Electron Transfer Complex between Nitrous Oxide Reductase and Cytochrome c552 from Pseudomonas nautica: Kinetic, Nuclear Magnetic Resonance, and Docking Studies

Contribution of nitrous oxide to the carbon footprint of full-scale wastewater treatment plants and mitigation strategies- a critical review

Nitrous oxide (N₂O), a potent greenhouse gas, significantly contributes to the carbon footprint of wastewater treatment plants (WWTPs) and contributes significantly to global climate change and to the deterioration of the natural environment. Our understanding of N₂O generation mechanisms has significantly improved in the last decade, but the development of effective N₂O emission mitigation strategies has lagged owing to the complexity of parameter regulation, substandard monitoring activities, and inadequate policy criteria. Based on critically screened published studies on N₂O control in full-scale WWTPs, this review elucidates N₂O generation pathway identifications and emission mechanisms and summarizes the impact of N₂O on the total carbon footprint of WWTPs. In particular, a linear relationship was established between N₂O emission factors and total nitrogen removal efficiencies in WWTPs located in China. Promising N₂O mitigation options were proposed, which focus on optimizing operating conditions and implementation of innovative treatment processes. Furthermore, the sustainable operation of WWTPs has been anticipated to convert WWTPs into absolute greenhouse gas reducers as a result of the refinement and improvement of on-site monitoring activities, mitigation mechanisms, regulation of operational parameters, modeling, and policies.

Molecular interplay of an assembly machinery for nitrous oxide reductase

Emissions of the critical ozone-depleting and greenhouse gas nitrous oxide (N₂O) from soils and industrial processes have increased considerably over the last decades^1-3. As the final step of bacterial denitrification, N₂O is reduced to chemically inert N₂ (refs. ^1,4) in a reaction that is catalysed by the copper-dependent nitrous oxide reductase (N₂OR) (ref. ⁵). The assembly of its unique [4Cu:2S] active site cluster Cu_Z requires both the ATP-binding-cassette (ABC) complex NosDFY and the membrane-anchored copper chaperone NosL (refs. ^4,6). Here we report cryo-electron microscopy structures of Pseudomonas stutzeri NosDFY and its complexes with NosL and N₂OR, respectively. We find that the periplasmic NosD protein contains a binding site for a Cu⁺ ion and interacts specifically with NosL in its nucleotide-free state, whereas its binding to N₂OR requires a conformational change that is triggered by ATP binding. Mutually exclusive structures of NosDFY in complex with NosL and with N₂OR reveal a sequential metal-trafficking and assembly pathway for a highly complex copper site. Within this pathway, NosDFY acts as a mechanical energy transducer rather than as a transporter. It links ATP hydrolysis in the cytoplasm to a conformational transition of the NosD subunit in the periplasm, which is required for NosDFY to switch its interaction partner so that copper ions are handed over from the chaperone NosL to the enzyme N₂OR.

Coordination chemistry of the CuZ site in nitrous oxide reductase and its synthetic mimics

Atmospheric nitrous oxide (N2O) has garnered significant attention recently due to its dual roles as an ozone depletion agent and a potent greenhouse gas. Anthropogenic N2O emissions occur primarily through agricultural disruption of nitrogen homeostasis causing N2O to build up in the atmosphere. The enzyme responsible for N2O fixation within the geochemical nitrogen cycle is nitrous oxide reductase (N2OR), which catalyzes 2H+/2e- reduction of N2O to N2 and H2O at a tetranuclear active site, CuZ. In this review, the coordination chemistry of CuZ is reviewed. Recent advances in the understanding of biological CuZ coordination chemistry is discussed, as are significant breakthroughs in synthetic modeling of CuZ that have emerged in recent years. The latter topic includes both structurally faithful, synthetic [Cu4(µ4-S)] clusters that are able to reduce N2O, as well as dicopper motifs that shed light on reaction pathways available to the critical CuI-CuIV cluster edge of CuZ. Collectively, these advances in metalloenzyme studies and synthetic model systems provide meaningful knowledge about the physiologically relevant coordination chemistry of CuZ but also open new questions that will pose challenges in the near future.

The effect of pH on Marinobacter hydrocarbonoclasticus denitrification pathway and nitrous oxide reductase

Increasing atmospheric concentration of N₂O has been a concern, as it is a potent greenhouse gas and promotes ozone layer destruction. In the N-cycle, release of N₂O is boosted upon a drop of pH in the environment. Here, Marinobacter hydrocarbonoclasticus was grown in batch mode in the presence of nitrate, to study the effect of pH in the denitrification pathway by gene expression profiling, quantification of nitrate and nitrite, and evaluating the ability of whole cells to reduce NO and N₂O. At pH 6.5, accumulation of nitrite in the medium occurs and the cells were unable to reduce N₂O. In addition, the biochemical properties of N₂O reductase isolated from cells grown at pH 6.5, 7.5 and 8.5 were compared for the first time. The amount of this enzyme at acidic pH was lower than that at pH 7.5 and 8.5, pinpointing to a post-transcriptional regulation, though pH did not affect gene expression of N₂O reductase accessory genes. N₂O reductase isolated from cells grown at pH 6.5 has its catalytic center mainly as CuZ(4Cu1S), while that from cells grown at pH 7.5 or 8.5 has it as CuZ(4Cu2S). This study evidences that an in vivo secondary level of regulation is required to maintain N₂O reductase in an active state.

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.

Proton-coupled electron transfer mechanisms of the copper centres of nitrous oxide reductase from Marinobacter hydrocarbonoclasticus - An electrochemical study

Reduction of N₂O to N₂ is catalysed by nitrous oxide reductase in the last step of the denitrification pathway. This multicopper enzyme has an electron transferring centre, CuA, and a tetranuclear copper-sulfide catalytic centre, "CuZ", which exists as CuZ*(4Cu1S) or CuZ(4Cu2S). The redox behaviour of these metal centres in Marinobacter hydrocarbonoclasticus nitrous oxide reductase was investigated by potentiometry and for the first time by direct electrochemistry. The reduction potential of CuA and CuZ(4Cu2S) was estimated by potentiometry to be +275 ± 5 mV and +65 ± 5 mV vs SHE, respectively, at pH 7.6. A proton-coupled electron transfer mechanism governs CuZ(4Cu2S) reduction potential, due to the protonation/deprotonation of Lys397 with a pK_ox of 6.0 ± 0.1 and a pK_red of 9.2 ± 0.1. The reduction potential of CuA, in enzyme samples with CuZ*(4Cu1S), is controlled by protonation of the coordinating histidine residues in a two-proton coupled electron transfer process. In the cyclic voltammograms, two redox pairs were identified corresponding to CuA and CuZ(4Cu2S), with no additional signals being detected that could be attributed to CuZ*(4Cu1S). However, an enhanced cathodic signal for the activated enzyme was observed under turnover conditions, which is explained by the binding of nitrous oxide to CuZ⁰(4Cu1S), an intermediate species in the catalytic cycle.

Bacterial nitrous oxide respiration: electron transport chains and copper transfer reactions

Biologically catalyzed nitrous oxide (N₂O, laughing gas) reduction to dinitrogen gas (N₂) is a desirable process in the light of ever-increasing atmospheric concentrations of this important greenhouse gas and ozone depleting substance. A diverse range of bacterial species produce the copper cluster-containing enzyme N₂O reductase (NosZ), which is the only known enzyme that converts N₂O to N₂. Based on phylogenetic analyses, NosZ enzymes have been classified into clade I or clade II and it has turned out that this differentiation is also applicable to nos gene clusters (NGCs) and some physiological traits of the corresponding microbial cells. The NosZ enzyme is the terminal reductase of anaerobic N₂O respiration, in which electrons derived from a donor substrate are transferred to NosZ by means of an electron transport chain (ETC) that conserves energy through proton motive force generation. This chapter presents models of the ETCs involved in clade I and clade II N₂O respiration as well as of the respective NosZ maturation and maintenance processes. Despite differences in NGCs and growth yields of N₂O-respiring microorganisms, the deduced bioenergetic framework in clade I and clade II N₂O respiration is assumed to be equivalent. In both cases proton motive quinol oxidation by N₂O is thought to be catalyzed by the Q cycle mechanism of a membrane-bound Rieske/cytochrome bc complex. However, clade I and clade II organisms are expected to differ significantly in terms of auxiliary electron transport processes as well as NosZ active site maintenance and repair.

Genomic organization, gene expression and activity profile of Marinobacter hydrocarbonoclasticus denitrification enzymes

Background

Denitrification is one of the main pathways of the N-cycle, during which nitrate is converted to dinitrogen gas, in four consecutive reactions that are each catalyzed by a different metalloenzyme. One of the intermediate metabolites is nitrous oxide, which has a global warming impact greater then carbon dioxide and which atmospheric concentration has been increasing in the last years. The four denitrification enzymes have been isolated and biochemically characterized from Marinobacter hydrocarbonoclasticus in our lab.

Methods

Bioinformatic analysis of the M. hydrocarbonoclasticus genome to identify the genes involved in the denitrification pathway. The relative gene expression of the gene encoding the catalytic subunits of those enzymes was analyzed during the growth under microoxic conditions. The consumption of nitrate and nitrite, and the reduction of nitric oxide and nitrous oxide by whole-cells was monitored during anoxic and microoxic growth in the presence of 10 mM sodium nitrate at pH 7.5.

Results

The bioinformatic analysis shows that genes encoding the enzymes and accessory factors required for each step of the denitrification pathway are clustered together. An unusual feature is the co-existence of genes encoding a q- and a c-type nitric oxide reductase, with only the latter being transcribed at similar levels as the ones encoding the catalytic subunits of the other denitrifying enzymes, when cells are grown in the presence of nitrate under microoxic conditions. Using either a batch- or a closed system, nitrate is completely consumed in the beginning of the growth, with transient formation of nitrite, and whole-cells can reduce nitric oxide and nitrous oxide from mid-exponential phase until being collected (time-point 50 h).

Discussion

M. hydrocarbonoclasticus cells can reduce nitric and nitrous oxide in vivo, indicating that the four denitrification steps are active. Gene expression profile together with promoter regions analysis indicates the involvement of a cascade regulatory mechanism triggered by FNR-type in response to low oxygen tension, with nitric oxide and nitrate as secondary effectors, through DNR and NarXL, respectively. This global characterization of the denitrification pathway of a strict marine bacterium, contributes to the understanding of the N-cycle and nitrous oxide release in marine environments.

Gene cloning, expression, and reducing property enhancement of nitrous oxide reductase from Alcaligenes denitrificans strain TB

Nitrous oxide (N₂O) is a potent greenhouse gas and tends to accumulate as an intermediate in the process of bacteria denitrification. To achieve complete reduction of nitrogen oxide (NO_x) in bacteria denitrification, the structural gene nosZ encoding nitrous oxide reductase (N₂OR) was cloned from Alcaligenes denitrificans strain TB (GenBank JQ044686). The recombinant plasmid containing the nosZ gene was built, and the expression of nosZ gene in Escherichia coli was determined. Results show that the nosZ gene consisting of 1917 nucleotides achieves heterologous expression successfully by codon optimization strategy under optimal conditions (pre-induction inoculum OD₆₀₀ of 0.67, final IPTG concentration of 0.5 mM, inducing time of 6 h, and inducing temperature of 28 °C). Determination result of gas chromatography confirms that N₂O degradation efficiency of recombinant E. coli is strengthened by at least 1.92 times compared with that of original strain TB when treated with N₂O as substrate. Moreover, N₂OR activity in recombinant strain is 2.09 times higher than that in wild strain TB, which validates the aforementioned result and implies that the recombinant E. coli BL21 (DE3)-pET28b-nosZ is a potential candidate to control N₂O accumulation and alleviate greenhouse effect. In addition, the N₂OR structure and the possible N₂O binding site in Alcaligenes sp. TB are predicted, which open an avenue for further research on the relationship between N₂OR activity and its structure.

Interaction between Neisseria gonorrhoeae bacterial peroxidase and its electron donor, the lipid-modified azurin

The Neisseria gonorrhoeae bacterial cytochrome c peroxidase plays a key role in detoxifying the cells from H₂ O₂ by reducing it to water using the lipid-modified azurin, LAz, a small type 1 copper protein, as electron donor. Here, the interaction between these two proteins was characterized by steady-state kinetics, two-dimensional NMR and molecular docking simulations. LAz is an efficient electron donor capable of activating this enzyme. This electron transfer complex is weak with a hydrophobic character, with LAz binding close to the electron transferring heme of the enzyme. The high catalytic rate (39 ± 0.03 s^-1 ) is explained by the LAz pre-orientation, due to a positive dipole moment, and by the fast-dynamic ensemble of orientations, suggested by the small chemical shifts.

The catalytic cycle of nitrous oxide reductase - The enzyme that catalyzes the last step of denitrification

The reduction of the potent greenhouse gas nitrous oxide requires a catalyst to overcome the large activation energy barrier of this reaction. Its biological decomposition to the inert dinitrogen can be accomplished by denitrifiers through nitrous oxide reductase, the enzyme that catalyzes the last step of the denitrification, a pathway of the biogeochemical nitrogen cycle. Nitrous oxide reductase is a multicopper enzyme containing a mixed valence CuA center that can accept electrons from small electron shuttle proteins, triggering electron flow to the catalytic sulfide-bridged tetranuclear copper "CuZ center". This enzyme has been isolated with its catalytic center in two forms, CuZ*(4Cu1S) and CuZ(4Cu2S), proven to be spectroscopic and structurally different. In the last decades, it has been a challenge to characterize the properties of this complex enzyme, due to the different oxidation states observed for each of its centers and the heterogeneity of its preparations. The substrate binding site in those two "CuZ center" forms and which is the active form of the enzyme is still a matter of debate. However, in the last years the application of different spectroscopies, together with theoretical calculations have been useful in answering these questions and in identifying intermediate species of the catalytic cycle. An overview of the spectroscopic, kinetics and structural properties of the two forms of the catalytic "CuZ center" is given here, together with the current knowledge on nitrous oxide reduction mechanism by nitrous oxide reductase and its intermediate species.

Periplasmic Nicotine Dehydrogenase NdhAB Utilizes Pseudoazurin as Its Physiological Electron Acceptor in Agrobacterium tumefaciens S33

Agrobacterium tumefaciens S33 can grow with nicotine as the sole source of carbon, nitrogen, and energy via a novel hybrid of the pyridine pathway and the pyrrolidine pathway. Characterization of the enzymes involved in the hybrid pathway is important for understanding its biochemical mechanism. Here, we report that the molybdenum-containing nicotine dehydrogenase (NdhAB), which catalyzes the initial step of nicotine degradation, is located in the periplasm of strain S33, while the 6-hydroxynicotine oxidase and 6-hydroxypseudooxynicoine oxidase are in the cytoplasm. This is consistent with the fact that NdhA has a Tat signal peptide. Interestingly, an open reading frame (ORF) adjacent to the ndhAB gene was verified to encode a copper-containing electron carrier, pseudoazurin (Paz), which has a signal peptide typical of bacterial Paz proteins. Both were transported into the periplasm after being produced in the cytoplasm. We purified NdhAB from the periplasmic fraction of strain S33 and found that with Paz as the physiological electron acceptor, NdhAB catalyzed the hydroxylation of nicotine at a specific rate of 110.52 ± 8.09 μmol · min^-1 · mg of protein^-1, where the oxygen atom in the hydroxyl group of the product 6-hydroxynicotine was derived from H₂O. The apparent K_m values for nicotine and Paz were 1.64 ± 0.07 μM and 3.61 ± 0.23 μM, respectively. NAD(P)⁺, O₂, and ferredoxin could not serve as electron acceptors. Disruption of the paz gene disabled the strain for nicotine degradation, indicating that Paz is required for nicotine catabolism in the strain. These findings help our understanding of electron transfer during nicotine degradation in bacteria.IMPORTANCE Nicotine is a toxic and addictive N-heterocyclic aromatic alkaloid produced in tobacco. Its catabolism in organisms and degradation in tobacco wastes have become major concerns for human health and the environment. Bacteria usually decompose nicotine using the classical strategy of hydroxylating the pyridine ring with the help of activated oxygen by nicotine dehydrogenase, which binds one molybdopterin, two [2Fe2S] clusters, and usually one flavin adenine dinucleotide (FAD) as well. However, the physiological electron acceptor for the reaction is still unknown. In this study, we found that the two-component nicotine dehydrogenase from Agrobacterium tumefaciens S33, naturally lacking an FAD-binding domain, is located in the periplasmic space and uses a copper-containing electron carrier, pseudoazurin, as its physiological electron acceptor. We report here the role of pseudoazurin in a reaction catalyzed by a molybdopterin-containing hydroxylase occurring in the periplasmic space. These results provide new biochemical knowledge on microbial degradation of N-heterocyclic aromatic compounds.

Spectroscopic Definition of the CuZ° Intermediate in Turnover of Nitrous Oxide Reductase and Molecular Insight into the Catalytic Mechanism

Spectroscopic methods and density functional theory (DFT) calculations are used to determine the geometric and electronic structure of Cu_Z°, an intermediate form of the Cu₄S active site of nitrous oxide reductase (N₂OR) that is observed in single turnover of fully reduced N₂OR with N₂O. Electron paramagnetic resonance (EPR), absorption, and magnetic circular dichroism (MCD) spectroscopies show that Cu_Z° is a 1-hole (i.e., 3Cu^ICu^II) state with spin density delocalized evenly over Cu_I and Cu_IV. Resonance Raman spectroscopy shows two Cu-S vibrations at 425 and 413 cm^-1, the latter with a -3 cm^-1 O¹⁸ solvent isotope shift. DFT calculations correlated to these spectral features show that Cu_Z° has a terminal hydroxide ligand coordinated to Cu_IV, stabilized by a hydrogen bond to a nearby lysine residue. Cu_Z° can be reduced via electron transfer from Cu_A using a physiologically relevant reductant. We obtain a lower limit on the rate of this intramolecular electron transfer (IET) that is >10⁴ faster than the unobserved IET in the resting state, showing that Cu_Z° is the catalytically relevant oxidized form of N₂OR. Terminal hydroxide coordination to Cu_IV in the Cu_Z° intermediate yields insight into the nature of N₂O binding and reduction, specifying a molecular mechanism in which N₂O coordinates in a μ-1,3 fashion to the fully reduced state, with hydrogen bonding from Lys397, and two electrons are transferred from the fully reduced μ₄S^2- bridged tetranuclear copper cluster to N₂O via a single Cu atom to accomplish N-O bond cleavage.

Predicting Protein-Protein Interactions Using BiGGER: Case Studies

The importance of understanding interactomes makes preeminent the study of protein interactions and protein complexes. Traditionally, protein interactions have been elucidated by experimental methods or, with lower impact, by simulation with protein docking algorithms. This article describes features and applications of the BiGGER docking algorithm, which stands at the interface of these two approaches. BiGGER is a user-friendly docking algorithm that was specifically designed to incorporate experimental data at different stages of the simulation, to either guide the search for correct structures or help evaluate the results, in order to combine the reliability of hard data with the convenience of simulations. Herein, the applications of BiGGER are described by illustrative applications divided in three Case Studies: (Case Study A) in which no specific contact data is available; (Case Study B) when different experimental data (e.g., site-directed mutagenesis, properties of the complex, NMR chemical shift perturbation mapping, electron tunneling) on one of the partners is available; and (Case Study C) when experimental data are available for both interacting surfaces, which are used during the search and/or evaluation stage of the docking. This algorithm has been extensively used, evidencing its usefulness in a wide range of different biological research fields.

Transcriptional and metabolic regulation of denitrification in Paracoccus denitrificans allows low but significant activity of nitrous oxide reductase under oxic conditions

Oxygen is known to repress denitrification at the transcriptional and metabolic levels. It has been a common notion that nitrous oxide reductase (N2 OR) is the most sensitive enzyme among the four N-oxide reductases involved in denitrification, potentially leading to increased N2 O production under suboxic or fluctuating oxygen conditions. We present detailed gas kinetics and transcription patterns from batch culture experiments with Paracoccus denitrificans, allowing in vivo estimation of e(-) -flow to O2 and N2 O under various O2 regimes. Transcription of nosZ took place concomitantly with that of narG under suboxic conditions, whereas transcription of nirS and norB was inhibited until O2 levels approached 0 μM in the liquid. Catalytically functional N2 OR was synthesized and active in aerobically raised cells transferred to vials with 7 vol% O2 in headspace, but N2 O reduction rates were 10 times higher when anaerobic pre-cultures were subjected to the same conditions. Upon oxygen exposure, there was an incomplete and transient inactivation of N2 OR that could be ascribed to its lower ability to compete for electrons compared with terminal oxidases. The demonstrated reduction of N2 O at high O2 partial pressure and low N2 O concentrations by a bacterium not known as a typical aerobic denitrifier may provide one clue to the understanding of why some soils appear to act as sinks rather than sources for atmospheric N2 O.

Abundance, composition and activity of denitrifier communities in metal polluted paddy soils

Denitrification is one of the most important soil microbial processes leading to the production of nitrous oxide (N₂O). The potential changes with metal pollution in soil microbial community for N₂O production and reduction are not well addressed. In this study, topsoil samples were collected both from polluted and non-polluted rice paddy fields and denitrifier communities were characterized with molecular fingerprinting procedures. All the retrieved nirK sequences could be grouped into neither α- nor β- proteobacteria, while most of the nosZ sequences were affiliated with α-proteobacteria. The abundances of the nirK and nosZ genes were reduced significantly in the two polluted soils. Thus, metal pollution markedly affected composition of both nirK and nosZ denitrifiers. While the total denitrifying activity and N₂O production rate were both reduced under heavy metal pollution of the two sites, the N₂O reduction rate showed no significant change. These findings suggest that N₂O production activity could be sensitive to heavy metal pollution, which could potentially lead to a decrease in N₂O emission in polluted paddies. Therefore, metal pollution could have potential impacts on soil N transformation and thus on N₂O emission from paddy soils.

Structural basis of interprotein electron transfer in bacterial sulfite oxidation

Interprotein electron transfer underpins the essential processes of life and relies on the formation of specific, yet transient protein-protein interactions. In biological systems, the detoxification of sulfite is catalyzed by the sulfite-oxidizing enzymes (SOEs), which interact with an electron acceptor for catalytic turnover. Here, we report the structural and functional analyses of the SOE SorT from Sinorhizobium meliloti and its cognate electron acceptor SorU. Kinetic and thermodynamic analyses of the SorT/SorU interaction show the complex is dynamic in solution, and that the proteins interact with Kd = 13.5 ± 0.8 μM. The crystal structures of the oxidized SorT and SorU, both in isolation and in complex, reveal the interface to be remarkably electrostatic, with an unusually large number of direct hydrogen bonding interactions. The assembly of the complex is accompanied by an adjustment in the structure of SorU, and conformational sampling provides a mechanism for dissociation of the SorT/SorU assembly.

The structure of ferricytochrome c552 from the psychrophilic marine bacterium Colwellia psychrerythraea 34H

Approximately 40% of all proteins are metalloproteins, and approximately 80% of Earth's ecosystems are at temperatures ≤5 °C, including 90% of the global ocean. Thus, an essential aspect of marine metallobiochemistry is an understanding of the structure, dynamics, and mechanisms of cold adaptation of metalloproteins from marine microorganisms. Here, the molecular structure of the electron-transfer protein cytochrome c552 from the psychrophilic marine bacterium Colwellia psychrerythraea 34H has been determined by X-ray crystallography (PDB: ). The structure is highly superimposable with that of the homologous cytochrome from the mesophile Marinobacter hydrocarbonoclasticus. Based on structural analysis and comparison of psychrophilic, psychrotolerant, and mesophilic sequences, a methionine-based ligand-substitution mechanism for psychrophilic protein stabilization is proposed.

Conversion of nitrous oxide to nitrogen by cobalt-substituted myoglobin

Turning myoglobin into a nitrous oxide reductase.

Determination of the active form of the tetranuclear copper sulfur cluster in nitrous oxide reductase

N2OR has been found to have two structural forms of its tetranuclear copper active site, the 4CuS Cu(Z)* form and the 4Cu2S Cu(Z) form. EPR, resonance Raman, and MCD spectroscopies have been used to determine the redox states of these sites under different reductant conditions, showing that the Cu(Z)* site accesses the 1-hole and fully reduced redox states, while the Cu(Z) site accesses the 2-hole and 1-hole redox states. Single-turnover reactions of N2OR for Cu(Z) and Cu(Z)* poised in these redox states and steady-state turnover assays with different proportions of Cu(Z) and Cu(Z)* show that only fully reduced Cu(Z)* is catalytically competent in rapid turnover with N2O.

No laughing matter: the unmaking of the greenhouse gas dinitrogen monoxide by nitrous oxide reductase

The gas nitrous oxide (N₂O) is generated in a variety of abiotic, biotic, and anthropogenic processes and it has recently been under scrutiny for its role as a greenhouse gas. A single enzyme, nitrous oxide reductase, is known to reduce N₂O to uncritical N₂, in a two-electron reduction process that is catalyzed at two unusual metal centers containing copper. Nitrous oxide reductase is a bacterial metalloprotein from the metabolic pathway of denitrification, and it forms a 130 kDa homodimer in which the two metal sites CuA and CuZ from opposing monomers are brought into close contact to form the active site of the enzyme. CuA is a binuclear, valence-delocalized cluster that accepts and transfers a single electron. The CuA site of nitrous oxide reductase is highly similar to that of respiratory heme-copper oxidases, but in the denitrification enzyme the site additionally undergoes a conformational change on a ligand that is suggested to function as a gate for electron transfer from an external donor protein. CuZ, the tetranuclear active center of nitrous oxide reductase, is isolated under mild and anoxic conditions as a unique [4Cu:2S] cluster. It is easily desulfurylated to yield a [4Cu:S] state termed CuZ (*) that is functionally distinct. The CuZ form of the cluster is catalytically active, while CuZ (*) is inactive as isolated in the [3Cu(1+):1Cu(2+)] state. However, only CuZ (*) can be reduced to an all-cuprous state by sodium dithionite, yielding a form that shows higher activities than CuZ. As the possibility of a similar reductive activation in the periplasm is unconfirmed, the mechanism and the actual functional state of the enzyme remain under debate. Using enzyme from anoxic preparations with CuZ in the [4Cu:2S] state, N2O was shown to bind between the CuA and CuZ sites, suggesting direct electron transfer from CuA to the substrate after its activation by CuZ.

Biochemical characterization of the purple form of Marinobacter hydrocarbonoclasticus nitrous oxide reductase

Nitrous oxide reductase (N(2)OR) catalyses the final step of the denitrification pathway-the reduction of nitrous oxide to nitrogen. The catalytic centre (CuZ) is a unique tetranuclear copper centre bridged by inorganic sulphur in a tetrahedron arrangement that can have different oxidation states. Previously, Marinobacter hydrocarbonoclasticus N(2)OR was isolated with the CuZ centre as CuZ*, in the [1Cu(2+) : 3Cu(+)] redox state, which is redox inert and requires prolonged incubation under reductive conditions to be activated. In this work, we report, for the first time, the isolation of N(2)OR from M. hydrocarbonoclasticus in the 'purple' form, in which the CuZ centre is in the oxidized [2Cu(2+) : 2Cu(+)] redox state and is redox active. This form of the enzyme was isolated in the presence of oxygen from a microaerobic culture in the presence of nitrate and also from a strictly anaerobic culture. The purple form of the enzyme was biochemically characterized and was shown to be a redox active species, although it is still catalytically non-competent, as its specific activity is lower than that of the activated fully reduced enzyme and comparable with that of the enzyme with the CuZ centre in either the [1Cu(2+) : 3Cu(+)] redox state or in the redox inactive CuZ* state.

The electron transfer complex between nitrous oxide reductase and its electron donors

Identifying redox partners and the interaction surfaces is crucial for fully understanding electron flow in a respiratory chain. In this study, we focused on the interaction of nitrous oxide reductase (N(2)OR), which catalyzes the final step in bacterial denitrification, with its physiological electron donor, either a c-type cytochrome or a type 1 copper protein. The comparison between the interaction of N(2)OR from three different microorganisms, Pseudomonas nautica, Paracoccus denitrificans, and Achromobacter cycloclastes, with their physiological electron donors was performed through the analysis of the primary sequence alignment, electrostatic surface, and molecular docking simulations, using the bimolecular complex generation with global evaluation and ranking algorithm. The docking results were analyzed taking into account the experimental data, since the interaction is suggested to have either a hydrophobic nature, in the case of P. nautica N(2)OR, or an electrostatic nature, in the case of P. denitrificans N(2)OR and A. cycloclastes N(2)OR. A set of well-conserved residues on the N(2)OR surface were identified as being part of the electron transfer pathway from the redox partner to N(2)OR (Ala495, Asp519, Val524, His566 and Leu568 numbered according to the P. nautica N(2)OR sequence). Moreover, we built a model for Wolinella succinogenes N(2)OR, an enzyme that has an additional c-type-heme-containing domain. The structures of the N(2)OR domain and the c-type-heme-containing domain were modeled and the full-length structure was obtained by molecular docking simulation of these two domains. The orientation of the c-type-heme-containing domain relative to the N(2)OR domain is similar to that found in the other electron transfer complexes.

The tetranuclear copper active site of nitrous oxide reductase: the CuZ center

This review focuses on the novel CuZ center of nitrous oxide reductase, an important enzyme owing to the environmental significance of the reaction it catalyzes, reduction of nitrous oxide, and the unusual nature of its catalytic center, named CuZ. The structure of the CuZ center, the unique tetranuclear copper center found in this enzyme, opened a novel area of research in metallobiochemistry. In the last decade, there has been progress in defining the structure of the CuZ center, characterizing the mechanism of nitrous oxide reduction, and identifying intermediates of this reaction. In addition, the determination of the structure of the CuZ center allowed a structural interpretation of the spectroscopic data, which was supported by theoretical calculations. The current knowledge of the structure, function, and spectroscopic characterization of the CuZ center is described here. We would like to stress that although many questions have been answered, the CuZ center remains a scientific challenge, with many hypotheses still being formed.

Mitigating release of the potent greenhouse gas N(2)O from the nitrogen cycle - could enzymic regulation hold the key?

When faced with a shortage of oxygen, many bacterial species use nitrate to support respiration via the process of denitrification. This takes place extensively in nitrogen-rich soils and generates the gaseous products nitric oxide (NO), nitrous oxide (N(2)O) and dinitrogen (N(2)). The denitrifying bacteria protect themselves from the endogenous cytotoxic NO produced by converting it to N(2)O, which can be released into the atmosphere. However, N(2)O is a potent greenhouse gas and hence the activity of the enzyme that breaks down N(2)O has a crucial role in restricting its atmospheric levels. Here, we review the current understanding of the process by which N(2)O is produced and destroyed and discuss the potential for feeding this into new approaches for combating N(2)O release.