Theoretical study of the reduction of nitric oxide in an A-type flavoprotein

NO Reduction to N2O Triggered by a Dinuclear Dinitrosyl Iron Complex via the Associated Pathways of Hyponitrite Formation and NO Disproportionation

In spite of the comprehensive study of the metal-mediated conversion of NO to N2O disclosing the conceivable processes/mechanism in biological and biomimetic studies, in this study, the synthesis cycles and mechanism of NO reduction to N2O triggered by the electronically localized dinuclear {Fe(NO)2}10-{Fe(NO)2}9 dinitrosyl iron complex (DNIC) [Fe(NO)2(μ-bdmap)Fe(NO)2(THF)] (1) (bdmap = 1,3- bis(dimethylamino)-2-propanolate) were investigated in detail. Reductive conversion of NO to N2O triggered by complex 1 in the presence of exogenous ·NO occurs via the simultaneous formation of hyponitrite-bound {[Fe2(NO)4(μ-bdmap)]2(κ4-N2O2)} (2) and [NO2]--bridged [Fe2(NO)4(μ-bdmap)(μ-NO2)] (3) (NO disproportionation yielding N2O and complex 3). EPR/IR spectra, single-crystal X-ray diffraction, and the electrochemical study uncover the reversible redox transformation of {Fe(NO)2}9-{Fe(NO)2}9 [Fe2(NO)4(μ-bdmap)(μ-OC4H8)]+ (7) ↔ {Fe(NO)2}10-{Fe(NO)2}9 1 ↔ {Fe(NO)2}10-{Fe(NO)2}10 [Fe(NO)2(μ-bdmap)Fe(NO)2]- (6) and characterize the formation of complex 1. Also, the synthesis study and DFT computation feature the detailed mechanism of electronically localized {Fe(NO)2}10-{Fe(NO)2}9 DNIC 1 reducing NO to N2O via the associated hyponitrite-formation and NO-disproportionation pathways. Presumably, the THF-bound {Fe(NO)2}9 unit of electronically localized {Fe(NO)2}10-{Fe(NO)2}9 complex 1 served as an electron buffering reservoir for accommodating electron redistribution, and the {Fe(NO)2}10 unit of complex 1 acted as an electron-transfer channel to drive exogeneous ·NO coordination to yield proposed relay intermediate κ2-N,O-[NO]--bridged [Fe2(NO)4(μ-bdmap)(μ-NO)] (A) for NO reduction to N2O.

NO Coupling by Nonclassical Dinuclear Dinitrosyliron Complexes to Form N2O Dictated by Hemilability

Selective coupling of NO by a nonclassical dinuclear dinitrosyliron complex (D-DNIC) to form N₂O is reported. The coupling is facilitated by the pyridinediimine (PDI) ligand scaffold, which enables the necessary denticity changes to produce mixed-valent, electron-deficient tethered DNICs. One-electron oxidation of the [{Fe(NO)₂}]₂^10/10 complex Fe₂(^PyrrPDI)(NO)₄ (4) results in NO coupling to form N₂O via the mixed-valent {[Fe(NO)₂]₂}^9/10 species, which possesses an electron-deficient four-coordinate {Fe(NO)₂}¹⁰ site, crucial in N-N bond formation. The hemilability of the PDI scaffold dictates the selectivity in N-N bond formation because stabilization of the five-coordinate {Fe(NO)₂}⁹ site in the mixed-valent [{Fe(NO)₂}]₂^9/10 species, [Fe₂(^Pyr2PDI)(NO)₄][PF₆] (6), does not result in an electron-deficient, four-coordinate {Fe(NO)₂}¹⁰ site, and hence no N-N coupling is observed.

A Monohydrosulfidodinitrosyldiiron Complex That Generates N2O as a Model for Flavodiiron Nitric Oxide Reductases: Reaction Mechanism and Electronic Structure

Flavodiiron nitric oxide reductases (FNORs) protect microbes from nitrosative stress under anaerobic conditions by mediating the reduction of nitric oxide (NO) to nitrous oxide (N2O). The proposed mechanism for the catalytic reduction of NO by FNORs involves a dinitrosyldiiron intermediate with a [hs-{FeNO}7]2 formulation, which produces N2O and a diferric species. Moreover, both NO and hydrogen sulfide (H2S) have been implicated in several similar physiological functions in biology and are also known to cross paths in cell signaling. Here we report the synthesis, spectroscopic and theoretical characterization, and N2O production activity of an unprecedented monohydrosulfidodinitrosyldiiron compound, with a [(HS)hs-{FeNO}7/hs-{FeNO}7] formulation, that models the key dinitrosyl intermediate of FNORs. The generation of N2O from this unique compound follows a semireduced pathway, where one-electron reduction generates a reactive hs-{FeNO}8 center via the occupation of an Fe-NO antibonding orbital. In contrast to the well-known reactivity of H2S and NO, the coordinated hydrosulfide remains unreactive toward NO and acts only as a spectator ligand during the NO reduction process.

A Nonheme Mononuclear {FeNO}7 Complex that Produces N2 O in the Absence of an Exogenous Reductant

A new nonheme iron(II) complex, Fe^II (Me₃ TACN)((OSi^Ph2 )₂ O) (1), is reported. Reaction of 1 with NO_(g) gives a stable mononitrosyl complex Fe(NO)(Me₃ TACN)((OSi^Ph2 )₂ O) (2), which was characterized by Mössbauer (δ=0.52 mm s^-1 , |ΔE_Q |=0.80 mm s^-1 ), EPR (S=3/2), resonance Raman (RR) and Fe K-edge X-ray absorption spectroscopies. The data show that 2 is an {FeNO}⁷ complex with an S=3/2 spin ground state. The RR spectrum (λ_exc =458 nm) of 2 combined with isotopic labeling (¹⁵ N, ¹⁸ O) reveals ν(N-O)=1680 cm^-1 , which is highly activated, and is a nearly identical match to that seen for the reactive mononitrosyl intermediate in the nonheme iron enzyme FDPnor (ν(NO)=1681 cm^-1 ). Complex 2 reacts rapidly with H₂ O in THF to produce the N-N coupled product N₂ O, providing the first example of a mononuclear nonheme iron complex that is capable of converting NO to N₂ O in the absence of an exogenous reductant.

Reduction of NO by diiron complexes in relation to flavodiiron nitric oxide reductases

Reduction of nitric oxide (NO) to nitrous oxide (N₂O) is associated with immense biological and health implications. Flavodiiron nitric oxide reductases (FNORs) are diiron containing enzymes that catalyze the two electron reduction of NO to N₂O and help certain pathogenic bacteria to survive under "nitrosative stress" in anaerobic growth conditions. Consequently, invading bacteria can proliferate inside the body of mammals by bypassing the immune defense mechanism involving NO and may thus lead to harmful infections. Various mechanisms, namely the direct reduction, semireduction, superreduction and hyponitrite mechanisms, have been proposed over time for catalytic NO reduction by FNORs. Model studies in relation to the diiron active site of FNORs have immensely helped to replicate the minimal structure-reactivity relationship and to understand the mechanism of NO reduction. A brief overview of the FNOR activity and the proposed reaction mechanisms followed by a systematic description and detailed analysis of the model studies is presented, which describes the development in the area of NO reduction by diiron complexes and its implications. A great deal of successful modeling chemistry as well as the shortcomings related to the synthesis and reactivity studies is discussed in detail. Finally, future prospects in this particular area of research are proposed, which in due course may bring more clarity in the understanding of this important redox reaction.

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

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

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.

How superoxide reductases and flavodiiron proteins combat oxidative stress in anaerobes

Microbial anaerobes are exposed in the natural environment and in their hosts, even if transiently, to fluctuating concentrations of oxygen and its derived reactive species, which pose a considerable threat to their anoxygenic lifestyle. To counteract these stressful conditions, they contain a multifaceted array of detoxifying systems that, in conjugation with cellular repairing mechanisms and in close crosstalk with metal homeostasis, allow them to survive in the presence of O₂ and reactive oxygen species. Some of these systems are shared with aerobes, but two families of enzymes emerged more recently that, although not restricted to anaerobes, are predominant in anaerobic microbes. These are the iron-containing superoxide reductases, and the flavodiiron proteins, endowed with O₂ and/or NO reductase activities, which are the subject of this Review. A detailed account of their physicochemical, physiological and molecular mechanisms will be presented, highlighting their unique properties in allowing survival of anaerobes in oxidative stress conditions, and comparing their properties with the most well-known detoxifying systems.

Spectroscopy and DFT Calculations of Flavo-Diiron Nitric Oxide Reductase Identify Bridging Structures of NO-Coordinated Diiron Intermediates

Flavo-diiron proteins (FDPs) are widespread in anaerobic bacteria, archaea, and protozoa, where they serve as the terminal components of dioxygen and nitric oxide reductive scavenging pathways. FDPs contain an N,O-ligated diiron site adjacent to a flavin mononucleotide (FMN) cofactor. The diiron site is structurally similar to those in hemerythrin, ribonucleotide reductase, and methane monooxygenase. However, only FDPs turn over NO to N2O at significant rates and yields. Previous studies revealed sequential binding of two NO molecules to the diferrous site, forming mono- and dinitrosyl intermediates leading to N2O formation. In the present work, these mono- and dinitrosyl intermediates have been characterized by EPR and Mössbauer spectroscopies and DFT calculations. Our results show that the iron proximal to the cofactor binds the first NO to form the diiron mononitrosyl complex, implying the iron distal to the FMN binds the second NO to form the diiron dinitrosyl intermediate. The exchange-coupling constants, J (H = JS1·S2), were found to differ substantially, +17 cm-1 for the diiron mononitrosyl and +60 cm-1 for the diiron dinitrosyl. Notwithstanding this large difference, our findings indicate retention of at least one hydroxo bridge throughout the NOR catalytic cycle. The Mossbauer hyperfine parameters and DFT calculations confirmed a semibridging NO- ligand in the mononitrosyl intermediate that lowers the exchange parameter. The DFT calculations on the dinitrosyl intermediate suggest a contribution to J from direct exchange between the S = 1 spins on the NO- ligands, which could initiate N-N bond formation. Our results provide insight into why FDPs are the only known nonheme diiron enzymes that competently turn over NO to N2O.

A Nonheme, High-Spin {FeNO}8 Complex that Spontaneously Generates N2O

One-electron reduction of [Fe(NO)-(N3PyS)]BF₄ (1) leads to the production of the metastable nonheme {FeNO}⁸ complex, [Fe(NO)(N3PyS)] (3). Complex 3 is a rare example of a high-spin (S = 1) {FeNO}⁸ and is the first example, to our knowledge, of a mononuclear nonheme {FeNO}⁸ species that generates N₂O. A second, novel route to 3 involves addition of Piloty's acid, an HNO donor, to an Fe^II precursor. This work provides possible new insights regarding the mechanism of nitric oxide reductases.

The dual function of flavodiiron proteins: oxygen and/or nitric oxide reductases

Flavodiiron proteins have emerged in the last two decades as a newly discovered family of oxygen and/or nitric oxide reductases widespread in the three life domains, and present in both aerobic and anaerobic organisms. Herein we present the main features of these fascinating enzymes, with a particular emphasis on the metal sites, as more appropriate for this special issue in memory of the exceptional bioinorganic scientist R. J. P. Williams who pioneered the notion of (metal) element availability-driven evolution. We also compare the flavodiiron proteins with the other oxygen and nitric oxide reductases known until now, highlighting how throughout evolution Nature arrived at different solutions for similar functions, in some cases adding extra features, such as energy conservation. These enzymes are an example of the (bioinorganic) unpredictable diversity of the living world.

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.

Recent Advances in Multinuclear Metal Nitrosyl Complexes

The coordination chemistry of metal nitrosyls has expanded rapidly in the past decades due to major advances of nitric oxide and its metal compounds in biology. This review article highlights advances made in the area of multinuclear metal nitrosyl complexes, including Roussin's salts and their ester derivatives from 2003 to present. The review article focuses on isolated multinuclear metal nitrosyl complexes and is organized into different sections by the number of metal centers and bridging ligands.

Light-induced N₂O production from a non-heme iron-nitrosyl dimer

Two non-heme iron–nitrosyl species, [Fe₂(N-Et-HPTB)(O₂CPh)(NO)₂](BF₄)₂ (1a) and [Fe₂(N-Et-HPTB)(DMF)₂(NO)(OH)](BF₄)₃ (2a), are characterized by FTIR and resonance Raman spectroscopy. Binding of NO is reversible in both complexes, which are prone to NO photolysis under visible light illumination. Photoproduction of N₂O occurs in high yield for 1a but not 2a. Low-temperature FTIR photolysis experiments with 1a in acetonitrile do not reveal any intermediate species, but in THF at room temperature, a new {FeNO}⁷ species quickly forms under illumination and exhibits a ν(NO) vibration indicative of nitroxyl-like character. This metastable species reacts further under illumination to produce N₂O. A reaction mechanism is proposed, and implications for NO reduction in flavodiiron proteins are discussed.

The nitric oxide reductase mechanism of a flavo-diiron protein: identification of active-site intermediates and products

The unique active site of flavo-diiron proteins (FDPs) consists of a nonheme diiron-carboxylate site proximal to a flavin mononucleotide (FMN) cofactor. FDPs serve as the terminal components for reductive scavenging of dioxygen or nitric oxide to combat oxidative or nitrosative stress in bacteria, archaea, and some protozoan parasites. Nitric oxide is reduced to nitrous oxide by the four-electron reduced (FMNH2-Fe(II)Fe(II)) active site. In order to clarify the nitric oxide reductase mechanism, we undertook a multispectroscopic presteady-state investigation, including the first Mössbauer spectroscopic characterization of diiron redox intermediates in FDPs. A new transient intermediate was detected and determined to be an antiferromagnetically coupled diferrous-dinitrosyl (S = 0, [{FeNO}(7)]2) species. This species has an exchange energy, J ≥ 40 cm(-1) (JS1 ° S2), which is consistent with a hydroxo or oxo bridge between the two irons. The results show that the nitric oxide reductase reaction proceeds through successive formation of diferrous-mononitrosyl (S = ½, Fe(II){FeNO}(7)) and the S = 0 diferrous-dinitrosyl species. In the rate-determining process, the diferrous-dinitrosyl converts to diferric (Fe(III)Fe(III)) and by inference N2O. The proximal FMNH2 then rapidly rereduces the diferric site to diferrous (Fe(II)Fe(II)), which can undergo a second 2NO → N2O turnover. This pathway is consistent with previous results on the same deflavinated and flavinated FDP, which detected N2O as a product (Hayashi Biochemistry 2010, 49, 7040). Our results do not support other proposed mechanisms, which proceed either via "super-reduction" of [{FeNO}(7)]2 by FMNH2 or through Fe(II){FeNO}(7) directly to a diferric-hyponitrite intermediate. The results indicate that an S = 0 [{FeNO}(7)}]2 complex is a proximal precursor to N-N bond formation and N-O bond cleavage to give N2O and that this conversion can occur without redox participation of the FMN cofactor.

Nitric oxide generation from heme/copper assembly mediated nitrite reductase activity

Nitric oxide (NO) as a cellular signaling molecule and vasodilator regulates a range of physiological and pathological processes. Nitrite (NO2 (-)) is recycled in vivo to generate nitric oxide, particularly in physiologic hypoxia and ischemia. The cytochrome c oxidase binuclear heme a 3/CuB active site is one entity known to be responsible for conversion of cellular nitrite to nitric oxide. We recently reported that a partially reduced heme/copper assembly reduces nitrite ion, producing nitric oxide; the heme serves as the reductant and the cupric ion provides a Lewis acid interaction with nitrite, facilitating nitrite (N-O) bond cleavage (Hematian et al., J. Am. Chem. Soc. 134:18912-18915, 2012). To further investigate this nitrite reductase chemistry, copper(II)-nitrito complexes with tridentate and tetradentate ligands were used in this study, where either O,O'-bidentate or O-unidentate modes of nitrite binding to the cupric center are present. To study the role of the reducing ability of the ferrous heme center, two different tetraarylporphyrinate-iron(II) complexes, one with electron-donating para-methoxy peripheral substituents and the other with electron-withdrawing 2,6-difluorophenyl substituents, were used. The results show that differing modes of nitrite coordination to the copper(II) ion lead to differing kinetic behavior. Here, also, the ferrous heme is in all cases the source of the reducing equivalent required to convert nitrite to nitric oxide, but the reduction ability of the heme center does not play a key role in the observed overall reaction rate. On the basis of our observations, reaction mechanisms are proposed and discussed in terms of heme/copper heterobinuclear structures.

Non-heme mononitrosyldiiron complexes: importance of iron oxidation state in controlling the nature of the nitrosylated products

Mononitrosyldiiron complexes having either an [Fe(II)·{FeNO}(7)] or an [Fe(III)·{FeNO}(7)] core formulation have been synthesized by methods that rely on redox-state-induced differentiation of the diiron starting materials in an otherwise symmetrical dinucleating ligand environment. The synthesis, X-ray structures, Mössbauer spectroscopy, cyclic voltammetry, and dioxygen reactivity of [Fe(III)·{FeNO}(7)] are described.

The functional model complex [Fe2(BPMP)(OPr)(NO)2](BPh4)2 provides insight into the mechanism of flavodiiron NO reductases

Flavodiiron nitric oxide reductases (FNORs), found in many pathogenic bacteria, are able to detoxify NO by reducing it to N2O. In this way, FNORs equip these pathogens with immunity to NO, which is a central immune defense agent in humans. Hence, FNORs are thought to promote infection of the human body, leading to chronic diseases. Despite this importance of FNORs for bacterial pathogenesis, the mechanism of NO reduction by these enzymes is not well understood. Here we present the synthesis and spectroscopic characterization of the diiron dinitrosyl model complex [Fe2(BPMP)(OPr)(NO)2](BPh4)2. The crystal structure of this complex shows two end-on-coordinated {FeNO}(7) units that, based on spectroscopic and electrochemical results, are only weakly electronically coupled. Importantly, reduction of this complex by two electrons leads to the clean formation of N2O in quantitative yield. This complex therefore represents the first example of a functional model system for FNORs. The results provide key mechanistic insight into the mechanism of FNORs and, in particular, represent strong support for the proposed "super-reduced" mechanism for these enzymes.

Histidine ligand variants of a flavo-diiron protein: effects on structure and activities

Flavo-diiron proteins (FDPs) contain non-heme diiron and proximal flavin mononucleotide (FMN) active sites and function as terminal components of a nitric oxide reductase (NOR) and/or a four-electron dioxygen reductase (O(2)R). While most FDPs show similar structural, spectroscopic, and redox properties, O(2)R and NOR activities vary significantly among FDPs. A potential source of this variability is the iron ligation status of a conserved His residue that provides an iron ligand in all known FDP structures but one, where this His residue is rotated away from iron and replaced by a solvent ligand. In order to test the effect of this His ligation status, we changed this ligating His residue (H90) in Thermotoga maritima (Tm) FDP to either Asn or Ala. The wild-type Tm FDP shows significantly higher O(2)R than NOR activity. Single crystal X-ray crystallography revealed a remarkably conserved diiron site structure in the H90N and -A variants, differing mainly by either Asn or solvent coordination, respectively, in place of H90. The steady-state activities were minimally affected by the H90 substitutions, remaining significantly higher for O(2)R versus NOR. The pre-steady-state kinetics of the fully reduced FDP with O(2) were also minimally affected by the H90 substitutions. The results indicate that the coordination status of this His ligand does not significantly modulate the O(2)R or NOR activities, and that FDPs can retain these activities when the individual iron centers are differentiated by His ligand substitution. This differentiation may have implications for the O(2)R and NOR mechanisms of FDPs.

Vibrational analysis of mononitrosyl complexes in hemerythrin and flavodiiron proteins: relevance to detoxifying NO reductase

Flavodiiron proteins (FDPs) play important roles in the microbial nitrosative stress response in low-oxygen environments by reductively scavenging nitric oxide (NO). Recently, we showed that FMN-free diferrous FDP from Thermotoga maritima exposed to 1 equiv NO forms a stable diiron-mononitrosyl complex (deflavo-FDP(NO)) that can react further with NO to form N(2)O [Hayashi, T.; Caranto, J. D.; Wampler, D. A; Kurtz, D. M., Jr.; Moënne-Loccoz, P. Biochemistry 2010, 49, 7040-7049]. Here we report resonance Raman and low-temperature photolysis FTIR data that better define the structure of this diiron-mononitrosyl complex. We first validate this approach using the stable diiron-mononitrosyl complex of hemerythrin, Hr(NO), for which we observe a ν(NO) at 1658 cm(-1), the lowest ν(NO) ever reported for a nonheme {FeNO}(7) species. Both deflavo-FDP(NO) and the mononitrosyl adduct of the flavinated FPD (FDP(NO)) show ν(NO) at 1681 cm(-1), which is also unusually low. These results indicate that, in Hr(NO) and FDP(NO), the coordinated NO is exceptionally electron rich, more closely approaching the Fe(III)(NO(-)) resonance structure. In the case of Hr(NO), this polarization may be promoted by steric enforcement of an unusually small FeNO angle, while in FDP(NO), the Fe(III)(NO(-)) structure may be due to a semibridging electrostatic interaction with the second Fe(II) ion. In Hr(NO), accessibility and steric constraints prevent further reaction of the diiron-mononitrosyl complex with NO, whereas in FDP(NO) the increased nucleophilicity of the nitrosyl group may promote attack by a second NO to produce N(2)O. This latter scenario is supported by theoretical modeling [Blomberg, L. M.; Blomberg, M. R.; Siegbahn, P. E. J. Biol. Inorg. Chem. 2007, 12, 79-89]. Published vibrational data on bioengineered models of denitrifying heme-nonheme NO reductases [Hayashi, T.; Miner, K. D.; Yeung, N.; Lin, Y.-W.; Lu, Y.; Moënne-Loccoz, P. Biochemistry 2011, 50, 5939-5947 ] support a similar mode of activation of a heme {FeNO}(7) species by the nearby nonheme Fe(II).

Insights into the nitric oxide reductase mechanism of flavodiiron proteins from a flavin-free enzyme

Flavodiiron proteins (FDPs) catalyze reductive scavenging of dioxygen and nitric oxide in air-sensitive microorganisms. FDPs contain a distinctive non-heme diiron/flavin mononucleotide (FMN) active site. Alternative mechanisms for the nitric oxide reductase (NOR) activity consisting of either protonation of a diiron-bridging hyponitrite or "super-reduction" of a diferrous-dinitrosyl by the proximal FMNH(2) in the rate-determining step have been proposed. To test these alternative mechanisms, we examined a deflavinated FDP (deflavo-FDP) from Thermotoga maritima. The deflavo-FDP retains an intact diiron site but does not exhibit multiturnover NOR or O(2) reductase (O(2)R) activity. Reactions of the reduced (diferrous) deflavo-FDP with nitric oxide were examined by UV-vis absorption, EPR, resonance Raman, and FTIR spectroscopies. Anaerobic addition of nitric oxide up to one NO per diferrous deflavo-FDP results in formation of a diiron-mononitrosyl complex characterized by a broad S = (1)/(2 )EPR signal arising from antiferromagnetic coupling of an S = (3)/(2) {FeNO}(7) with an S = 2 Fe(II). Further addition of NO results in two reaction pathways, one of which produces N(2)O and the diferric site and the other of which produces a stable diiron-dinitrosyl complex. Both NO-treated and as-isolated deflavo-FDPs regain full NOR and O(2)R activities upon simple addition of FMN. The production of N(2)O upon addition of NO to the mononitrosyl deflavo-FDP supports the hyponitrite mechanism, but the concomitant formation of a stable diiron-dinitrosyl complex in the deflavo-FDP is consistent with a super-reduction pathway in the flavinated enzyme. We conclude that a diiron-mononitrosyl complex is an intermediate in the NOR catalytic cycle of FDPs.

A structure-consistent mechanism for dioxygen formation in photosystem II

In recent DFT studies a new mechanism for O-O bond formation at the oxygen evolving center (OEC) in photosystem II has been suggested. With the structure of the S(4) state required for that mechanism, the structures of the lower S states are investigated herein by adding protons and electrons. A model was used including the full amino acids for the ones ligating the OEC, and in which the backbone positions were held fixed from the X-ray structure. The only charged second-shell ligand Arg357 was also included. An optimized structure for the S(1) state was reached with a large similarity to one of those suggested by EXAFS. A full catalytic cycle was derived which can rationalize the structural relaxation in the S(2) to S(3) transition, and the fact that only an electron leaves in the transition before. Water is suggested to bind to the OEC in the S(2) to S(3), and S(4) to S(0) transitions. A new possibility for water exchange is suggested from the final energy diagram. The optimal O-O bond formation occurs between an oxygen radical and an oxo ligand. The alternative mechanism, where the oxygen radical reacts with an external water, has a barrier about 20 kcal mol(-1) higher.