Characterization of a Bimetallic-Bridging Intermediate in the Reduction of NO to N2O: a Density Functional Theory Study

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.

Discrete Ligand Binding and Electron Transfer Properties of ba3-Cytochrome c Oxidase from Thermus thermophilus: Evolutionary Adaption to Low Oxygen and High Temperature Environments

Cytochrome c oxidase (C cO) couples the oxidation of cytochrome c to the reduction of molecular oxygen to water and links these electron transfers to proton translocation. The redox-driven C cO conserves part of the released free energy generating a proton motive force that leads to the synthesis of the main biological energy source ATP. Cytochrome ba₃ oxidase is a B-type oxidase from the extremely thermophilic eubacterium Thermus thermophilus with high O₂ affinity, expressed under elevated temperatures and limited oxygen supply and possessing discrete structural, ligand binding, and electron transfer properties. The origin and the cause of the peculiar, as compared to other C cOs, thermodynamic and kinetic properties remain unknown. Fourier transform infrared (FTIR) and time-resolved step-scan FTIR (TRS²-FTIR) spectroscopies have been employed to investigate the origin of the binding and electron transfer properties of cytochrome ba₃ oxidase in both the fully reduced (FR) and mixed valence (MV) forms. Several independent and not easily separated factors leading to increased thermostability and high O₂ affinity have been determined. These include (i) the increased hydrophobicity of the active center, (ii) the existence of a ligand input channel, (iii) the high affinity of Cu_B for exogenous ligands, (iv) the optimized electron transfer (ET) pathways, (v) the effective proton-input channel and water-exit pathway as well the proton-loading/exit sites, (vi) the specifically engineered protein structure, and (vii) the subtle thermodynamic and kinetic regulation. We correlate the unique ligand binding and electron transfer properties of cytochrome ba₃ oxidase with the existence of an adaption mechanism which is necessary for efficient function. These results suggest that a cascade of structural factors have been optimized by evolution, through protein architecture, to ensure the conversion of cytochrome ba₃ oxidase into a high O₂-affinity enzyme that functions effectively in its extreme native environment. The present results show that ba₃-cytochrome c oxidase uses a unique structural pattern of energy conversion that has taken into account all the extreme environmental factors that affect the function of the enzyme and is assembled in such a way that its exclusive functions are secured. Based on the available data of CcOs, we propose possible factors including the rigidity and nonpolar hydrophobic interactions that contribute to the behavior observed in cytochrome ba₃ oxidase.

Reversible temperature-dependent high- to low-spin transition in the heme Fe–Cu binuclear center of cytochrome ba3 oxidase

A reversible temperature-dependent high-spin to low-spin transition with T_1/2 = −60 °C has been observed in the resonance Raman spectra of the equilibrium reduced and photoreduced heme a₃ of the thermophilic ba₃ heme–copper oxidoreductase. The transition is based on the frequency shifts of the spin-state marker bands ν₂ (C_bC_b) and ν₁₀ (C_aC_m) and is attributed to the displacement of the heme iron along the heme normal as a consequence of the Fe–Np repulsion at temperature below −40 °C which will increase the ligand field strength forcing the pairing of d electrons into the lower energy orbitals.

A reversible temperature-dependent high- to low-spin transition with T_1/2 = −60 °C has been observed in the resonance Raman spectra of the equilibrium reduced and photoreduced heme a₃ of the thermophilic ba₃ heme–copper oxidoreductase.

A trans-Hyponitrite Intermediate in the Reductive Coupling and Deoxygenation of Nitric Oxide by a Tricopper-Lewis Acid Complex

The reduction of nitric oxide (NO) to nitrous oxide (N2O) is a process relevant to biological chemistry as well as to the abatement of certain environmental pollutants. One of the proposed key intermediates in NO reduction is hyponitrite (N2O2(2-)), the product of reductive coupling of two NO molecules. We report the reductive coupling of NO by an yttrium-tricopper complex generating a trans-hyponitrite moiety supported by two μ-O-bimetallic (Y,Cu) cores, a previously unreported coordination mode. Reaction of the hyponitrite species with Brønsted acids leads to the generation of N2O, demonstrating the viability of the hyponitrite complex as an intermediate in NO reduction to N2O. The additional reducing equivalents stored in each tricopper unit are employed in a subsequent step for N2O reduction to N2, for an overall (partial) conversion of NO to N2. The combination of Lewis acid and multiple redox active metals facilitates this four electron conversion via an isolable hyponitrite intermediate.

Proton-Assisted Mechanism of NO Reduction on a Dinuclear Ruthenium Complex

Density-functional-theory (DFT) calculations are performed for the proposal of a plausible mechanism on the reduction of NO to N2O by a dinuclear ruthenium complex, reported by Arikawa and co-workers [J. Am. Chem. Soc. 2007, 129, 14160]. On the basis of the experimental fact that the reduction proceeds under strongly acidic conditions, the role of protons in the mechanistic pathways is investigated with model complexes, where one or two NO ligands are protonated. The reaction mechanism of the NO reduction is partitioned into three steps: reorientation of N2O2 (cis-NO dimer), O-N bond cleavage, and N2O elimination. A key finding is that the protonation of the NO ligand(s) significantly reduces the activation barrier in the rate-determining reorientation step. The activation energy of 43.1 kcal/mol calculated for the proton-free model is reduced to 30.2 and 17.6 kcal/mol for the mono- and diprotonated models, respectively. The protonation induces the electron transfer from the Ru(II)Ru(II) core to the O═N-N═O moiety to give a Ru(III)Ru(III) core and a hyponitrite (O-N═N-O)(2-) species. The formation of the hyponitrite species provides an alternative pathway for the N2O2 reorientation, resulting in the lower activation energies in the presence of proton(s). The protonation also has a marginal effect on the O-N bond cleavage and the N2O elimination steps. Our calculations reveal a remarkable role of protons in the NO reduction via N2O formation and provide new insights into the mechanism of NO reduction catalyzed by metalloenzymes such as nitric oxide reductase (NOR) that contains a diiron active site.

Nitric oxide activation by caa3 oxidoreductase from Thermus thermophilus

We present UV-Raman evidence for the formation of the hyponitrite (HO–NN–O⁻) species in the binuclear heme a₃ Fe–Cu_B center (νN–N = 1330 cm⁻¹) of caa₃ heme-copper oxidoreductase from Thermus thermophilus.

Nitric oxide coupling mediated by iron porphyrins: the N-N bond formation step is facilitated by electrons and a proton

The coupling of two NO molecules catalyzed by iron porphyrins is of biological importance. We use density functional theory calculations to examine the factors that control the fundamental N-N bond formation step mediated by a single iron porphyrin. The presence of an axial Im ligand, extra electrons, and most importantly a proton, enhance the N-N bond formation step in our model.

Observation of ligand transfer in ba3 oxidase from Thermus thermophilus: simultaneous FTIR detection of photolabile heme a3(2+)-CN and transient Cu(B)(2+)-CN complexes

FTIR and light-minus-dark FTIR spectroscopy have been employed to investigate the reaction of oxidized and fully reduced ba(3) oxidase with cyanide. The characterization of the structures of the bound CN(-) in the binuclear heme Fe-Cu(B) center is essential, given that a central issue in the function of ba(3) oxidase is the extent to which the partially reduced substrates interact with the two metals. In the reaction of oxidized ba(3) oxidase with cyanide the initially formed heme a(3)(3+)-C≡N-Cu(B)(2+) species with ν(CN) frequency at 2152 cm(-1) was replaced by a photolabile complex with a frequency at 2075 cm(-1) characteristic of heme a(3)(2+)-CN(-). Photolysis of the heme a(3)(2+)-CN(-) adduct produced a band at 2146 cm(-1) attributed to the formation of a transient Cu(B)(2+)-CN(-) complex. All forms are pH independent between pH 5.5-9.5 and at pD 7.5 indicating the absence of ionizable groups that influence the properties of the cyanide complexes. In contrast to previous reports, our results show that CN(-) does not bind simultaneously to both heme a(3)(2+) and Cu(B)(2+) to form the mixed valence a(3)(2+)-CN·Cu(B)(2+)CN species. The photolysis products of the heme a(3)(2+)-CN(-)/Cu(B)(2+) and heme a(3)(2+)-CN(-)/Cu(B)(1+) species are different suggesting that relaxation dynamics in the binuclear center following ligand photodissociation are dependent on the oxidation state of Cu(B).

Spectroscopic characterization of mononitrosyl complexes in heme--nonheme diiron centers within the myoglobin scaffold (Fe(B)Mbs): relevance to denitrifying NO reductase

Denitrifying NO reductases are evolutionarily related to the superfamily of heme--copper terminal oxidases. These transmembrane protein complexes utilize a heme-nonheme diiron center to reduce two NO molecules to N(2)O. To understand this reaction, the diiron site has been modeled using sperm whale myoglobin as a scaffold and mutating distal residues Leu-29 and Phe-43 to histidines and Val-68 to a glutamic acid to create a nonheme Fe(B) site. The impact of incorporation of metal ions at this engineered site on the reaction of the ferrous heme with one NO was examined by UV-vis absorption, EPR, resonance Raman, and FTIR spectroscopies. UV--vis absorption and resonance Raman spectra demonstrate that the first NO molecule binds to the ferrous heme, but while the apoproteins and Cu(I)- or Zn(II)-loaded proteins show characteristic EPR signatures of S = 1/2 six-coordinate heme {FeNO}(7) species that can be observed at liquid nitrogen temperature, the Fe(II)-loaded proteins are EPR silent at ≥30 K. Vibrational modes from the heme [Fe-N-O] unit are identified in the RR and FTIR spectra using (15)NO and (15)N(18)O. The apo and Cu(I)-bound proteins exhibit ν(FeNO) and ν(NO) that are only marginally distinct from those reported for native myoglobin. However, binding of Fe(II) at the Fe(B) site shifts the heme ν(FeNO) by 17 cm(-1) and the ν(NO) by -50 cm(-1) to 1549 cm(-1). This low ν(NO) is without precedent for a six-coordinate heme {FeNO}(7) species and suggests that the NO group adopts a strong nitroxyl character stabilized by electrostatic interaction with the nearby nonheme Fe(II). Detection of a similarly low ν(NO) in the Zn(II)-loaded protein supports this interpretation.

Bioinspired heme, heme/nonheme diiron, heme/copper, and inorganic NOx chemistry: *NO((g)) oxidation, peroxynitrite-metal chemistry, and *NO((g)) reductive coupling

The focus of this Forum Article highlights work from our own laboratories and those of others in the area of biochemical and biologically inspired inorganic chemistry dealing with nitric oxide [nitrogen monoxide, *NO((g))] and its biological roles and reactions. The latter focus is on (i) oxidation of *NO((g)) to nitrate by nitric oxide dioxygenases (NODs) and (ii) reductive coupling of two molecules of *NO((g)) to give N(2)O(g). In the former case, NODs are described, and the highlighting of possible peroxynitrite/heme intermediates and the consequences of this are given by a discussion of recent works with myoglobin and a synthetic heme model system for NOD action. Summaries of recent copper complex chemistries with *NO((g)) and O(2)(g), leading to peroxynitrite species, are given. The coverage of biological reductive coupling of *NO((g)) deals with bacterial nitric oxide reductases (NORs) with heme/nonheme diiron active sites and on heme/copper oxidases such as cytochrome c oxidase, which can mediate the same chemistry. Recently designed protein and synthetic model compounds (heme/nonheme/diiron or heme/copper) as functional mimics are discussed in some detail. We also highlight examples from the chemical literature, not necessarily involving biologically relevant metal ions, that describe the oxidation of *NO((g)) to nitrate (or nitrite) and possible peroxynitrite intermediates or reductive coupling of *NO((g)) to give nitrous oxide.

Linkage isomerization in heme-NOx compounds: understanding NO, nitrite, and hyponitrite interactions with iron porphyrins

Nitric oxide (NO) and its derivatives such as nitrite and hyponitrite are biologically important species of relevance to human health. Much of their physiological relevance stems from their interactions with the iron centers in heme proteins. The chemical reactivities displayed by the heme-NOx species (NOx = NO, nitrite, hyponitrite) are a function of the binding modes of the NOx ligands. Hence, an understanding of the types of binding modes extant in heme-NOx compounds is important if we are to unravel the inherent chemical properties of these NOx metabolites. In this Forum Article, the experimentally characterized linkage isomers of heme-NOx models and proteins are presented and reviewed. Nitrosyl linkage isomers of synthetic iron and ruthenium porphyrins have been generated by photolysis at low temperatures and characterized by spectroscopy and density functional theory calculations. Nitrite linkage isomers in synthetic metalloporphyrin derivatives have been generated from photolysis experiments and in low-temperature matrices. In the case of nitrite adducts of heme proteins, both N and O binding have been determined crystallographically, and the role of the distal H-bonding residue in myoglobin in directing the O-binding mode of nitrite has been explored using mutagenesis. To date, only one synthetic metalloporphyrin complex containing a hyponitrite ligand (displaying an O-binding mode) has been characterized by crystallography. This is contrasted with other hyponitrite binding modes experimentally determined for coordination compounds and computationally for NO reductase enzymes. Although linkage isomerism in heme-NOx derivatives is still in its infancy, opportunities now exist for a detailed exploration of the existence and stabilities of the metastable states in both heme models and heme proteins.

CO impedes superfast O2 binding in ba3 cytochrome oxidase from Thermus thermophilus

Kinetic studies of heme-copper terminal oxidases using the CO flow-flash method are potentially compromised by the fate of the photodissociated CO. In this time-resolved optical absorption study, we compared the kinetics of dioxygen reduction by ba(3) cytochrome c oxidase from Thermus thermophilus in the absence and presence of CO using a photolabile O(2)-carrier. A novel double-laser excitation is introduced in which dioxygen is generated by photolyzing the O(2)-carrier with a 355 nm laser pulse and the fully reduced CO-bound ba(3) simultaneously with a second 532-nm laser pulse. A kinetic analysis reveals a sequential mechanism in which O(2) binding to heme a(3) at 90 μM O(2) occurs with lifetimes of 9.3 and 110 μs in the absence and presence of CO, respectively, followed by a faster cleavage of the dioxygen bond (4.8 μs), which generates the P intermediate with the concomitant oxidation of heme b. The second-order rate constant of 1 × 10(9) M(-1) s(-1) for O(2) binding to ba(3) in the absence of CO is 10 times greater than observed in the presence of CO as well as for the bovine heart enzyme. The O(2) bond cleavage in ba(3) of 4.8 μs is also approximately 10 times faster than in the bovine enzyme. These results suggest important structural differences between the accessibility of O(2) to the active site in ba(3) and the bovine enzyme, and they demonstrate that the photodissociated CO impedes access of dioxygen to the heme a(3) site in ba(3), making the CO flow-flash method inapplicable.

Electronic structure and dynamics of nitrosyl porphyrins

Nitric oxide (NO) is a signaling molecule employed to regulate essential physiological processes. Thus, there is great interest in understanding the interaction of NO with heme, which is found at the active site of many proteins that recognize NO, as well as those involved in its creation and elimination. We summarize what we have learned from investigations of the structure, vibrational properties, and conformational dynamics of NO complexes with ferrous porphyrins, as well as computational investigations in support of these experimental studies. Multitemperature crystallographic data reveal variations in the orientational disorder of the nitrosyl ligand. In some cases, equilibria among NO orientations can be analyzed using the van't Hoff relationship and the free energy and enthalpy of the solid-state transitions evaluated experimentally. Density functional theory (DFT) calculations predict that intrinsic barriers to torsional rotation are smaller than thermal energies at physiological temperatures, and the coincidence of observed NO orientations with minima in molecular mechanics potentials indicates that nonbonded interactions with other chemical groups control the conformational freedom of the bound NO. In favorable cases, reduced disorder at low temperatures exposes subtle structural features including off-axis tilting of the Fe-NO bond and anisotropy of the equatorial Fe-N bonds. We also present the results of nuclear resonance vibrational spectroscopy measurements on oriented single crystals of [Fe(TPP)(NO)] and [Fe(TPP)(1-MeIm)(NO)]. These describe the anisotropic vibrational motion of iron in five- and six-coordinate heme-NO complexes and reveal vibrations of all Fe-ligand bonds as well as low-frequency molecular distortions associated with the doming of the heme upon ligand binding. A quantitative comparison with predicted frequencies, amplitudes, and directions facilitates identification of the vibrational modes but also suggests that commonly used DFT functionals are not fully successful at capturing the trans interaction between the axial NO and imidazole ligands. This supports previous conclusions that heme-NO complexes exhibit an unusual degree of variability with respect to the computational method, and we speculate that this variability hints at a genuine electronic instability that a protein can exploit to tune its reactivity. We anticipate that ongoing characterization of heme-NO complexes will deepen our understanding of their structure, dynamics, and reactivity.

Reductive coupling of nitrogen monoxide (*NO) facilitated by heme/copper complexes

The interactions of nitrogen monoxide (*NO; nitric oxide) with transition metal centers continue to be of great interest, in part due to their importance in biochemical processes. Here, we describe *NO((g)) reductive coupling chemistry of possible relevance to that process (i.e., nitric oxide reductase (NOR) biochemistry), which occurs at the heme/Cu active site of cytochrome c oxidases (CcOs). In this report, heme/Cu/*NO((g)) activity is studied using 1:1 ratios of heme and copper complex components, (F(8))Fe (F(8) = tetrakis(2,6-difluorophenyl)porphyrinate(2-)) and [(tmpa)Cu(I)(MeCN)](+) (TMPA = tris(2-pyridylmethyl)amine). The starting point for heme chemistry is the mononitrosyl complex (F(8))Fe(NO) (lambda(max) = 399 (Soret), 541 nm in acetone). Variable-temperature (1)H and (2)H NMR spectra reveal a broad peak at delta = 6.05 ppm (pyrrole) at room temperature (RT), which gives rise to asymmetrically split pyrrole peaks at 9.12 and 8.54 ppm at -80 degrees C. A new heme dinitrosyl species, (F(8))Fe(NO)(2), obtained by bubbling (F(8))Fe(NO) with *NO((g)) at -80 degrees C, could be reversibly formed, as monitored by UV-vis (lambda(max) = 426 (Soret), 538 nm in acetone), EPR (silent), and NMR spectroscopies; that is, the mono-NO complex was regenerated upon warming to RT. (F(8))Fe(NO)(2) reacts with [(tmpa)Cu(I)(MeCN)](+) and 2 equiv of acid to give [(F(8))Fe(III)](+), [(tmpa)Cu(II)(solvent)](2+), and N(2)O((g)), fitting the stoichiometric *NO((g)) reductive coupling reaction: 2*NO((g)) + Fe(II) + Cu(I) + 2H(+) --> N(2)O((g)) + Fe(III) + Cu(II) + H(2)O, equivalent to one enzyme turnover. Control reaction chemistry shows that both iron and copper centers are required for the NOR-type chemistry observed and that, if acid is not present, half the *NO is trapped as a (F(8))Fe(NO) complex, while the remaining nitrogen monoxide undergoes copper complex promoted disproportionation chemistry. As part of this study, [(F(8))Fe(III)]SbF(6) was synthesized and characterized by X-ray crystallography, along with EPR (77 K: g = 5.84 and 6.12 in CH(2)Cl(2) and THF, respectively) and variable-temperature NMR spectroscopies. These structural and physical properties suggest that at RT this complex consists of an admixture of high and intermediate spin states.

Accommodation of two diatomic molecules in cytochrome bo: insights into NO reductase activity in terminal oxidases

Bacterial heme-copper terminal oxidases react quickly with NO to form a heme-nitrosyl complex, which, in some of these enzymes, can further react with a second NO molecule to produce N(2)O. Previously, we characterized the heme a(3)-NO complex formed in cytochrome ba(3) from Thermus thermophilus and the product of its low-temperature illumination. We showed that the photolyzed NO group binds to Cu(B)(I) to form an end-on NO-Cu(B) or a side-on copper-nitrosyl complex, which is likely to represent the binding characteristics of the second NO molecule at the heme-copper active site. Here we present a comparative study with cytochrome bo(3) from Escherichia coli. Both terminal oxidases are shown to catalyze the same two-electron reduction of NO to N(2)O. The EPR and resonance Raman signatures of the heme o(3)-NO complex are comparable to those of the a(3)-NO complex. However, low-temperature FTIR experiments reveal that photolysis of the heme o(3)-NO complex does not produce a Cu(B)-nitrosyl complex, but that instead, the NO remains unbound in the active-site cavity. Additional FTIR photolysis experiments on the heme-nitrosyl complexes of these terminal oxidases, in the presence of CO, demonstrate that an [o(3)-NO.OC-Cu(B)] tertiary complex can form in bo(3) but not in ba(3). We assign these differences to a greater iron-copper distance in the reduced form of bo(3) compared to that of ba(3). Because this difference in metal-metal distance does not appear to affect the NO reductase activity, our results suggest that the coordination of the second NO to Cu(B) is not an essential step of the reaction mechanism.

Fourier transform infrared characterization of a CuB-nitrosyl complex in cytochrome ba3 from Thermus thermophilus: relevance to NO reductase activity in heme-copper terminal oxidases

The two heme-copper terminal oxidases of Thermus thermophilus have been shown to catalyze the two-electron reduction of nitric oxide (NO) to nitrous oxide (N2O) [Giuffre, A.; Stubauer, G.; Sarti, P.; Brunori, M.; Zumft, W. G.; Buse, G.; Soulimane, T. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 14718-14723]. While it is well-established that NO binds to the reduced heme a3 to form a low-spin heme {FeNO}7 species, the role CuB plays in the binding of the second NO remains unclear. Here we present low-temperature FTIR photolysis experiments carried out on the NO complex formed by addition of NO to fully reduced cytochrome ba3. Low-temperature UV-vis, EPR, and RR spectroscopies confirm the binding of NO to the heme a3 and the efficiency of the photolysis at 30 K. The nu(NO) modes from the light-induced FTIR difference spectra are isolated from other perturbed vibrations using 15NO and 15N18O. The nu(N-O)a3 is observed at 1622 cm-1, and upon photolysis, it is replaced by a new nu(N-O) at 1589 cm-1 assigned to a CuB-nitrosyl complex. This N-O stretching frequency is more than 100 cm-1 lower than those reported for Cu-NO models with three N-ligands and for CuB+-NO in bovine aa3. Because the UV-vis and RR data do not support a bridging configuration between CuB and heme a3 for the photolyzed NO, we assign the exceptionally low nu(NO) to an O-bound (eta1-O) or a side-on (eta2-NO) CuB-nitrosyl complex. From this study, we propose that, after binding of a first NO molecule to the heme a3 of fully reduced Tt ba3, the formation of an N-bound {CuNO}11 is prevented, and the addition of a second NO produces an O-bond CuB-hyponitrite species bridging CuB and Fea3. In contrast, bovine cytochrome c oxidase is believed to form an N-bound CuB-NO species; the [{FeNO}7{CuNO}11] complex is suggested here to be an inhibitory complex.

Spectroscopic characterization of heme iron-nitrosyl species and their role in NO reductase mechanisms in diiron proteins

Nitric oxide (NO) plays an important role in cell signalling and in the mammalian immune response to infection. On its own, NO is a relatively inert radical, and when it is used as a signalling molecule, its concentration remains within the picomolar range. However, at infection sites, the NO concentration can reach the micromolar range, and reactions with other radical species and transition metals lead to a broad toxicity. Under aerobic conditions, microorganisms cope with this nitrosative stress by oxidizing NO to nitrate (NO3−). Microbial hemoglobins play an essential role in this NO-detoxifying process. Under anaerobic conditions, detoxification occurs via a 2-electron reduction of two NO molecules to N2O. In many bacteria and archaea, this NO-reductase reaction is catalyzed by diiron proteins. Despite the importance of this reaction in providing microorganisms with a resistance to the mammalian immune response, its mechanism remains ill-defined. Because NO is an obligatory intermediate of the denitrification pathway, respiratory NO reductases also provide resistance to toxic concentrations of NO. This family of enzymes is the focus of this review. Respiratory NO reductases are integral membrane protein complexes that contain a norB subunit evolutionarily related to subunit I of cytochrome c oxidase (Cc O). NorB anchors one high-spin heme b3 and one non-heme iron known as FeB, i.e ., analogous to CuB in Cc O. A second group of diiron proteins with NO-reductase activity is comprised of the large family of soluble flavoprotein A found in strict and facultative anaerobic bacteria and archaea. These soluble detoxifying NO reductases contain a non-heme diiron cluster with a Fe–Fe distance of 3.4 Å and are only briefly mentioned here as a promising field of research. This article describes possible mechanisms of NO reduction to N2O in denitrifying NO-reductase (NOR) proteins and critically reviews recent experimental results. Relevant theoretical model calculations and spectroscopic studies of the NO-reductase reaction in heme/copper terminal oxidases are also overviewed.