Principles of structure, bonding, and reactivity for metal nitrosyl complexes

Enemark, John H.; Feltham, Robert D.

doi:10.1016/s0010-8545(00)80259-3

Cited by 1,605 publications

(1,374 citation statements)

References 106 publications

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“…The reduction of cyt c was monitored by the increase in absorption at 550 nm (e 550 = 19,600 M −1 ·cm −1 (39). These experiments show that 3 electrons are required to oxidize Fe III -NH 2 OH to the 455-nm intermediate, suggesting an {FeNO} 6 or Fe II -ONOH species (33). Consistent with this observation, treatment of Fe III -OH 2 with NO supplied either as the gas or via the NO donor PROLI-NONOate [1-(hydroxy-NNOazoxy)-L-proline] produces an EPR-silent species with an absorption spectrum identical to the absorption spectrum of the 455-nm intermediate (Fig.…”

Section: Resultsmentioning

confidence: 83%

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Nitrosomonas europaea cytochrome P460 is a direct link between nitrification and nitrous oxide emission

Caranto

Vilbert

Lancaster

2016

Proc. Natl. Acad. Sci. U.S.A.

170

212

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Ammonia oxidizing bacteria (AOB) are major contributors to the emission of nitrous oxide (N 2 O). It has been proposed that N 2 O is produced by reduction of NO. Here, we report that the enzyme cytochrome (cyt) P460 from the AOB Nitrosomonas europaea converts hydroxylamine (NH 2 OH) quantitatively to N 2 O under anaerobic conditions. Previous literature reported that this enzyme oxidizes NH 2 OH to nitrite (NO − 2 ) under aerobic conditions. Although we observe NO − 2 formation under aerobic conditions, its concentration is not stoichiometric with the NH 2 OH concentration. By contrast, under anaerobic conditions, the enzyme uses 4 oxidizing equivalents (eq) to convert 2 eq of NH 2 OH to N 2 O. Enzyme kinetics coupled to UV/visible absorption and electron paramagnetic resonance (EPR) spectroscopies support a mechanism in which an Fe III -NH 2 OH adduct of cyt P460 is oxidized to an {FeNO} 6 unit. This species subsequently undergoes nucleophilic attack by a second equivalent of NH 2 OH, forming the N-N bond of N 2 O during a bimolecular, rate-determining step. We propose that NO − 2 results when nitric oxide (NO) dissociates from the {FeNO} 6 intermediate and reacts with dioxygen. Thus, NO − 2 is not a direct product of cyt P460 activity. We hypothesize that the cyt P460 oxidation of NH 2 OH contributes to NO and N 2 O emissions from nitrifying microorganisms. possesses a global warming potential nearly 300-fold greater than carbon dioxide (1). Atmospheric N 2 O concentrations have increased ∼120% since the preindustrial era, largely due to the widespread use of fertilizers required to produce sustenance for humans and livestock. N 2 O is a byproduct of the microbial metabolism of fertilizer components, including ammonia (NH 3 ) and nitrate (NO − 3 ); consequently, agricultural soils account for an estimated 60-75% of global N 2 O emissions. The metabolic pathway by which microorganisms oxidize NH 3 , nitrification, occurs in two phases, both of which are mediated by autotrophic microorganisms. In the first, NH 3 -oxidizing bacteria (AOB) or archaea (AOA) oxidize NH 3 to nitrite (NO − 2 ). In the second, NO − 2 is subsequently oxidized to NO − 3 by NO − 2 -oxidizing bacteria. NH 3 -oxidizing microbes contribute substantially to global N 2 O emissions, whereas NO − 2 -oxidizing bacteria produce negligible N 2 O (2, 3). AOB are proposed to emit N 2 O either as a byproduct of the nitrification pathway or as a product of the nitrifier denitrification pathway (i.e., the reduction of NO − 2 ) (4-6). Nitrification of NH 3 to NO − 2 occurs in two steps (7,8). The first step is catalyzed by NH 3 monooxygenase, which uses copper (Cu) and dioxygen (O 2 ) to hydroxylate NH 3 to hydroxylamine (NH 2 OH) (9). In AOB, the second step is thought to be the four-electron oxidation of NH 2 OH to NO − 2 by NH 2 OH oxidoreductase (HAO). HAO is a multiheme enzyme with eight c-type hemes per subunit: seven are electron transfer cofactors, and the eighth is the so-called P460 active site that contains a unique tyrosine cross-link to the ...

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Section: Resultsmentioning

confidence: 83%

“…Treatment of cyt P460 with 100 mM NH 2 OH results in the disappearance of the S = 5 / 2 Fe III -OH 2 signal and the appearance of two rhombic S = 1 / 2 EPR signals ( (33). This {FeNO} 7 species can be independently generated by treating cyt P460 with the HNO donor, disodium diazen-1-ium-1,2,2 triolate (Na 2 N 2 O 3 , Angeli's salt).…”

Section: Resultsmentioning

confidence: 99%

Nitrosomonas europaea cytochrome P460 is a direct link between nitrification and nitrous oxide emission

Caranto

Vilbert

Lancaster

2016

Proc. Natl. Acad. Sci. U.S.A.

170

212

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“…2b) was interpreted as a combination of a (bme-dach)Fe(NO) unit, a known product of [(bme-dach)Fe] 2 dimer cleavage by free, gaseous NO ref. 18), that serves as a metallodithiolate donor ligand to an {Fe(NO) 2 } 9 unit (Enemark-Feltham notation 19 ). Such an electron count represents the oxidized form of a dinitrosyl iron complex (DNIC).…”

Section: Synthesis and Characterizationmentioning

confidence: 99%

Redox active iron nitrosyl units in proton reduction electrocatalysis

et al. 2014

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Base metal, molecular catalysts for the fundamental process of conversion of protons and electrons to dihydrogen, remain a substantial synthetic goal related to a sustainable energy future. Here we report a diiron complex with bridging thiolates in the butterfly shape of the 2Fe2S core of the [FeFe]-hydrogenase active site but with nitrosyl rather than carbonyl or cyanide ligands. This binuclear [(NO)Fe(N 2 S 2 )Fe(NO) 2 ] þ complex maintains structural integrity in two redox levels; it consists of a (N 2 S 2 )Fe(NO) complex (N 2 S 2 ¼ N,N 0 -bis(2-mercaptoethyl)-1,4-diazacycloheptane) that serves as redox active metallodithiolato bidentate ligand to a redox active dinitrosyl iron unit, Fe(NO) 2 . Experimental and theoretical methods demonstrate the accommodation of redox levels in both components of the complex, each involving electronically versatile nitrosyl ligands. An interplay of orbital mixing between the Fe(NO) and Fe(NO) 2 sites and within the iron nitrosyl bonds in each moiety is revealed, accounting for the interactions that facilitate electron uptake, storage and proton reduction.

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“…Under Enemark͞Feltham notation (33), these species are conveniently represented as {FeNO} 7 , which treats the iron-nitrosyl as a bracketed unit with a superscript to represent the total number of Fe d and NO * electrons. Detailed spectroscopic studies and electronic structure calculations for complexes containing S ϭ 3͞2 {FeNO} 7 units (31,32) have shown that the ground-and excited-state properties are best rationalized in terms of a limiting formulation involving high-spin Fe 3ϩ (S ϭ 5͞2) antiferromagnetically coupled to NO Ϫ (S ϭ 1).…”

Section: Eprmentioning

confidence: 99%

Nitric oxide binding at the mononuclear active site of reduced Pyrococcus furiosus superoxide reductase

Clay

Cosper

Jenney

et al. 2003

Proc. Natl. Acad. Sci. U.S.A.

133

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O ver the past 4 yr, evidence has accumulated for a novel pathway for detoxification of reactive oxygen species that is specific for anaerobic and microaerophilic microorganisms (1-3). The key enzyme in this pathway is superoxide reductase (SOR), which catalyzes the reduction of superoxide to hydrogen peroxide (4), with rubredoxin as the probable physiological electron donor (2, 5). Although all SORs have a ␤-barrel domain containing a unique type of mononuclear Fe center that serves as the site for superoxide reduction (6, 7), some have an additional N-terminal domain that contains an intrinsic rubredoxin-like mononuclear Fe site, which is ligated by four cysteines in a distorted tetrahedral arrangement analogous to that found in desulforedoxin (6). These two classes are conveniently referred to as 1Fe-and 2Fe-SORs, but they are also known by their trivial names, neelaredoxins and desulfoferrodoxins, respectively.Spectroscopic and crystallographic studies of the 1Fe-SOR from Pyrococcus furiosus (7-9) and the 2Fe-SOR from Desulfovibrio desulfuricans (6, 10, 11) have shown that the mononuclear iron active site is ligated by four equatorial histidines (3 N and 1␦N) and one axial cysteinate in a square-pyramidal arrangement. The sixth coordination site appears to be occupied by a monodentate glutamate in the oxidized state but is vacant or occupied by a weakly coordinated water molecule in the reduced state, thereby providing a site for superoxide binding and reduction. These structural studies, combined with recent mutagenesis and pulse radiolysis kinetic results (12-16), have led to the proposal for the catalytic mechanism shown in Fig. 1 Characterization of enzymatic intermediates is required for both detailed understanding of the mechanism of SOR and addressing the key questions of how and why the SOR active site preferentially catalyzes reduction rather than dismutation of superoxide. However, these intermediates are short lived and difficult to study experimentally. Nitric oxide (NO) has been extensively used as a substrate analog of molecular oxygen to form stable nitrosyl derivatives that provide insight into oxygen transport and activation intermediates in many heme (18-21) and nonheme (22-29) iron enzymes. Hence, we report here the formation and spectroscopic characterization of a stable NO-bound derivative of the reduced 1Fe-SOR from P. furiosus using the combination of EPR, UV-visible absorption, and variable-temperature, variable-field magnetic CD (VTVH MCD), resonance Raman, and Fourier transform IR (FTIR) spectroscopies. The structural and electronic characterization of the NO adduct of SOR facilitates understanding of how the active site is tuned for oxidative addition of superoxide and release rather than intraligand cleavage of the peroxide product. Materials and MethodsBiochemical Techniques and Sample Preparation. Recombinant natural abundance and 34 S globally enriched P. furiosus SOR was This paper was submitted directly (Track II) to the PNAS office.Abbreviations: SOR, superoxide reductase; V...

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Principles of structure, bonding, and reactivity for metal nitrosyl complexes

Cited by 1,605 publications

References 106 publications

Nitrosomonas europaea cytochrome P460 is a direct link between nitrification and nitrous oxide emission

Nitrosomonas europaea cytochrome P460 is a direct link between nitrification and nitrous oxide emission

Redox active iron nitrosyl units in proton reduction electrocatalysis

Nitric oxide binding at the mononuclear active site of reduced Pyrococcus furiosus superoxide reductase

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