guide the treatment of severe disease presentations.

Nitric Oxide Reductase Activity in Heme-Nonheme Binuclear Engineered Myoglobins through a One-Electron Reduction Cycle

Fe_BMbs are structural and functional models of native bacterial nitric oxide reductases (NORs) generated through engineering of myoglobin. These biosynthetic models replicate the heme-nonheme diiron site of NORs and allow substitutions of metal centers and heme cofactors. Here, we provide evidence for multiple NOR turnover in monoformyl-heme-containing Fe_BMb1 proteins loaded with Fe^II, Co^II, or Zn^II metal ions at the Fe_B site (Fe^II/Co^II/Zn^II-Fe_BMb1(MF-heme)). FTIR detection of the ν(NNO) band of N₂O at 2231 cm^-1 provides a direct quantitative measurement of the product in solution. A maximum number of turnover is observed with Fe^II-Fe_BMb1(MF-heme), but the NOR activity is retained when the Fe_B site is loaded with Zn^II. These data support the viability of a one-electron semireduced pathway for the reduction of NO at binuclear centers in reducing conditions.

A Novel Eukaryotic Denitrification Pathway in Foraminifera

Benthic foraminifera are unicellular eukaryotes inhabiting sediments of aquatic environments. Several species were shown to store and use nitrate for complete denitrification, a unique energy metabolism among eukaryotes. The population of benthic foraminifera reaches high densities in oxygen-depleted marine habitats, where they play a key role in the marine nitrogen cycle. However, the mechanisms of denitrification in foraminifera are still unknown, and the possibility of a contribution of associated bacteria is debated. Here, we present evidence for a novel eukaryotic denitrification pathway that is encoded in foraminiferal genomes. Large-scale genome and transcriptomes analyses reveal the presence of a denitrification pathway in foraminifera species of the genus Globobulimina. This includes the enzymes nitrite reductase (NirK) and nitric oxide reductase (Nor) as well as a wide range of nitrate transporters (Nrt). A phylogenetic reconstruction of the enzymes' evolutionary history uncovers evidence for an ancient acquisition of the foraminiferal denitrification pathway from prokaryotes. We propose a model for denitrification in foraminifera, where a common electron transport chain is used for anaerobic and aerobic respiration. The evolution of hybrid respiration in foraminifera likely contributed to their ecological success, which is well documented in palaeontological records since the Cambrian period.

Bacterial denitrifying nitric oxide reductases and aerobic respiratory terminal oxidases use similar delivery pathways for their molecular substrates

The superfamily of heme‑copper oxidoreductases (HCOs) include both NO and O₂ reductases. Nitric oxide reductases (NORs) are bacterial membrane enzymes that catalyze an intermediate step of denitrification by reducing nitric oxide (NO) to nitrous oxide (N₂O). They are structurally similar to heme‑copper oxygen reductases (HCOs), which reduce O₂ to water. The experimentally observed apparent bimolecular rate constant of NO delivery to the deeply buried catalytic site of NORs was previously reported to approach the diffusion-controlled limit (10⁸-10⁹ M^-1 s^-1). Using the crystal structure of cytochrome-c dependent NOR (cNOR) from Pseudomonas aeruginosa, we employed several protocols of molecular dynamics (MD) simulation, which include flooding simulations of NO molecules, implicit ligand sampling and umbrella sampling simulations, to elucidate how NO in solution accesses the catalytic site of this cNOR. The results show that NO partitions into the membrane, enters the enzyme from the lipid bilayer and diffuses to the catalytic site via a hydrophobic tunnel that is resolved in the crystal structures. This is similar to what has been found for O₂ diffusion through the closely related O₂ reductases. The apparent second order rate constant approximated using the simulation data is ~5 × 10⁸ M^-1 s^-1, which is optimized by the dynamics of the amino acid side chains lining in the tunnel. It is concluded that both NO and O₂ reductases utilize well defined hydrophobic tunnels to assure that substrate diffusion to the buried catalytic sites is not rate limiting under physiological conditions.

The insertion of the non-heme FeB cofactor into nitric oxide reductase from P. denitrificans depends on NorQ and NorD accessory proteins

Bacterial NO reductases (NOR) catalyze the reduction of NO into N₂O, either as a step in denitrification or as a detoxification mechanism. cNOR from Paracoccus (P.) denitrificans is expressed from the norCBQDEF operon, but only the NorB and NorC proteins are found in the purified NOR complex. Here, we established a new purification method for the P. denitrificans cNOR via a His-tag using heterologous expression in E. coli. The His-tagged enzyme is both structurally and functionally very similar to non-tagged cNOR. We were also able to express and purify cNOR from the structural genes norCB only, in absence of the accessory genes norQDEF. The produced protein is a stable NorCB complex containing all hemes and it can bind gaseous ligands (CO) to heme b₃, but it is catalytically inactive. We show that this deficient cNOR lacks the non-heme iron cofactor Fe_B. Mutational analysis of the nor gene cluster revealed that it is the norQ and norD genes that are essential to form functional cNOR. NorQ belongs to the family of MoxR P-loop AAA+ ATPases, which are in general considered to facilitate enzyme activation processes often involving metal insertion. Our data indicates that NorQ and NorD work together in order to facilitate non-heme Fe insertion. This is noteworthy since in many cases Fe cofactor binding occurs spontaneously. We further suggest a model for NorQ/D-facilitated metal insertion into cNOR.

Pseudomonas aeruginosa overexpression system of nitric oxide reductase for in vivo and in vitro mutational analyses

Membrane-integrated nitric oxide reductase (NOR) reduces nitric oxide (NO) to nitrous oxide (N₂O) with protons and electrons. This process is essential for the elimination of the cytotoxic NO that is produced from nitrite (NO₂^-) during microbial denitrification. A structure-guided mutagenesis of NOR is required to elucidate the mechanism for NOR-catalyzed NO reduction. We have already solved the crystal structure of cytochrome c-dependent NOR (cNOR) from Pseudomonas aeruginosa. In this study, we then constructed its expression system using cNOR-gene deficient and wild-type strains for further functional study. Characterizing the variants of the five conserved Glu residues located around the heme/non-heme iron active center allowed us to establish how the anaerobic growth rate of cNOR-deficient strains expressing cNOR variants correlates with the in vitro enzymatic activity of the variants. Since bacterial strains require active cNOR to eliminate cytotoxic NO and to survive under denitrification conditions, the anaerobic growth rate of a strain with a cNOR variant is a good indicator of NO decomposition capability of the variants and a marker for the screening of functionally important residues without protein purification. Using this in vivo screening system, we examined the residues lining the putative proton transfer pathways for NO reduction in cNOR, and found that the catalytic protons are likely transferred through the Glu57 located at the periplasmic protein surface. The homologous cNOR expression system developed here is an invaluable tool for facile identification of crucial residues in vivo, and for further in vitro functional and structural studies.

Characterization of the quinol-dependent nitric oxide reductase from the pathogen Neisseria meningitidis, an electrogenic enzyme

Bacterial nitric oxide reductases (NORs) catalyse the reduction of NO to N₂O and H₂O. NORs are found either in denitrification chains, or in pathogens where their primary role is detoxification of NO produced by the immune defense of the host. Although NORs belong to the heme-copper oxidase superfamily, comprising proton-pumping O₂-reducing enzymes, the best studied NORs, cNORs (cytochrome c-dependent), are non-electrogenic. Here, we focus on another type of NOR, qNOR (quinol-dependent). Recombinant qNOR from Neisseria meningitidis, a human pathogen, purified from Escherichia coli, showed high catalytic activity and spectroscopic properties largely similar to cNORs. However, in contrast to cNOR, liposome-reconstituted qNOR showed respiratory control ratios above two, indicating that NO reduction by qNOR was electrogenic. Further, we determined a 4.5 Å crystal structure of the N. meningitidis qNOR, allowing exploration of a potential proton transfer pathway from the cytoplasm by mutagenesis. Most mutations had little effect on the activity, however the E-498 variants were largely inactive, while the corresponding substitution in cNOR was previously shown not to induce significant effects. We thus suggest that, contrary to cNOR, the N. meningitidis qNOR uses cytoplasmic protons for NO reduction. Our results allow possible routes for protons to be discussed.

De Novo Design of Tetranuclear Transition Metal Clusters Stabilized by Hydrogen-Bonded Networks in Helical Bundles

De novo design provides an attractive approach to test the mechanism by which metalloproteins define the geometry and reactivity of their metal ion cofactors. While there has been considerable progress in designing proteins that bind transition metal ions including iron-sulfur clusters, the design of tetranuclear clusters with oxygen-rich environments has not been accomplished. Here, we describe the design of tetranuclear clusters, consisting of four Zn2+ and four carboxylate oxygens situated at the vertices of a distorted cube-like structure. The tetra-Zn2+ clusters are bound at a buried site within a four-helix bundle, with each helix donating a single carboxylate (Glu or Asp) and imidazole (His) ligand, as well as second- and third-shell ligands. Overall, the designed site consists of four Zn2+ and 16 polar side chains in a fully connected hydrogen-bonded network. The designed proteins have apolar cores at the top and bottom of the bundle, which drive the assembly of the liganding residues near the center of the bundle. The steric bulk of the apolar residues surrounding the binding site was varied to determine how subtle changes in helix-helix packing affect the binding site. The crystal structures of two of four proteins synthesized were in good agreement with the overall design; both formed a distorted cuboidal site stabilized by flanking second- and third-shell interactions that stabilize the primary ligands. A third structure bound a single Zn2+ in an unanticipated geometry, and the fourth bound multiple Zn2+ at multiple sites at partial occupancy. The metal-binding and conformational properties of the helical bundles in solution, probed by circular dichroism spectroscopy, analytical ultracentrifugation, and NMR, were consistent with the crystal structures.

Oxygen Activation and Energy Conservation by Cytochrome c Oxidase

This review focuses on the type A cytochrome c oxidases (CcO), which are found in all mitochondria and also in several aerobic bacteria. CcO catalyzes the respiratory reduction of dioxygen (O₂) to water by an intriguing mechanism, the details of which are fairly well understood today as a result of research for over four decades. Perhaps even more intriguingly, the membrane-bound CcO couples the O₂ reduction chemistry to translocation of protons across the membrane, thus contributing to generation of the electrochemical proton gradient that is used to drive the synthesis of ATP as catalyzed by the rotary ATP synthase in the same membrane. After reviewing the structure of the core subunits of CcO, the active site, and the transfer paths of electrons, protons, oxygen, and water, we describe the states of the catalytic cycle and point out the few remaining uncertainties. Finally, we discuss the mechanism of proton translocation and the controversies in that area that still prevail.

Analysis of multiple haloarchaeal genomes suggests that the quinone-dependent respiratory nitric oxide reductase is an important source of nitrous oxide in hypersaline environments

Microorganisms, including Bacteria and Archaea, play a key role in denitrification, which is the major mechanism by which fixed nitrogen returns to the atmosphere from soil and water. While the enzymology of denitrification is well understood in Bacteria, the details of the last two reactions in this pathway, which catalyse the reduction of nitric oxide (NO) via nitrous oxide (N₂ O) to nitrogen (N₂ ), are little studied in Archaea, and hardly at all in haloarchaea. This work describes an extensive interspecies analysis of both complete and draft haloarchaeal genomes aimed at identifying the genes that encode respiratory nitric oxide reductases (Nors). The study revealed that the only nor gene found in haloarchaea is one that encodes a single subunit quinone dependent Nor homologous to the qNor found in bacteria. This surprising discovery is considered in terms of our emerging understanding of haloarchaeal bioenergetics and NO management.

An Unsaturated Quinolone N-Oxide of Pseudomonas aeruginosa Modulates Growth and Virulence of Staphylococcus aureus

The pathogen Pseudomonas aeruginosa produces over 50 different quinolones, 16 of which belong to the class of 2-alkyl-4-quinolone N-oxides (AQNOs) with various chain lengths and degrees of saturation. We present the first synthesis of a previously proposed unsaturated compound that is confirmed to be present in culture extracts of P. aeruginosa, and its structure is shown to be trans-Δ¹ -2-(non-1-enyl)-4-quinolone N-oxide. This compound is the most active agent against S.  aureus, including MRSA strains, by more than one order of magnitude whereas its cis isomer is inactive. At lower concentrations, the compound induces small-colony variants of S. aureus, reduces the virulence by inhibiting hemolysis, and inhibits nitrate reductase activity under anaerobic conditions. These studies suggest that this unsaturated AQNO is one of the major agents that are used by P. aeruginosa to modulate competing bacterial species.

Genome-Resolved Meta-Omics Ties Microbial Dynamics to Process Performance in Biotechnology for Thiocyanate Degradation

Remediation of industrial wastewater is important for preventing environmental contamination and enabling water reuse. Biological treatment for one industrial contaminant, thiocyanate (SCN^-), relies upon microbial hydrolysis, but this process is sensitive to high loadings. To examine the activity and stability of a microbial community over increasing SCN^- loadings, we established and operated a continuous-flow bioreactor fed increasing loadings of SCN^-. A second reactor was fed ammonium sulfate to mimic breakdown products of SCN^-. Biomass was sampled from both reactors for metagenomics and metaproteomics, yielding a set of genomes for 144 bacteria and one rotifer that constituted the abundant community in both reactors. We analyzed the metabolic potential and temporal dynamics of these organisms across the increasing loadings. In the SCN^- reactor, Thiobacillus strains capable of SCN^- degradation were highly abundant, whereas the ammonium sulfate reactor contained nitrifiers and heterotrophs capable of nitrate reduction. Key organisms in the SCN^- reactor expressed proteins involved in SCN^- degradation, sulfur oxidation, carbon fixation, and nitrogen removal. Lower performance at higher loadings was linked to changes in microbial community composition. This work provides an example of how meta-omics can increase our understanding of industrial wastewater treatment and inform iterative process design and development.

Insights into proton translocation in cbb3 oxidase from MD simulations

Heme-copper oxidases are membrane protein complexes that catalyse the final step of the aerobic respiration, namely the reduction of oxygen to water. The energy released during catalysis is coupled to the active translocation of protons across the membrane, which contributes to the establishment of an electrochemical gradient that is used for ATP synthesis. The distinctive C-type (or cbb₃) cytochrome c oxidases, which are mostly present in proteobacteria, exhibit a number of unique structural and functional features, including high catalytic activity at low oxygen concentrations. At the moment, the functioning mechanism of C-type oxidases, in particular the proton transfer/pumping mechanism presumably via a single proton channel, is still poorly understood. In this work we used all-atom molecular dynamics simulations and continuum electrostatics calculations to obtain atomic-level insights into the hydration and dynamics of a cbb₃ oxidase. We provide the details of the water dynamics and proton transfer pathways for both the "chemical" and "pumped" protons, and show that formation of protonic connections is strongly affected by the protonation state of key residues, namely H243, E323 and H337.

Unexpected Diversity and High Abundance of Putative Nitric Oxide Dismutase (Nod) Genes in Contaminated Aquifers and Wastewater Treatment Systems

It has recently been suggested that oxygenic dismutation of NO into N2 and O2 may occur in the anaerobic methanotrophic "Candidatus Methylomirabilis oxyfera" and the alkane-oxidizing gammaproteobacterium HdN1. It may represent a new pathway in microbial nitrogen cycling catalyzed by a putative NO dismutase (Nod). The formed O2 enables microbes to employ aerobic catabolic pathways in anoxic habitats, suggesting an ecophysiological niche space of substantial appeal for bioremediation and water treatment. However, it is still unknown whether this physiology is limited to "Ca Methylomirabilis oxyfera" and HdN1 and whether it can be coupled to the oxidation of electron donors other than alkanes. Here, we report insights into an unexpected diversity and remarkable abundance of nod genes in natural and engineered water systems. Phylogenetically diverse nod genes were recovered from a range of contaminated aquifers and N-removing wastewater treatment systems. Together with nod genes from "Ca Methylomirabilis oxyfera" and HdN1, the novel environmental nod sequences formed no fewer than 6 well-supported phylogenetic clusters, clearly distinct from canonical NO reductase (quinol-dependent NO reductase [qNor] and cytochrome c-dependent NO reductase [cNor]) genes. The abundance of nod genes in the investigated samples ranged from 1.6 × 107 to 5.2 × 1010 copies · g-1 (wet weight) of sediment or sludge biomass, accounting for up to 10% of total bacterial 16S rRNA gene counts. In essence, NO dismutation could be a much more widespread physiology than currently perceived. Understanding the controls of this emergent microbial capacity could offer new routes for nitrogen elimination or pollutant remediation in natural and engineered water systems.

IMPORTANCE

NO dismutation into N2 and O2 is a novel process catalyzed by putative NO dismutase (Nod). To date, only two bacteria, the anaerobic methane-oxidizing bacterium "Ca Methylomirabilis oxyfera" and the alkane-oxidizing gammaproteobacterium HdN1, are known to harbor nod genes. In this study, we report efficient molecular tools that can detect and quantify a wide diversity of nod genes in environmental samples. A surprisingly high diversity and abundance of nod genes were found in contaminated aquifers as well as wastewater treatment systems. This evidence indicates that NO dismutation may be a much more widespread physiology in natural and man-made environments than currently perceived. The molecular tools presented here will facilitate further studies on these enigmatic microbes in the future.

Heterologous expression of Halomonas halodenitrificans nitric oxide reductase and its N-terminally truncated NorC subunit in Escherichia coli

Halomonas halodenitrificans nitric oxide reductase (NOR) is the membrane-bound heterodimer complex of NorC, which contains a low-spin heme c center, and NorB, which contains a low-spin heme b center, a high-spin heme b₃ center, and a non-heme Fe_B center. The soluble domain of NorC, NorC* (ΔMet1-Val37) was heterologously expressed in Escherichia coli using expression plasmids harboring the truncated norC gene deleted of its 84 5'-terminal nucleotides. Analogous scission of the N-terminal helix as the membrane anchor took place when the whole norC gene was used. NorC* exhibited spectra typical of a low-spin heme c. In addition, NorC* functioned as the acceptor of an electron from a cytochrome c isolated from the periplasm of H. halodenitrificans and small reducing reagents. The redox potential of NorC* shifted ca. 40mV in the negative direction from that of NorC. Unlike NorC, recombinant NorB was not heterologously expressed. However, recombinant NOR (rNOR) could be expressed in E. coli by using a plasmid harboring all genes in the nor operon, norCBQDX, from which the three hairpin loops (mRNA) were deleted, and by using the ccm genes for the maturation of C-type heme. rNOR exhibited the same spectroscopic properties and reactivity to NO and O₂ as NOR, although its enzymatic activity toward NO was considerably decreased. These results on the expression of rNOR and NorC* will allow us to develop more profound studies on the properties of the four Fe centers and the reaction mechanism of NOR from this halophilic denitrifying bacterium.

Nitric oxide coupling to generate N2O promoted by a single-heme system as examined by density functional theory

Bacteria utilize a heme/non-heme enzyme system to detoxify nitric oxide (NO) to N₂O. In order to probe the capacity of a single-heme system to mediate this NO-to-N₂O transformation, various scenarios for addition of electrons, protons, and a second NO molecule to a heme nitrosyl to generate N₂O were explored by density functional theory calculations. We describe, utilizing this single-heme system, several stepwise intermediates along pathways that enable the critical N-N bond formation step yielding the desired Fe-N₂O product. We also report a hitherto unreported directional second protonation that results in either productive N₂O formation with loss of water, or formation of a non-productive hyponitrous acid adduct Fe{HONNOH}.

Helix perturbations in membrane proteins assist in inter-helical interactions and optimal helix positioning in the bilayer

Transmembrane (TM) helices in integral membrane proteins are primarily α-helical in structure. Here we analyze 1134 TM helices in 90 high resolution membrane proteins and find that apart from the widely prevalent α-helices, TM regions also contain stretches of 3₁₀ (3 to 8 residues) and π-helices (5 to 19 residues) with distinct sequence signatures. The various helix perturbations in TM regions comprise of helices with kinked geometry, as well as those with an interspersed 3₁₀/π-helical fragment and show high occurrence in a few membrane proteins. Proline is frequently present at sites of these perturbations, but it is neither a necessary nor a sufficient requirement. Helix perturbations are also conserved within a family of membrane proteins despite low sequence identity in the perturbed region. Furthermore, a perturbation influences the geometry of the TM helix, mediates inter-helical interactions within and across protein chains and avoids hydrophobic mismatch of the helix termini with the bilayer. An analysis of π-helices in the TM regions of the heme copper oxidase superfamily shows that interspersed π-helices can vary in length from 6 to 19 amino acids or be entirely absent, depending upon the protein function. The results presented here would be helpful for prediction of 3₁₀ and π-helices in TM regions and can assist the computational design of membrane proteins.

Distribution and dynamics of quinones in the lipid bilayer mimicking the inner membrane of mitochondria

Quinone and its analogues (Q) constitute an important class of compounds that perform key electron transfer reactions in oxidative- and photo-phosphorylation. In the inner membrane of mitochondria, ubiquinone molecules undergo continuous redox transitions enabling electron transfer between the respiratory complexes. In such a dynamic system undergoing continuous turnover for ATP synthesis, an uninterrupted supply of substrate molecules is absolutely necessary. In the current work, we have performed atomistic molecular dynamics simulations and free energy calculations to assess the structure, dynamics, and localization of quinone and its analogues in a lipid bilayer, whose composition mimics the one in the inner mitochondrial membrane. The results show that there is a strong tendency of both quinone and quinol molecules to localize in the vicinity of the lipids' acyl groups, right under the lipid head group region. Additionally, we observe a second location in the middle of the bilayer where quinone molecules tend to stabilize. Translocation of quinone through a lipid bilayer is very fast and occurs in 10-100ns time scale, whereas the translocation of quinol is at least an order of magnitude slower. We suggest that this has important mechanistic implications given that the localization of Q ensures maximal occupancy of the Q-binding sites or Q-entry points in electron transport chain complexes, thereby maintaining an optimal turnover rate for ATP synthesis.

Exploring membrane respiratory chains

Acquisition of energy is central to life. In addition to the synthesis of ATP, organisms need energy for the establishment and maintenance of a transmembrane difference in electrochemical potential, in order to import and export metabolites or to their motility. The membrane potential is established by a variety of membrane bound respiratory complexes. In this work we explored the diversity of membrane respiratory chains and the presence of the different enzyme complexes in the several phyla of life. We performed taxonomic profiles of the several membrane bound respiratory proteins and complexes evaluating the presence of their respective coding genes in all species deposited in KEGG database. We evaluated 26 quinone reductases, 5 quinol:electron carriers oxidoreductases and 18 terminal electron acceptor reductases. We further included in the analyses enzymes performing redox or decarboxylation driven ion translocation, ATP synthase and transhydrogenase and we also investigated the electron carriers that perform functional connection between the membrane complexes, quinones or soluble proteins. Our results bring a novel, broad and integrated perspective of membrane bound respiratory complexes and thus of the several energetic metabolisms of living systems. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.