Activity assays of NnlA homologs suggest the natural product N-nitroglycine is degraded by diverse bacteria.

Linear nitramines (R-N(R')NO2; R' = H or alkyl) are toxic compounds, some with environmental relevance, while others are rare natural product nitramines. One of these natural product nitramines is N-nitroglycine (NNG), which is produced by some Streptomyces strains and exhibits antibiotic activity towards Gram-negative bacteria. An NNG degrading heme enzyme, called NnlA, has recently been discovered in the genome of Variovorax sp. strain JS1663 (Vs NnlA). Evidence is presented that NnlA and therefore, NNG degradation activity is widespread. To achieve this objective, we characterized and tested the NNG degradation activity of five Vs NnlA homologs originating from bacteria spanning several classes and isolated from geographically distinct locations. E. coli transformants containing all five homologs converted NNG to nitrite. Four of these five homologs were isolated and characterized. Each isolated homolog exhibited similar oligomerization and heme occupancy as Vs NnlA. Reduction of this heme was shown to be required for NnlA activity in each homolog, and each homolog degraded NNG to glyoxylate, NO2- and NH4+ in accordance with observations of Vs NnlA. It was also shown that NnlA cannot degrade the NNG analog 2-nitroaminoethanol. The combined data strongly suggest that NnlA enzymes specifically degrade NNG and are found in diverse bacteria and environments. These results imply that NNG is also produced in diverse environments and NnlA may act as a detoxification enzyme to protect bacteria from exposure to NNG.

The hemerythrin-like diiron protein from Mycobacterium kansasii is a nitric oxide peroxidase.

The hemerythrin-like protein from Mycobacterium kansasii (Mka HLP) is a member of a distinct class of oxo-bridged diiron proteins that are found only in mycobacterial species that cause respiratory disorders in humans. Because it had been shown to exhibit weak catalase activity and a change in absorbance on exposure to nitric oxide (NO), the reactivity of Mka HLP toward NO was examined under a variety of conditions. Under anaerobic conditions, we found that NO was converted to nitrite (NO2-) via an intermediate, which absorbed light at 520 nm. Under aerobic conditions NO was converted to nitrate (NO3-). In each of these two cases, the maximum amount of nitrite or nitrate formed was at best stoichiometric with the concentration of Mka HLP. When incubated with NO and H2O2, we observed NO peroxidase activity yielding nitrite and water as reaction products. Steady-state kinetic analysis of NO consumption during this reaction yielded a Km for NO of 0.44 µM and a kcat/Km of 2.3 × 105 M-1s-1. This high affinity for NO is consistent with a physiological role for Mka HLP in deterring nitrosative stress. This is the first example of a peroxidase that uses an oxo-bridged diiron center and a rare example of a peroxidase utilizing NO as an electron donor and cosubstrate. This activity provides a mechanism by which the infectious Mycobacterium may combat against the cocktail of NO and superoxide (O2•-) generated by macrophages to defend against bacteria, as well as to produce NO2- to adapt to hypoxic conditions.

Synergistic Steric and Electronic Effects on the Photoredox Catalysis by a Multivariate Library of Titania Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) that display photoredox activity are attractive materials for sustainable photocatalysis. The ability to tune both their pore sizes and electronic structures based solely on the choice of the building blocks makes them amenable for systematic studies based on physical organic and reticular chemistry principles with high degrees of synthetic control. Here, we present a library of eleven isoreticular and multivariate (MTV) photoredox-active MOFs, UCFMOF-n, and UCFMTV-n-x% with a formula Ti6O9[links]3, where the links are linear oligo-p-arylene dicarboxylates with n number of p-arylene rings and x mol% of multivariate links containing electron-donating groups (EDGs). The average and local structures of UCFMOFs were elucidated from advanced powder X-ray diffraction (XRD) and total scattering tools, consisting of parallel arrangements of one-dimensional (1D) [Ti6O9(CO2)6]∞ nanowires connected through the oligo-arylene links with the topology of the edge-2-transitive rod-packed hex net. Preparation of an MTV library of UCFMOFs with varying link sizes and amine EDG functionalization enabled us to study both their steric (pore size) and electronic (highest occupied molecular orbital-lowest unoccupied molecular orbital, HOMO-LUMO, gap) effects on the substrate adsorption and photoredox transformation of benzyl alcohol. The observed relationship between the substrate uptake and reaction kinetics with the molecular traits of the links indicates that longer links, as well as increased EDG functionalization, exhibit impressive photocatalytic rates, outperforming MIL-125 by almost 20-fold. Our studies relating photocatalytic activity with pore size and electronic functionalization demonstrate how these are important parameters to consider when designing new MOF photocatalysts.

Reduction of a Heme Cofactor Initiates N-Nitroglycine Degradation by NnlA.

Linear nitramines are potentially carcinogenic environmental contaminants. The NnlA enzyme from Variovorax sp. strain JS1663 degrades the nitramine N-nitroglycine (NNG)-a natural product produced by some bacteria-to glyoxylate and nitrite (NO2-). Ammonium (NH4+) was predicted as the third product of this reaction. A source of nonheme FeII was shown to be required for initiation of NnlA activity. However, the role of this FeII for NnlA activity was unclear. This study reveals that NnlA contains a b-type heme cofactor. Reduction of this heme-either by a nonheme iron source or dithionite-is required to initiate NnlA activity. Therefore, FeII is not an essential substrate for holoenzyme activity. Our data show that reduced NnlA (FeII-NnlA) catalyzes at least 100 turnovers and does not require O2. Finally, NH4+ was verified as the third product, accounting for the complete nitrogen mass balance. Size exclusion chromatography showed that NnlA is a dimer in solution. Additionally, FeII-NnlA is oxidized by O2 and NO2- and stably binds carbon monoxide (CO) and nitric oxide (NO). These are characteristics shared with heme-binding PAS domains. Furthermore, a structural homology model of NnlA was generated using the PAS domain from Pseudomonas aeruginosa Aer2 as a template. The structural homology model suggested His73 is the axial ligand of the NnlA heme. Site-directed mutagenesis of His73 to alanine decreased the heme occupancy of NnlA and eliminated NNG activity, validating the homology model. We conclude that NnlA forms a homodimeric heme-binding PAS domain protein that requires reduction for initiation of the activity. IMPORTANCE Linear nitramines are potential carcinogens. These compounds result from environmental degradation of high-energy cyclic nitramines and as by-products of carbon capture technologies. Mechanistic understanding of the biodegradation of these compounds is critical to inform strategies for their remediation. Biodegradation of NNG by NnlA from Variovorax sp. strain JS 1663 requires nonheme iron, but its role is unclear. This study shows that nonheme iron is unnecessary. Instead, our study reveals that NnlA contains a heme cofactor, the reduction of which is critical for activating NNG degradation activity. These studies constrain the proposals for NnlA reaction mechanisms, thereby informing mechanistic studies of degradation of anthropogenic nitramine contaminants. In addition, these results will inform future work to design biocatalysts to degrade these nitramine contaminants.

Particle Swarm Fitting of Spin Hamiltonians: Magnetic Circular Dichroism of Reduced and NO-Bound Flavodiiron Protein.

A particle swarm optimization (PSO) algorithm is described for the fitting of ground-state spin Hamiltonian parameters from variable-temperature/variable-field (VTVH) magnetic circular dichroism (MCD) data. This PSO algorithm is employed to define the ground state of two catalytic intermediates from a flavodiiron protein (FDP), a class of enzymes with nitric oxide reductase activity. The bimetallic iron active site of this enzyme proceeds through a biferrous intermediate and a mixed ferrous-{FeNO}7 intermediate during the catalytic cycle, and the MCD spectra of these intermediates are presented and analyzed. The fits of the spin Hamiltonians are shown to provide important geometric and electronic insight into these species that is compared and contrasted with previous reports.

The Ferric-Superoxo Intermediate of the TxtE Nitration Pathway Resists Reduction, Facilitating Its Reaction with Nitric Oxide.

TxtE is a cytochrome P450 (CYP) homologue that mediates the nitric oxide (NO)-dependent direct nitration of l-tryptophan (Trp) to form 4-nitro-l-tryptophan (4-NO2-Trp). A recent report showed evidence that TxtE activity requires NO to react with a ferric-superoxo intermediate. Given this minimal mechanism, it is not clear how TxtE avoids Trp hydroxylation, a mechanism that also traverses the ferric-superoxo intermediate. To provide insight into canonical CYP intermediates that TxtE can access, electron coupling efficiencies to form 4-NO2-Trp under single- or limited-turnover conditions were measured and compared to steady-state efficiencies. As previously reported, Trp nitration by TxtE is supported by the engineered self-sufficient variant, TB14, as well as by reduced putidaredoxin. Ferrous (FeII) TxtE exhibits excellent electron coupling (70%), which is 50-fold higher than that observed under turnover conditions. In addition, two- or four-electron reduced TB14 exhibits electron coupling (∼6%) that is 2-fold higher than that of one-electron reduced TB14 (3%). The combined results suggest (1) autoxidation is the sole TxtE uncoupling pathway and (2) the TxtE ferric-superoxo intermediate cannot be reduced by these electron transfer partners. The latter conclusion is further supported by ultraviolet-visible absorption spectral time courses showing neither spectral nor kinetic evidence for reduction of the ferric-superoxo intermediate. We conclude that resistance of the ferric-superoxo intermediate to reduction is a key feature of TxtE that increases the lifetime of the intermediate and enables its reaction with NO and efficient nitration activity.

Nitric oxide is an obligate bacterial nitrification intermediate produced by hydroxylamine oxidoreductase.

Ammonia (NH3)-oxidizing bacteria (AOB) emit substantial amounts of nitric oxide (NO) and nitrous oxide (N2O), both of which contribute to the harmful environmental side effects of large-scale agriculture. The currently accepted model for AOB metabolism involves NH3 oxidation to nitrite (NO2-) via a single obligate intermediate, hydroxylamine (NH2OH). Within this model, the multiheme enzyme hydroxylamine oxidoreductase (HAO) catalyzes the four-electron oxidation of NH2OH to NO2- We provide evidence that HAO oxidizes NH2OH by only three electrons to NO under both anaerobic and aerobic conditions. NO2- observed in HAO activity assays is a nonenzymatic product resulting from the oxidation of NO by O2 under aerobic conditions. Our present study implies that aerobic NH3 oxidation by AOB occurs via two obligate intermediates, NH2OH and NO, necessitating a mediator of the third enzymatic step.

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

Ammonia oxidizing bacteria (AOB) are major contributors to the emission of nitrous oxide (N2O). It has been proposed that N2O is produced by reduction of NO. Here, we report that the enzyme cytochrome (cyt) P460 from the AOB Nitrosomonas europaea converts hydroxylamine (NH2OH) quantitatively to N2O under anaerobic conditions. Previous literature reported that this enzyme oxidizes NH2OH to nitrite ([Formula: see text]) under aerobic conditions. Although we observe [Formula: see text] formation under aerobic conditions, its concentration is not stoichiometric with the NH2OH concentration. By contrast, under anaerobic conditions, the enzyme uses 4 oxidizing equivalents (eq) to convert 2 eq of NH2OH to N2O. Enzyme kinetics coupled to UV/visible absorption and electron paramagnetic resonance (EPR) spectroscopies support a mechanism in which an FeIII-NH2OH adduct of cyt P460 is oxidized to an {FeNO}6 unit. This species subsequently undergoes nucleophilic attack by a second equivalent of NH2OH, forming the N-N bond of N2O during a bimolecular, rate-determining step. We propose that [Formula: see text] results when nitric oxide (NO) dissociates from the {FeNO}6 intermediate and reacts with dioxygen. Thus, [Formula: see text] is not a direct product of cyt P460 activity. We hypothesize that the cyt P460 oxidation of NH2OH contributes to NO and N2O emissions from nitrifying microorganisms.

Spectroscopy and DFT Calculations of a Flavo-diiron Enzyme Implicate New Diiron Site Structures.

Flavo-diiron proteins (FDPs) are non-heme iron containing enzymes that are widespread in anaerobic bacteria, archaea, and protozoa, serving as the terminal components to dioxygen and nitric oxide reductive scavenging pathways in these organisms. FDPs contain a dinuclear iron active site similar to that in hemerythrin, ribonucleotide reductase, and methane monooxygenase, all of which can bind NO and O2. However, only FDP competently turns over NO to N2O. Here, EPR and Mössbauer spectroscopies allow electronic characterization of the diferric and diferrous species of FDP. The exchange-coupling constant J (Hex = JS1·S2) was found to increase from +20 cm-1 to +32 cm-1 upon reduction of the diferric to the diferrous species, indicative of (1) at least one hydroxo bridge between the iron ions for both states and (2) a change to the diiron core structure upon reduction. In comparison to characterized diiron proteins and synthetic complexes, the experimental values were consistent with a dihydroxo bridged diferric core, which loses one hydroxo bridge upon reduction. DFT calculations of these structures gave values of J and Mössbauer parameters in agreement with experiment. Although the crystal structure shows a hydrogen bond between the iron bound aspartate and the bridging solvent molecule, the DFT calculations of structures consistent with the crystal structure gave calculated values of J incompatible with the spectroscopic results. We conclude that the crystal structure of the diferric state does not represent the frozen solution structure and that a mono-µ-hydroxo diferrous species is the catalytically functional state that reacts with NO and O2. The new EPR spectroscopic probe of the diferric state indicated that the diferric structure of FDP prior to and immediately after turnover with NO are flavin mononucleotide (FMN) dependent, implicating an additional proton transfer role for FMN in turnover of NO.

Dioxygen and nitric oxide scavenging by Treponema denticola flavodiiron protein: a mechanistic paradigm for catalysis.

Flavodiiron proteins (FDPs) contain a unique active site consisting of a non-heme diiron carboxylate site proximal to a flavin mononucleotide (FMN). FDPs serve as the terminal components for reductive scavenging of dioxygen (to water) or nitric oxide (to nitrous oxide), which combats oxidative or nitrosative stress in many bacteria. Characterizations of FDPs from spirochetes or from any oral microbes have not been previously reported. Here, we report characterization of an FDP from the anaerobic spirochete, Treponema (T.) denticola, which is associated with chronic periodontitis. The isolated T. denticola FDP exhibited efficient four-electron dioxygen reductase activity and lower but significant anaerobic nitric oxide reductase activity. A mutant T. denticola strain containing the inactivated FDP-encoding gene was significantly more air-sensitive than the wild-type strain. Single turnover reactions of the four-electron-reduced FDP (FMNH2-Fe(II)Fe(II)) (FDPred) with O2 monitored on the milliseconds to seconds time scale indicated initial rapid formation of a spectral feature consistent with a cis-µ-1,2-peroxo-diferric intermediate, which triggered two-electron oxidation of FMNH2. Reaction of FDPred with NO showed apparent cooperativity between binding of the first and second NO to the diferrous site. The resulting diferrous dinitrosyl complex triggered two-electron oxidation of the FMNH2. Our cumulative results on this and other FDPs indicate that smooth two-electron FMNH2 oxidation triggered by the FDPred/substrate complex and overall four-electron oxidation of FDPred to FDPox constitutes a mechanistic paradigm for both dioxygen and nitric oxide reductase activities of FDPs. Four-electron reductive O2 scavenging by FDPs could contribute to oxidative stress protection in many other oral bacteria.

A diferrous-dinitrosyl intermediate in the N2O-generating pathway of a deflavinated flavo-diiron protein.

Flavo-diiron proteins (FDPs) function as anaerobic nitric oxide scavengers in some microorganisms, catalyzing reduction of nitric to nitrous oxide. The FDP from Thermotoga maritima can be prepared in a deflavinated form with an intact diferric site (deflavo-FDP). Hayashi et al. [(2010) Biochemistry 49, 7040-7049] reported that reaction of NO with reduced deflavo-FDP produced substoichiometric N2O. Here we report a multispectroscopic approach to identify the iron species in the reactions of deflavo-FDP with NO. Mössbauer spectroscopy identified two distinct ferrous species after reduction of the antiferromagnetically coupled diferric site. Approximately 60% of the total ferrous iron was assigned to a diferrous species associated with the N2O-generating pathway. This pathway proceeds through successive diferrous-mononitrosyl (S = (1)/2 Fe(II){FeNO}(7)) and diferrous-dinitrosyl (S = 0 [{FeNO}(7)]2) species that form within ∼100 ms of mixing of the reduced protein with NO. The diferrous-dinitrosyl intermediate converted to an antiferromagnetically coupled diferric species that was spectroscopically indistinguishable from that in the starting deflavinated protein. These diiron species closely resembled those reported for the flavinated FDP [Caranto et al. (2014) J. Am. Chem. Soc. 136, 7981-7992], and the time scales of their formation and decay were consistent with the steady state turnover of the flavinated protein. The remaining ∼40% of ferrous iron was inactive in N2O generation but reversibly bound NO to give an S = (3)/2 {FeNO}(7) species. The results demonstrate that N2O formation in FDPs can occur via conversion of S = 0 [{FeNO}(7)]2 to a diferric form without participation of the flavin cofactor.

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.

Studies of iron(III) porphyrinates containing silanethiolate ligands.

The chemistry of several iron(III) porphyrinates containing silanethiolate ligands is described. The complexes are prepared by protonolysis reactions of silanethiols with the iron(III) precursors, [Fe(OMe)(TPP)] and [Fe(OH)(H2O)(TMP)] (TPP = dianion of meso-tetraphenylporphine; TMP = dianion of meso-tetramesitylporphine). Each of the compounds has been fully characterized in solution and the solid state. The stability of the silanethiolate complexes versus other iron(III) porphyrinate complexes containing sulfur-based ligands allows for an examination of their reactivity with several biologically relevant small molecules including H2S, NO, and 1-methylimidazole. Electrochemically, the silanethiolate complexes display a quasi-reversible one-electron oxidation event at potentials higher than that observed for an analogous arenethiolate complex. The behavior of these complexes versus other sulfur-ligated iron(III) porphyrinates is discussed.

Treponema denticola superoxide reductase: in vivo role, in vitro reactivities, and a novel [Fe(Cys)(4)] site.

In vitro and in vivo results are presented demonstrating that superoxide reductase (SOR) from the air-sensitive oral spirochete, Treponema denticola (Td), is a principal enzymatic scavenger of superoxide in this organism. This SOR contains the characteristic non-heme [Fe(His)(4)Cys] active sites. No other metal-binding domain has been annotated for Td SOR. However, we found that Td SOR also accommodates a [Fe(Cys)(4)] site whose spectroscopic and redox properties resemble those in so-called 2Fe-SORs. Spectroscopic comparisons of the wild type and engineered Cys → Ser variants indicate that three of the Cys ligands correspond to those in [Fe(Cys)(4)] sites of "canonical" 2Fe-SORs, whereas the fourth Cys ligand residue has no counterpart in canonical 2Fe-SORs or in any other known [Fe(Cys)(4)] protein. Structural modeling is consistent with iron ligation of the "noncanonical" Cys residue across subunit interfaces of the Td SOR homodimer. The Td SOR was isolated with only a small percentage of [Fe(Cys)(4)] sites. However, quantitative formation of stable [Fe(Cys)(4)] sites was readily achieved by exposing the as-isolated protein to an iron salt, a disulfide reducing agent and air. The disulfide/dithiol status and iron occupancy of the Td SOR [Fe(Cys)(4)] sites could, thus, reflect intracellular redox status, particularly during periods of oxidative stress.

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).

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.

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.

Adsorption kinetics of catalase to thin films of carbon nanotubes.

The adsorption conditions used to immobilize catalase onto thin films of carbon nanotubes were investigated to elucidate the conditions that produced films with maximum amounts of active catalase. The adsorption kinetics were monitored by spectroscopic ellipsometry, and the immobilized catalase films were then assayed for catalytic activity. The development of a volumetric optical model used to interpret the ellipsometric data is discussed. According to the results herein discussed, not only the adsorbed amount but also the initial adsorption rates determine the final catalytic activity of the adsorbed layer. The results described in this paper have direct implications on the rational design and analytical performance of enzymatic biosensors.

The emergence of nitric oxide in the biosynthesis of bacterial natural products.

Nitric oxide (NO) has a broad range of biological utilities including as a biosynthon for bacterial natural products (NPs). The biosyntheses of thaxtomin A and rufomycin require an NO-dependent nitration step where a bacterial NO synthase provides the necessary NO and nitration is catalyzed by a cytochrome P450 homolog. Undiscovered NO-dependent biosynthesis pathways are likely to have similar requirements: a pathway dedicated NO source and a metalloenzyme to catalyze the NO-dependent chemistry. This review discusses the current literature on NO-dependent nitration biosynthesis and relevant enzyme mechanisms. The proposal that NO is important for NN bond containing NPs is also explored, particularly for the formations of nitrosamine and nitramine NPs.

Influences of the heme-lysine crosslink in cytochrome P460 over redox catalysis and nitric oxide sensitivity.

Ammonia (NH3)-oxidizing bacteria (AOB) derive total energy for life from the multi-electron oxidation of NH3 to nitrite (NO2-). One obligate intermediate of this metabolism is hydroxylamine (NH2OH), which can be oxidized to the potent greenhouse agent nitrous oxide (N2O) by the AOB enzyme cytochrome (cyt) P460. We have now spectroscopically characterized a 6-coordinate (6c) {FeNO}7 intermediate on the NH2OH oxidation pathway of cyt P460. This species has two fates: it can either be oxidized to the {FeNO}6 that then undergoes attack by NH2OH to ultimately generate N2O, or it can lose its axial His ligand, thus generating a stable, off-pathway 5-coordinate (5c) {FeNO}7 species. We show that the wild type (WT) cyt P460 exhibits a slow nitric oxide (NO)-independent conversion (kHis-off = 2.90 × 10-3 s-1), whereas a cross-link-deficient Lys70Tyr cyt P460 mutant protein underwent His dissociation via both a NO-independent (kHis-off = 3.8 × 10-4 s-1) and a NO-dependent pathway [kHis-off(NO) = 790 M-1 s-1]. Eyring analyses of the NO-independent pathways for these two proteins revealed a significantly larger (ca. 27 cal mol-1 K-1) activation entropy (ΔS‡) in the cross-link-deficient mutant. Our results suggest that the Lys-heme cross-link confers rigidity to the positioning of the heme P460 cofactor to avoid the fast NO-dependent His dissociation pathway and subsequent formation of the off-pathway 5c {FeNO}7 species. The relevance of these findings to NO signaling proteins such as heme-nitric oxide/oxygen binding (H-NOX) is also discussed.