Biochemical characterization and solution structure of nitrous oxide reductase from Alcaligenes xylosoxidans (NCIMB 11015)

Enzymatic regulation of N2O production by denitrifying bacteria in the sludge of biological nitrogen removal process

This study analyzed the activities of all denitrifying enzymes involved in the denitrification process under different organic loads in a continuously operating sequencing batch reactor (SBR), to reveal how the denitrifying enzymes performed while the denitrifying bacteria facing changes in organic load, and leading to nitrous oxide (N₂O) production by fine-tuning enzyme activities. Results show that the activities of nitrate reductase (Nar), nitrite reductase (Nir), nitric oxide reductase (Nor) and nitrous oxide reductase (N₂OR) increased with the increase of organic loads, and the increase of the activity of different enzymes promoted by the organic load increase were as Nar > Nir > Nor > N₂OR. Compared with the Nar and Nir, the catalytic processes of the Nor and N₂OR were more susceptible to the influence of the substrate concentration and the content of internal and external carbon sources. The Nor usually maintained "excess" catalytic activity to ensure the smooth reduction of nitric oxide when the electron donor and substrate were sufficient. Otherwise, it reduced to a relatively lower catalytic activity and remained stable. The activities of the N₂OR were generally weaker than that of other denitrifying enzymes. More N₂O was produced in the period feeding with low organic loads (COD/NO₃^--N ≤ 4.9). The mechanism of the enzyme activities (Nor and N₂OR) regulating the total concentrations of N₂O was clarified. When the organic load was relatively low (COD/NO₃^--N ≤ 2.5), the N₂OR activity was inhibited due to its inability to acquire enough electrons, resulting the production of N₂O. When the organic load was moderate (2.5 < COD/NO₃^--N ≤ 4.9), the N₂OR activity was lower than the Nor activity due to the different activation rates of Nor and N₂OR by the substrate in bacteria, resulting the production of N₂O.

Analyzing the mechanism of nitrous oxide production in aerobic phase of anoxic/aerobic sequential batch reactor from the perspective of key enzymes

How the vast majority of nitrous oxide (N₂O) in the aerobic zone of nitrogen bio-removal process is produced is still a controversial issue. To solve this issue, this study measured the activities of two key denitrifying enzymes (nitric oxide reductase (Nor) and nitrous oxide reductase (N₂OR)) in an A/O SBR with different chemical nitrogen demand (COD)/total nitrogen (TN) ratios. By analyzing the Spearman's correlations between the N₂O production, the enzyme activities, and the factors, the main N₂O production process was identified. By comparing the activities of these enzymes, this study analyzed the reasons for the N₂O production. Results show that Nor activities had a linear relationship with total N₂O concentrations (y = 0.34749 + 31.31365x, R² = 0.83362) and were not affected by COD (r = 0.299, N = 15, P = 0.279 > 0.05), which showed that most of the N₂O released and produced came from the autotrophic denitrification. N₂OR activities had a positive correlation with COD (r = 0.692, N = 15, P = 0.004 < 0.01), which showed that heterotrophic denitrification played a role as an N₂O consumer. Nor activities were much higher than N₂OR activities and the gap between them increased when the total N₂O concentration increased, showing that the heterotrophic denitrification was difficult to consume all the N₂O produced by the autotrophic denitrification. Reducing autotrophic denitrification is the best way to reduce N₂O production in aerobic phase.

Nitrite removal by Acinetobacter sp.TX: a candidate of curbing N2O emission

The nitrite removal pathway in Acinetobacter sp. TX5 was explored through the key gene identification and the corresponding enzyme purification, after which the capability to reduce nitrite by immobilized beads was investigated in a fixed-bed reactor. Results revealed that a nosZ gene encoding nitrous oxide reductase (N₂OR) exists in TX5 cells, and a N₂OR responsible for the reduction of N₂O to N₂ was purified successfully with a molecular weight of 70.05 kDa, a purification fold of 16.30 and a recovery rate of 5.17%. For TX5 immobilization, the optimal values of polyvinyl alcohol (PVA), spent mushroom substrate (SMS) and Aci (TX5) obtained by response surface methodology (RSM) were 6.32%, 2.92% and 4.57%, respectively. In a fixed-bed reactor packed with immobilized TX5, the removal efficiency (RE) achieved 90% (at 50 h) for NO2--N and 85% (at 96 h) for total nitrogen (TN). On the basis of these results, a nitrite removal pathway in TX5 was proposed. Overall, Acinetobacter sp. TX5 might be a promising candidate for nitrite removal with an ability to suppress N₂O accumulation.

NosZ gene cloning, reduction performance and structure of Pseudomonas citronellolis WXP-4 nitrous oxide reductase

Nitrous oxide reductase (N₂OR) is the only known enzyme that can reduce the powerful greenhouse gas nitrous oxide (N₂O) to harmless nitrogen at the final step of bacterial denitrification. To alleviate the N₂O emission, emerging approaches aim at microbiome biotechnology. In this study, the genome sequence of facultative anaerobic bacteria Pseudomonas citronellolis WXP-4, which efficiently degrades N₂O, was obtained by de novo sequencing for the first time, and then, four key reductase structure coding genes related to complete denitrification were identified. The single structural encoding gene nosZ with a length of 1914 bp from strain WXP-4 was cloned in Escherichia coli BL21(DE3), and the N₂OR protein (76 kDa) was relatively highly efficiently expressed under the optimal inducing conditions of 1.0 mM IPTG, 5 h, and 30 °C. Denitrification experiment results confirmed that recombinant E. coli had strong denitrification ability and reduced 10 mg L⁻¹ of N₂O to N₂ within 15 h under the optimal conditions of pH 7.0 and 40 °C, its corresponding N₂O reduction rate was almost 2.3 times that of Alcaligenes denitrificans strain TB, but only 80% of that of wild strain WXP-4, meaning that nos gene cluster auxiliary gene deletion decreased the activity of N₂OR. The 3D structure of N₂OR predicted on the basis of sequence homology found that electron transfer center CuA had only five amino acid ligands, and the S2 of the catalytically active center CuZ only bound one Cu_I atom. The unique 3D structure was different from previous reports and may be closely related to the strong N₂O reduction ability of strain WXP-4 and recombinant E. coli. The findings show a potential application of recombinant E. coli in alleviating the greenhouse effect and provide a new perspective for researching the relationship between structure and function of N₂OR.

Nitrous oxide reductase (N₂OR) is the only known enzyme that can reduce the powerful greenhouse gas nitrous oxide (N₂O) to harmless nitrogen at the final step of bacterial denitrification. The recombinant E. coli and wild strain WXP-4 demonstrate strong N₂O reduction ability.

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.

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.

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 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.

Anaerobic purification, characterization and preliminary mechanistic study of recombinant nitrous oxide reductase from Achromobacter cycloclastes

An overexpression system for nitrous oxide reductase (N(2)OR), an enzyme that catalyzes the conversion of N(2)O to N(2) and H(2)O, has been developed in Achromobacter cycloclastes. Anaerobically purified A. cycloclastes recombinant N(2)OR (AcN(2)OR) has on average 4.5 Cu and 1.2 S per monomer. Upon reduction by methyl viologen, AcN(2)OR displays a high specific activity: 124 U/mg at 25 degrees C. Anaerobically purified AcN(2)OR displays a unique absorption spectrum. UV-visible and EPR spectra, combined with kinetics studies, indicate that the as-purified form of the enzyme is predominately a mixture of the fully-reduced Cu(Z)=[4Cu(I)] state and the Cu(Z)=[3Cu(I).Cu(II)] state, with the latter readily reducible by reduced forms of viologens. CD spectra of the as-purified AcN(2)OR over a range of pH values reveal perturbations of the protein conformation induced by pH variations, although the principal secondary structure elements are largely unaltered. Further, the activity of AcN(2)OR in D(2)O is significantly decreased compared with that in H(2)O, indicative of a significant solvent isotope effect on N(2)O reduction. These data are in good agreement with conclusions reached in recent studies on the effect of pH on catalysis by N(2)OR [K. Fujita, D.M. Dooley, Inorg. Chem. 46 (2007) 613-615].

Respiratory transformation of nitrous oxide (N2O) to dinitrogen by Bacteria and Archaea

N2O is a potent greenhouse gas and stratospheric reactant that has been steadily on the rise since the beginning of industrialization. It is an obligatory inorganic metabolite of denitrifying bacteria, and some production of N2O is also found in nitrifying and methanotrophic bacteria. We focus this review on the respiratory aspect of N2O transformation catalysed by the multicopper enzyme nitrous oxide reductase (N2OR) that provides the bacterial cell with an electron sink for anaerobic growth. Two types of Cu centres discovered in N2OR were both novel structures among the Cu proteins: the mixed-valent dinuclear Cu(A) species at the electron entry site of the enzyme, and the tetranuclear Cu(Z) centre as the first catalytically active Cu-sulfur complex known. Several accessory proteins function as Cu chaperone and ABC transporter systems for the biogenesis of the catalytic centre. We describe here the paradigm of Z-type N2OR, whose characteristics have been studied in most detail in the genera Pseudomonas and Paracoccus. Sequenced bacterial genomes now provide an invaluable additional source of information. New strains harbouring nos genes and capability of N2O utilization are being uncovered. This reveals previously unknown relationships and allows pattern recognition and predictions. The core nos genes, nosZDFYL, share a common phylogeny. Most principal taxonomic lineages follow the same biochemical and genetic pattern and share the Z-type enzyme. A modified N2OR is found in Wolinella succinogenes, and circumstantial evidence also indicates for certain Archaea another type of N2OR. The current picture supports the view of evolution of N2O respiration prior to the separation of the domains Bacteria and Archaea. Lateral nos gene transfer from an epsilon-proteobacterium as donor is suggested for Magnetospirillum magnetotacticum and Dechloromonas aromatica. In a few cases, nos gene clusters are plasmid borne. Inorganic N2O metabolism is associated with a diversity of physiological traits and biochemically challenging metabolic modes or habitats, including halorespiration, diazotrophy, symbiosis, pathogenicity, psychrophily, thermophily, extreme halophily and the marine habitat down to the greatest depth. Components for N2O respiration cover topologically the periplasm and the inner and outer membranes. The Sec and Tat translocons share the task of exporting Nos components to their functional sites. Electron donation to N2OR follows pathways with modifications depending on the host organism. A short chronology of the field is also presented.

Insight into Catalysis of Nitrous Oxide Reductase from High-resolution Structures of Resting and Inhibitor-bound Enzyme from Achromobacter cycloclastes

The difficult chemistry of nitrous oxide (N2O) reduction to gaseous nitrogen (N2) in biology is catalysed by the novel micro4-sulphide-bridged tetranuclear Cuz cluster of the N2O reductases (N2OR). Two spectroscopically distinct forms of this cluster have been identified as CuZ and CuZ*. We have obtained a 1.86 A resolution crystal structure of the pink-purple species of N2OR from Achromobacter cycloclastes (AcN2OR) isolated under aerobic conditions. This structure reveals a previously unobserved ligation with two oxygen atoms from H2O/OH- coordinated to Cu1 and Cu4 of the catalytic centre. We ascribe this structure to be that of the CuZ form of the cluster, since the previously reported structures of two blue species of N2ORs, also isolated aerobically, have characterised the redox inactive CuZ* form, revealing a single water molecule at Cu4. Exposure of the as-isolated AcN2OR to sodium iodide led to reduction of the electron-donating CuA site and the formation of a blue species. Structure determination of this adduct at 1.7 A resolution showed that iodide was bound at the CuZ site bridging the Cu1 and Cu4 ions. This structure represents the first observation of an inhibitor bound to the Cu1-Cu4 edge of the catalytic cluster, providing clear evidence for this being the catalytic edge in N2ORs. These structures, together with the published structural and spectroscopic data, give fresh insight into the mode of substrate binding, reduction and catalysis.

NosX function connects to nitrous oxide (N2O) reduction by affecting the CuZcenter of NosZ and its activity in vivo

The effect of loss of the 34-kDa periplasmic NosX protein on the properties of N2O reductase was investigated with an N2O-respiration negative, double mutant of the paralogous genes nosX and nirX of Paracoccus denitrificans. In spite of absence of whole-cell N2O-reducing activity, the purified reductase was catalytically active, which attributes NosX a physiological role in sustaining the reaction cycle. N2O reductase exhibited the spectroscopic features of Cu(A) and the redox-inert, paramagnetic state, Cu(Z)*, of the catalytic center. Cu(Z)*, hitherto considered the result of spontaneous reaction of the reductase with dioxygen, attains cellular significance.

Activation of N2O Reduction by the Fully Reduced μ4-Sulfide Bridged Tetranuclear CuZ Cluster in Nitrous Oxide Reductase

The tetranuclear CuZ cluster catalyzes the two-electron reduction of N2O to N2 and H2O in the enzyme nitrous oxide reductase. This study shows that the fully reduced 4CuI form of the cluster correlates with the catalytic activity of the enzyme. This is the first demonstration that the S = 1/2 form of CuZ can be further reduced. Complementary DFT calculations support the experimental findings and demonstrate that N2O binding in a bent mu-1,3-bridging mode to the 4CuI form is most efficient due to strong back-bonding from two reduced copper atoms. This back-donation activates N2O for electrophilic attack by a proton.

Crystal structure of nitrous oxide reductase from Paracoccus denitrificans at 1.6 A resolution

N2O is generated by denitrifying bacteria as a product of NO reduction. In denitrification, N2O is metabolized further by the enzyme N2O reductase (N2OR), a multicopper protein which converts N2O into dinitrogen and water. The structure of N2OR remained unknown until the recent elucidation of the structure of the enzyme isolated from Pseudomonas nautica. In the present paper, we report the crystal structure of a blue form of the enzyme that was purified under aerobic conditions from Paracoccus denitrificans. N2OR is a head-to-tail homodimer stabilized by a multitude of interactions including two calcium sites located at the intermonomeric surface. Each monomer is composed of two domains: a C-terminal cupredoxin domain that carries the dinuclear electron entry site known as Cu(A), and an N-terminal seven-bladed beta-propeller domain which hosts the active-site centre Cu(Z). The electrons are transferred from Cu(A) to Cu(Z) across the subunit interface. Cu(Z) is a tetranuclear copper cluster in which the four copper ions (Cu1 to Cu4) are ligated by seven histidine imidazoles, a hydroxyl or water oxygen and a bridging inorganic sulphide. A bound chloride ion near the Cu(Z) active site shares one of the ligand imidazoles of Cu1. This arrangement probably influences the redox potential of Cu1 so that this copper is stabilized in the cupric state. The treatment of N2OR with H2O2 or cyanide causes the disappearance of the optical band at 640 nm, attributed to the Cu(Z) centre. The crystal structure of the enzyme soaked with H2O2 or cyanide suggests that an average of one copper of the Cu(Z) cluster has been lost. The lowest occupancy is observed for Cu3 and Cu4. A docking experiment suggests that N(2)O binds between Cu1 and Cu4 so that the oxygen of N2O replaces the oxygen ligand of Cu4. Certain ligand imidazoles of Cu1 and Cu2, as well as of Cu4, are located at the dimer interface. Particularly those of Cu2 and Cu4 are parts of a bonding network which couples these coppers to the Cu(A) centre in the neighbouring monomer. This structure may provide an efficient electron transfer path for reduction of the bound N2O.

Multiple forms of the catalytic centre, CuZ, in the enzyme nitrous oxide reductase from Paracoccus pantotrophus

Nitrous oxide reductase catalyses the reduction of nitrous oxide to dinitrogen at a unique tetranuclear copper site, called Cu(Z), which has a central inorganic sulphide ligand. Limited incubation with oxygen during the preparation of nitrous oxide reductase from Paracoccus pantotrophus results in changed redox properties of the catalytic centre by comparison with anaerobic preparations. While the anaerobically purified enzyme has a catalytic centre which performs a single electron step at a midpoint potential of E(m)=+60 mV versus the standard hydrogen electrode (n=1), the altered centre shows no redox change under similar experimental conditions. Spectroscopic properties of this 'redox fixed' centre are similar to spectra of the reduced 'redox active' form of CuZ, although the positions and intensities of a number of transitions are changed in the optical spectrum. These observations are interpreted in terms of two forms of the catalytic centre, called CuZ and CuZ*. The structural relationship between these forms is unclear. EPR and magnetic circular dichroism spectra suggest that the basic Cu4S structure is common to both. Curiously, steady-state activity of the aerobic enzyme preparation is slightly increased despite the fact the catalytic centre does not undergo detectable redox changes.

Codenitrification and denitrification are dual metabolic pathways through which dinitrogen evolves from nitrate in Streptomyces antibioticus

We screened actinomycete strains for dinitrogen (N(2))-producing activity and discovered that Streptomyces antibioticus B-546 evolves N(2) and some nitrous oxide (N(2)O) from nitrate (NO(3)(-)). Most of the N(2) that evolved from the heavy isotope ([(15)N]NO(3)(-)) was (15)N(14)N, indicating that this nitrogen species consists of two atoms, one arising from NO(3)(-) and the other from different sources. This phenomenon is similar to codenitrification in fungi. The strain also evolved less, but significant, amounts of (15)N(15)N from [(15)N]NO(3)(-) in addition to (15)N(15)NO with concomitant cell growth. Prior to the production of N(2) and N(2)O, NO(3)(-) was rapidly reduced to nitrite (NO(2)(-)) accompanied by distinct cell growth, showing that the actinomycete strain is a facultative anaerobe that depends on denitrification and nitrate respiration for anoxic growth. The cell-free activities of denitrifying enzymes could be reconstituted, supporting the notion that the (15)N(15)N and (15)N(15)NO species are produced by denitrification from NO(3)(-) via NO(2)(-). We therefore demonstrated a unique system in an actinomycete that produces gaseous nitrogen (N(2) and N(2)O) through both denitrification and codenitrification. The predominance of codenitrification over denitrification along with oxygen tolerance is the key feature of nitrate metabolism in this actinomycete.

Structural investigations of the CuA centre of nitrous oxide reductase from Pseudomonas stutzeri by site-directed mutagenesis and X-ray absorption spectroscopy

Nitrous oxide reductase is the terminal component of a respiratory chain that utilizes N2O in lieu of oxygen. It is a homodimer carrying in each subunit the electron transfer site, CuA, and the substrate-reducing catalytic centre, CuZ. Spectroscopic data have provided robust evidence for CuA as a binuclear, mixed-valence metal site. To provide further structural information on the CuA centre of N2O reductase, site directed mutagenesis and Cu K-edge X-ray absorption spectroscopic investigation have been undertaken. Candidate amino acids as ligands for the CuA centre of the enzyme from Pseudomonas stutzeri ATCC14405 were substituted by evolutionary conserved residues or amino acids similar to the wild-type residues. The mutations identified the amino acids His583, Cys618, Cys622 and Met629 as ligands of Cu1, and Cys618, Cys622 and His626 as the minimal set of ligands for Cu2 of the CuA centre. Other amino acid substitutions indicated His494 as a likely ligand of CuZ, and an indirect role for Asp580, compatible with a docking function for the electron donor. Cu binding and spectroscopic properties of recombinant N2O reductase proteins point at intersubunit or interdomain interaction of CuA and CuZ. Cu K-edge X-ray absorption spectra have been recorded to investigate the local environment of the Cu centres in N2O reductase. Cu K-edge Extended X-ray Absorption Fine Structure (EXAFS) for binuclear Cu chemical systems show clear evidence for Cu backscattering at approximately 2.5 A. The Cu K-edge EXAFS of the CuA centre of N2O reductase is very similar to that of the CuA centre of cytochrome c oxidase and the optimum simulation of the experimental data involves backscattering from a histidine group with Cu-N of 1.92 A, two sulfur atoms at 2.24 A and a Cu atom at 2. 43 A, and allows for the presence of a further light atom (oxygen or nitrogen) at 2.05 A. The interpretation of the CuA EXAFS is in line with ligands assigned by site-directed mutagenesis. By a difference spectrum approach, using the Cu K-edge EXAFS of the holoenzyme and that of the CuA-only form, histidine was identified as a major contributor to the backscattering. A structural model for the CuA centre of N2O reductase has been generated on the basis of the atomic coordinates for the homologous domain of cytochrome c oxidase and incorporating our current results and previous spectroscopic data.