Ascorbate Peroxidase Compound II Is an Iron(IV) Oxo Species

Unusually Stable Synthetic Diheme Bis-Fe(IV)oxo: An Intermediate in Diheme Enzymes MauG and BthA

Widespread diheme enzymes MauG and BthA of the bacterial cytochrome c peroxidase (bCCP) superfamily produce an unusually stable bis-Fe(IV) intermediate upon 2e¯-oxidation. Herein, we report, for the first time, the synthesis and characterization of the unusually stable bis-Fe(IV)═O intermediate, as a synthetic mimic of the bis-Fe(IV) species generated in the catalytic cycle of the native diheme enzymes that display similar stability at room temperature. Various spectroscopic techniques, including UV-vis, ESI-MS, EPR, resonance Raman, and Mössbauer, were utilized to thoroughly characterize this fairly stable intermediate. The reaction of a diiron(III) porphyrin dimer with soluble iodosylbenzene (sPhIO) at -80 °C produces a red-colored solution of a hitherto unknown six-coordinate bis-Fe(III)porphyrin-sPhIO adduct which quickly undergoes O-I bond cleavage to yield the green bis-Fe(IV)═O intermediate. The reactivities of such a bis-Fe(IV)═O intermediate have also been demonstrated in the oxygen atom transfer (OAT) and C-H bond activation reactions. Computational studies revealed that the local electric field (LEF) of one heme exerted on the other heme unit is most likely the root cause of the unusual stability of the bis-Fe(IV)═O complex reported here. Indeed, the bis-Fe(IV)═O intermediate has been found to be stabilized significantly relative to its monomeric unit, and the stability of the dimeric system is maximized when the two porphyrin planes are relatively oriented by 20°, at which the LEF reaches its maximum value. The present work provides an excellent opportunity for the mechanistic investigation of the highly challenging and unexplored diheme enzymatic processes and will therefore have widespread practical applicability.

An Oxoiron(IV) Complex Supported by an N-Alkylated Cyclam Ligand System Containing a Pendant Alcohol Moiety

The effect of a pendant neutral alcohol moiety in the N-alkylated cyclam (1,4,8,11-tetraazacyclotetradecane) ligand backbone is examined for the non-heme mononuclear oxoiron(IV) unit in [FeIV(Osyn)(TMC-HOR)(NCCH3)]2+ (1-syn) (TMC-HOR=2-(4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecan1-yl)ethan-1-ol). Unlike in the related [FeIV(Oanti)(TMC-SR)]+ (3-anti) (TMC-SR=1-mercaptoethyl-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane) complex, bearing an axial mono-anionic thiolate ligand trans to the oxo unit, the alcohol moiety in 1-syn stays protonated and does not axially coordinate to iron. The protonation of the alcohol moiety is a prerequisite for the stabilization of the oxoiron(IV) core; it presumably serves as a hydrogen bonding donor to the oxoiron(IV) unit, which is positioned syn to the three methyl groups. Comparative reactivity studies reveal 1-syn to be a stronger hydrogen atom abstraction but weaker oxygen atom transfer agent relative to the [FeIV(Osyn)(TMC)(NCCH3)]2+ (2-syn) complex, bearing the N-tetramethylated cyclam (TMC) ligand.

Single-atom nanozymes for enhanced electrochemical biosensing: A review

Enzyme-based electrochemical biosensors have broad and significant applications in biomedical, environmental monitoring, and food safety fields. However, the application of natural enzymes is limited due to issues such as poor stability, complex preparation, and high cost. Single-atom nanozymes (SAzymes), with their unique catalytic properties and efficient enzyme-like activities, present a promising alternative in the field of electrochemical biosensing. Compared to traditional enzymes, SAzyme offer enhanced stability and controllability, making them particularly effective in complex detection environments. This work presents the first systematic review of the progress made since 2018 in the use of SAzymes as alternatives to natural enzymes in electrochemical biosensors, and presents the latest advancements in this area. The review begins with a discussion of various enzyme-like activities of single-atom materials, including peroxidase (POD)-like, oxidase (OXD)-like, catalase (CAT)-like, and superoxide dismutase (SOD)-like activities. It then explores the advantages of SAzymes in improving the performance of electrochemical biosensors from multiple perspectives. The review also summarizes the applications of SAzyme-based electrochemical sensors for reactive oxygen species (ROS), metabolites, neurotransmitters, and other analytes, highlighting specific examples to elucidate underlying catalytic mechanisms and understand fundamental structure-performance relationships. In the final section, the challenges faced by SAzyme-based electrochemical biosensing are discussed, along with potential solutions.

Stabilization of the Ferryl═Oxoheme Form of Staphylococcus aureus IsdG by Electron Transfer from a Second-Sphere Tryptophan

The ferryl heme forms of Staphylococcus aureus IsdG and IsdI have novel UV/vis absorption spectra that are distinct from those of the three forms of ferryl heme typically found in biological systems: compound I, compound II, and compound ES. In this work, the ferryl heme form of IsdG was characterized because it is an analogue for the immediate product of enzyme-catalyzed heme hydroxylation. The ferryl heme form of IsdG generated following the addition of meta-chloroperoxybenzoic acid to the ferric heme form of IsdG has a half-life of 4.0 ± 0.2 min, which is more than 100 times longer than the half-life for the ferryl heme form of human heme oxygenase (hHO). Magnetic circular dichroism characterization of the IsdG species yielded spectral data and zero-field splitting parameters consistent with either a compound II- or compound ES-like ferryl heme. Further characterization of isotopically enriched samples with electron paramagnetic resonance spectroscopy revealed the presence of a protein-based organic radical, as would be expected for compound ES. Finally, multiscale quantum mechanics/molecular mechanics and time-dependent density functional theory strongly suggest that the ferryl heme form of IsdG has a ruffled porphyrin ligand and an oxo ligand. Thus, the ferryl heme form of IsdG is assigned to a compound ES-like species with a Trp67-based radical. Electron transfer from Trp67 to porphyrin will stabilize the immediate product of heme hydroxylation and provide a thermodynamic driving force for the reaction. Furthermore, the ability to transfer an electron between Trp67 and the substrate may explain the differential reactivity of meso-hydroxyheme in IsdG and hHO.

Correlating Structure with Spectroscopy in Ascorbate Peroxidase Compound II

Structural and spectroscopic investigations of compound II in ascorbate peroxidase (APX) have yielded conflicting conclusions regarding the protonation state of the crucial Fe(IV) intermediate. Neutron diffraction and crystallographic data support an iron(IV)-hydroxo formulation, whereas Mössbauer, X-ray absorption (XAS), and nuclear resonance vibrational spectroscopy (NRVS) studies appear consistent with an iron(IV)-oxo species. Here we examine APX with spectroscopy-oriented QM/MM calculations and extensive exploration of the conformational space for both possible formulations of compound II. We establish that irrespective of variations in the orientation of a vicinal arginine residue and potential reorganization of proximal water molecules and hydrogen bonding, the Fe-O distances for the oxo and hydroxo forms consistently fall within distinct, narrow, and nonoverlapping ranges. The accuracy of geometric parameters is validated by coupled-cluster calculations with the domain-based local pair natural orbital approach, DLPNO-CCSD(T). QM/MM calculations of spectroscopic properties are conducted for all structural variants, encompassing Mössbauer, optical, X-ray absorption, and X-ray emission spectroscopies and NRVS. All spectroscopic observations can be assigned uniquely to an Fe(IV)═O form. A terminal hydroxy group cannot be reconciled with the spectroscopic data. Under no conditions can the Fe(IV)═O distance be sufficiently elongated to approach the crystallographically reported Fe-O distance. The latter is consistent only with a hydroxo species, either Fe(IV) or Fe(III). Our findings strongly support the Fe(IV)═O formulation of APX-II and highlight unresolved discrepancies in the nature of samples used across different experimental studies.

C-H bond activation by high-valent iron/cobalt-oxo complexes: a quantum chemical modeling approach

High-valent metal-oxo species serve as key intermediates in the activation of inert C-H bonds. Here, we present a comprehensive DFT analysis of the parameters that have been proposed as influencing factors in modeled high-valent metal-oxo mediated C-H activation reactions. Our approach involves utilizing DFT calculations to explore the electronic structures of modeled FeIVO (species 1) and CoIVO ↔ CoIII-O˙ (species 2), scrutinizing their capacity to predict improved catalytic activity. DFT and DLPNO-CCSD(T) calculations predict that the iron-oxo species possesses a triplet as the ground state, while the cobalt-oxo has a doublet as the ground state. Furthermore, we have investigated the mechanistic pathways for the first C-H bond activation, as well as the desaturation of the alkanes. The mechanism was determined to be a two-step process, wherein the first hydrogen atom abstraction (HAA) represents the rate-limiting step, involving the proton-coupled electron transfer (PCET) process. However, we found that the second HAA step is highly exothermic for both species. Our calculations suggest that the iron-oxo species (Fe-O = 1.672 Å) exhibit relatively sluggish behavior compared to the cobalt-oxo species (Co-O = 1.854 Å) in C-H bond activation, attributed to a weak metal-oxygen bond. MO, NBO, and deformation energy analysis reveal the importance of weakening the M-O bond in the cobalt species, thereby reducing the overall barrier to the reaction. This catalyst was found to have a C-H activation barrier relatively smaller than that previously reported in the literature.

Unravelling the Effect of Acid-Driven Electron Transfer in High-Valent FeIV =O/MnIV =O Species and Its Implications for Reactivity

The electron transfer (ET) step is one of the crucial processes in biochemical redox reactions that occur in nature and has been established as a key step in dictating the reactivity of high-valent metal-oxo species. Although metalloenzymes possessing metal-oxo units at their active site are typically associated with outer-sphere electron transfer (OSET) processes, biomimetic models, in contrast, have been found to manifest either an inner-sphere electron transfer (ISET) or OSET mechanism. This distinction is clearly illustrated through the behaviour of [(N4Py)MnIV (O)]2+ (1) and [(N4Py)FeIV (O)]2+ (2) complexes, where complex 1 showcases an OSET mechanism, while complex 2 exhibits an ISET mechanism, especially evident in their reactions involving C-H bond activation and oxygen atom transfer reactions in the presence of a Lewis/Bronsted acid. However, the precise reason for this puzzling difference remains elusive. This work unveils the origin of the perplexing inner-sphere vs outer-sphere electron transfer process (ISET vs OSET) in [(N4Py)MnIV (O)]2+ (1) and [(N4Py)FeIV (O)]2+ (2) species in the presence of Bronsted acid. The calculations indicate that when the substrate (toluene) approaches both 1 and 2 that is hydrogen bonded with two HOTf molecules (denoted as 1-HOTf and 2-HOTf, respectively), proton transfer from one of the HOTf molecules to the metal-oxo unit is triggered and a simultaneous electron transfer occurs from toluene to the metal centre. Interestingly, the preference for OSET by 1-HOTf is found to originate from the choice of MnIV =O centre to abstract spin-down (β) electron from toluene to its δ(dxy ) orbital. On the other hand, in 2-HOTf, a spin state inversion from triplet to quintet state takes place during the proton (from HOTf) coupled electron transfer (from toluene) preferring a spin-up (α) electron abstraction to its σ* (dz 2 ) orbital mediated by HOTf giving rise to ISET. In addition, 2-HOTf was calculated to possess a larger reorganisation energy, which facilitates the ISET process via the acid. The absence of spin-inversion and smaller reorganisation energy switch the mechanism to OSET for 1-HOTf. Therefore, for the first time, the significance of spin-state and spin-inversion in the electron transfer process has been identified and demonstrated within the realm of high-valent metal-oxo chemistry. This discovery holds implications for the potential involvement of high-valent Mn-oxo species in performing similar transformative processes within Photosystem II.

Cytochrome P460 Cofactor Maturation Proceeds via Peroxide-Dependent Post-translational Modification

Cytochrome P460s are heme enzymes that oxidize hydroxylamine to nitrous oxide. They bear specialized "heme P460" cofactors that are cross-linked to their host polypeptides by a post-translationally modified lysine residue. Wild-type N. europaea cytochrome P460 may be isolated as a cross-link-deficient proenzyme following anaerobic overexpression in E. coli. When treated with peroxide, this proenzyme undergoes maturation to active enzyme with spectroscopic and catalytic properties that match wild-type cyt P460. This maturation reactivity requires no chaperones─it is intrinsic to the protein. This behavior extends to the broader cytochrome c'β superfamily. Accumulated data reveal key contributions from the secondary coordination sphere that enable selective, complete maturation. Spectroscopic data support the intermediacy of a ferryl species along the maturation pathway.

A sorghum ascorbate peroxidase with four binding sites has activity against ascorbate and phenylpropanoids

In planta, H2O2 is produced as a by-product of enzymatic reactions and during defense responses. Ascorbate peroxidase (APX) is a key enzyme involved in scavenging cytotoxic H2O2. Here, we report the crystal structure of cytosolic APX from sorghum (Sorghum bicolor) (Sobic.001G410200). While the overall structure of SbAPX was similar to that of other APXs, SbAPX uniquely displayed four bound ascorbates rather than one. In addition to the ɣ-heme pocket identified in other APXs, ascorbates were bound at the δ-meso and two solvent-exposed pockets. Consistent with the presence of multiple binding sites, our results indicated that the H2O2-dependent oxidation of ascorbate displayed positive cooperativity. Bound ascorbate at two surface sites established an intricate proton network with ascorbate at the ɣ-heme edge and δ-meso sites. Based on crystal structures, steady-state kinetics, and site-directed mutagenesis results, both ascorbate molecules at the ɣ-heme edge and the one at the surface are expected to participate in the oxidation reaction. We provide evidence that the H2O2-dependent oxidation of ascorbate by APX produces a C2-hydrated bicyclic hemiketal form of dehydroascorbic acid at the ɣ-heme edge, indicating two successive electron transfers from a single-bound ascorbate. In addition, the δ-meso site was shared with several organic compounds, including p-coumaric acid and other phenylpropanoids, for the potential radicalization reaction. Site-directed mutagenesis of the critical residue at the ɣ-heme edge (R172A) only partially reduced polymerization activity. Thus, APX removes stress-generated H2O2 with ascorbates, and also uses this same H2O2 to potentially fortify cell walls via oxidative polymerization of phenylpropanoids in response to stress.

Rh single-atom nanozymes for efficient ascorbic acid oxidation and detection

The management of ascorbic acid (AA) in biological fluids is of significant importance for body functions and human health, yet challenging due to the lack of high-performance sensing catalysts. Herein, we report the design of Rh single-atom nanozymes (Rh SAzymes) by mimicking the active sites of ascorbate peroxidase toward efficient electrocatalytic oxidation and detection of AA. Benefiting from the enzyme-mimicking single-atom coordination, the Rh SAzyme exhibits an unprecedented electrocatalytic activity for AA oxidation with an onset potential as low as 0.02 V (vs. Ag/AgCl). Combined with the screen-printing technology, a miniaturized Rh SAzyme biosensor was firstly constructed for tracking dynamic trends of AA in the human subject and detecting AA content in nutritional products. The as-prepared biosensor exhibits excellent detection performances with a wide linear range of 10.0 μM-53.1 mM, a low detection limit of 0.26 μM, and a long stability of 28 days. This work opens a door for the design of artificial single-atom electrocatalysts to mimic natural enzymes and their subsequent application in biosensors.

QM Calculations Revealed that Outer-Sphere Electron Transfer Boosted O-O Bond Cleavage in the Multiheme-Dependent Cytochrome bd Oxygen Reductase

The cytochrome bd oxygen reductase catalyzes the four-electron reduction of dioxygen to two water molecules. The structure of this enzyme reveals three heme molecules in the active site, which differs from that of heme-copper cytochrome c oxidase. The quantum chemical cluster approach was used to uncover the reaction mechanism of this intriguing metalloenzyme. The calculations suggested that a proton-coupled electron transfer reduction occurs first to generate a ferrous heme b₅₉₅. This is followed by the dioxygen binding at the heme d center coupled with an outer-sphere electron transfer from the ferrous heme b₅₉₅ to the dioxygen moiety, affording a ferric ion superoxide intermediate. A second proton-coupled electron transfer produces a heme d ferric hydroperoxide, which undergoes efficient O-O bond cleavage facilitated by an outer-sphere electron transfer from the ferrous heme b₅₉₅ to the O-O σ* orbital and an inner-sphere proton transfer from the heme d hydroxyl group to the leaving hydroxide. The synergistic benefits of the two types of hemes rationalize the highly efficient oxygen reduction repertoire for the multi-heme-dependent cytochrome bd oxygen reductase family.

Activity of Cytosolic Ascorbate Peroxidase (APX) from Panicum virgatum against Ascorbate and Phenylpropanoids

APX is a key antioxidant enzyme in higher plants, scavenging H2O2 with ascorbate in several cellular compartments. Here, we report the crystal structures of cytosolic ascorbate peroxidase from switchgrass (Panicum virgatum L., Pvi), a strategic feedstock plant with several end uses. The overall structure of PviAPX was similar to the structures of other APX family members, with a bound ascorbate molecule at the ɣ-heme edge pocket as in other APXs. Our results indicated that the H2O2-dependent oxidation of ascorbate displayed positive cooperativity. Significantly, our study suggested that PviAPX can oxidize a broad range of phenylpropanoids with δ-meso site in a rather similar efficiency, which reflects its role in the fortification of cell walls in response to insect feeding. Based on detailed structural and kinetic analyses and molecular docking, as well as that of closely related APX enzymes, the critical residues in each substrate-binding site of PviAPX are proposed. Taken together, these observations shed new light on the function and catalysis of PviAPX, and potentially benefit efforts improve plant health and biomass quality in bioenergy and forage crops.

Comparative oxidative ability of mononuclear and dinuclear high-valent iron-oxo species towards the activation of methane: does the axial/bridge atom modulate the reactivity?

Over the years, mononuclear Fe^IVO species have been extensively studied, but the presence of dinuclear Fe^IVO species in soluble methane monooxygenase (sMMO) has inspired the development of biomimic models that could activate inert substrates such as methane. There are some successful attempts; particularly the [(Por)(m-CBA) Fe^IV(μ-N)Fe^IV(O)(Por˙⁺)]^- species has been reported to activate methane and yield decent catalytic turnover numbers and therefore regarded as the closest to the sMMO enzyme functional model, as no mononuclear Fe^IVO analogues could achieve this feat. In this work, we have studied a series of mono and dinuclear models using DFT and ab initio DLPNO-CCSD(T) calculations to probe the importance of nuclearity in enhancing the reactivity. We have probed the catalytic activities of four complexes: [(HO)Fe^IV(O)(Por)]^- (1), [(HO)Fe^IV(O)(Por˙⁺)] (2), μ-oxo dinuclear iron species [(Por)(m-CBA)Fe^IV(μ-O)Fe^IV(O) (Por˙⁺)]^- (3) and N-bridged dinuclear iron species [(Por)(m-CBA)Fe^IV(μ-N)Fe^IV(O)(Por˙⁺)]^- (4) towards the activation of methane. Additionally, calculations were performed on the mononuclear models [(X)Fe^IV(O)(Por˙⁺)]ⁿ {X = N 4a (n = -2), NH 4b (n = -1) and NH₂4c (n = 0)} to understand the role of nuclearity in the reactivity. DFT calculations performed on species 1-4 suggest an interesting variation among them, with species 1-3 possessing an intermediate spin (S = 1) as a ground state and species 4 possessing a high-spin (S = 2) as a ground state. Furthermore, the two Fe^IV centres in species 3 and 4 are antiferromagnetically coupled, yielding a singlet state with a distinct difference in their electronic structure. On the other hand, species 2 exhibits a ferromagnetic coupling between the Fe^IV and the Por˙⁺ moiety. Our calculations suggest that the higher barriers for the C-H bond activation of methane and the rebound step for species 1 and 3 are very high in energy, rendering them unreactive towards methane, while species 2 and 4 have lower barriers, suggesting their reactivity towards methane. Studies on the system reveal that model 4a has multiple FeN bonds facilitating greater reactivity, whereas the other two models have longer Fe-N bonds and less radical character with steeper barriers. Strong electronic cooperativity is found to be facilitated by the bridging nitride atom, and this cooperativity is suppressed by substituents such as oxygen, rendering them inactive. Thus, our study unravels that apart from enhancing the nuclearity, bridging atoms that facilitate strong cooperation between the metals are required to activate very inert substrates such as methane, and our results are broadly in agreement with earlier experimental findings.

17O Electron Nuclear Double Resonance Analysis of Compound I: Inverse Correlation between Oxygen Spin Population and Electron Donation

Although the activation of inert C-H bonds by metal-oxo complexes has been widely studied, important questions remain, particularly regarding the role of oxygen spin population (i.e., unpaired electrons on the oxo ligand) in facilitating C-H bond cleavage. In order to shed light on this issue, we have utilized 17O electron nuclear double resonance spectroscopy to measure the oxygen spin populations of three compound I intermediates in heme enzymes with different reactivities toward C-H bonds: chloroperoxidase, cytochrome P450, and a selenolate (selenocysteinyl)-ligated cytochrome P450. The experimental data suggest an inverse correlation between oxygen spin population and electron donation from the axial ligand. We have explored the implications of this result using a Hückel-type molecular orbital model and constrained density functional theory calculations. These investigations have allowed us to examine the relationship between oxygen spin population, oxygen charge, electron donation from the axial ligand, and reactivity.

Serial Femtosecond Crystallography Reveals the Role of Water in the One- or Two-Electron Redox Chemistry of Compound I in the Catalytic Cycle of the B-Type Dye-Decolorizing Peroxidase DtpB

Controlling the reactivity of high-valent Fe(IV)–O catalytic intermediates, Compounds I and II, generated in heme enzymes upon reaction with dioxygen or hydrogen peroxide, is important for function. It has been hypothesized that the presence (wet) or absence (dry) of distal heme pocket water molecules can influence whether Compound I undergoes sequential one-electron additions or a concerted two-electron reduction. To test this hypothesis, we investigate the role of water in the heme distal pocket of a dye-decolorizing peroxidase utilizing a combination of serial femtosecond crystallography and rapid kinetic studies. In a dry distal heme site, Compound I reduction proceeds through a mechanism in which Compound II concentration is low. This reaction shows a strong deuterium isotope effect, indicating that reduction is coupled to proton uptake. The resulting protonated Compound II (Fe(IV)–OH) rapidly reduces to the ferric state, giving the appearance of a two-electron transfer process. In a wet site, reduction of Compound I is faster, has no deuterium effect, and yields highly populated Compound II, which is subsequently reduced to the ferric form. This work provides a definitive experimental test of the hypothesis advanced in the literature that relates sequential or concerted electron transfer to Compound I in wet or dry distal heme sites.

Heme compound II models in chemoselectivity and disproportionation reactions

Heme compound II models bearing electron-deficient and -rich porphyrins, [FeIV(O)(TPFPP)(Cl)]- (1a) and [FeIV(O)(TMP)(Cl)]- (2a), respectively, are synthesized, spectroscopically characterized, and investigated in chemoselectivity and disproportionation reactions using cyclohexene as a mechanistic probe. Interestingly, cyclohexene oxidation by 1a occurs at the allylic C-H bonds with a high kinetic isotope effect (KIE) of 41, yielding 2-cyclohexen-1-ol product; this chemoselectivity is the same as that of nonheme iron(iv)-oxo intermediates. In contrast, as observed in heme compound I models, 2a yields cyclohexene oxide product with a KIE of 1, demonstrating a preference for C[double bond, length as m-dash]C epoxidation. The latter result is interpreted as 2a disproportionating to form [FeIV(O)(TMP+˙)]+ (2b) and FeIII(OH)(TMP), and 2b becoming the active oxidant to conduct the cyclohexene epoxidation. In contrast to 2a, 1a does not disproportionate under the present reaction conditions. DFT calculations confirm that compound II models prefer C-H bond hydroxylation and that disproportionation of compound II models is controlled thermodynamically by the porphyrin ligands. Other aspects, such as acid and base effects on the disproportionation of compound II models, have been discussed as well.

Inhibitory effect of ascorbate on tryptophan 2,3-dioxygenase

Tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO) catalyze the same reaction, oxidative cleavage of L-tryptophan (L-Trp) to N-formyl-kynurenine. In both enzymes, the ferric (FeIII) form is inactive, and ascorbate (Asc) is frequently used as a reductant in in vitro assays to activate the enzymes by reducing the heme iron. Recently, it has been reported that Asc activates IDO2 by acting as a reductant, however, it is also a competitive inhibitor of the enzyme. Here, the effect of Asc on human TDO (hTDO) is investigated. Similar to its interaction with IDO2, Asc acts as both a reductant and a competitive inhibitor of hTDO in the absence of catalase, and its inhibitory effect was enhanced by the addition of H2O2. Interestingly, however, no inhibitory effect of Asc was observed in the presence of catalase. TDO is known to be activated by H2O2 and a ferryl-oxo (FeIV=O) intermediate (Compound II) is generated during the activation process. The observation that Asc acts as a competitive inhibitor of hTDO only in the absence of catalase can be explained by assuming that the target of Asc is Compound II. Asc seems to compete with L-Trp in an unusual manner.

NRVS investigation of ascorbate peroxidase compound II: Observation of Iron(IV)oxo stretching

The protonation state of ascorbate peroxidase compound II (APX-II) has been a subject of debate. A combined X-ray/neutron crystallographic study reported that APX-II is best described as an iron(IV)hydroxide species with an FeO distance of 1.88 Å (Kwon, et al. Nat Commun2016, 7, 13,445), while X-ray absorption spectroscopy (XAS) experiments (utilizing extended X-ray absorption fine structure (EXAFS) and pre-edge analyses) indicate APX-II is an authentic iron(IV)oxo species with an FeO distance 1.68 Å (Ledray, et al. Journal of the American Chemical Society2020,142, 20,419). Previous debates concerning ferryl protonation states have been resolved through the application of Badger's rule, which correlates FeO bond distances with FeO vibrational frequencies. To obtain the required vibrational data, we have collected Nuclear Resonance Vibrational Spectroscopy (NRVS) data for APX-II. We observe a broad vibrational feature in the range associated with iron(IV)oxo stretching (700-800 cm-1). This feature appears to have two peaks at 732 cm-1 and 770 cm-1, corresponding to FeO distances of 1.69 and 1.67 Å, respectively. The broad vibrational envelope and the presence of multiple resonances could reflect a distribution of hydrogen bonding interactions within the active-site pocket.

Metalloprotein catalysis: structural and mechanistic insights into oxidoreductases from neutron protein crystallography

Metalloproteins catalyze a range of reactions, with enhanced chemical functionality due to their metal cofactor. The reaction mechanisms of metalloproteins have been experimentally characterized by spectroscopy, macromolecular crystallography and cryo-electron microscopy. An important caveat in structural studies of metalloproteins remains the artefacts that can be introduced by radiation damage. Photoreduction, radiolysis and ionization deriving from the electromagnetic beam used to probe the structure complicate structural and mechanistic interpretation. Neutron protein diffraction remains the only structural probe that leaves protein samples devoid of radiation damage, even when data are collected at room temperature. Additionally, neutron protein crystallography provides information on the positions of light atoms such as hydrogen and deuterium, allowing the characterization of protonation states and hydrogen-bonding networks. Neutron protein crystallography has further been used in conjunction with experimental and computational techniques to gain insight into the structures and reaction mechanisms of several transition-state metal oxidoreductases with iron, copper and manganese cofactors. Here, the contribution of neutron protein crystallography towards elucidating the reaction mechanism of metalloproteins is reviewed.

C-H Bond Cleavage by Bioinspired Nonheme Metal Complexes

The functionalization of C-H bonds is one of the most challenging transformations in synthetic chemistry. In biology, these processes are well-known and are achieved with a variety of metalloenzymes, many of which contain a single metal center within their active sites. The most well studied are those with Fe centers, and the emerging experimental data show that high-valent iron oxido species are the intermediates responsible for cleaving the C-H bond. This Forum Article describes the state of this field with an emphasis on nonheme Fe enzymes and current experimental results that provide insights into the properties that make these species capable of C-H bond cleavage. These parameters are also briefly considered in regard to manganese oxido complexes and Cu-containing metalloenzymes. Synthetic iron oxido complexes are discussed to highlight their utility as spectroscopic and mechanistic probes and reagents for C-H bond functionalization. Avenues for future research are also examined.

Aspartate or arginine? Validated redox state X-ray structures elucidate mechanistic subtleties of FeIV = O formation in bacterial dye-decolorizing peroxidases

Structure determination of proteins and enzymes by X-ray crystallography remains the most widely used approach to complement functional and mechanistic studies. Capturing the structures of intact redox states in metalloenzymes is critical for assigning the chemistry carried out by the metal in the catalytic cycle. Unfortunately, X-rays interact with protein crystals to generate solvated photoelectrons that can reduce redox active metals and hence change the coordination geometry and the coupled protein structure. Approaches to mitigate such site-specific radiation damage continue to be developed, but nevertheless application of such approaches to metalloenzymes in combination with mechanistic studies are often overlooked. In this review, we summarize our recent structural and kinetic studies on a set of three heme peroxidases found in the bacterium Streptomyces lividans that each belong to the dye decolourizing peroxidase (DyP) superfamily. Kinetically, each of these DyPs has a distinct reactivity with hydrogen peroxide. Through a combination of low dose synchrotron X-ray crystallography and zero dose serial femtosecond X-ray crystallography using an X-ray free electron laser (XFEL), high-resolution structures with unambiguous redox state assignment of the ferric and ferryl (Fe^IV = O) heme species have been obtained. Experiments using stopped-flow kinetics, solvent-isotope exchange and site-directed mutagenesis with this set of redox state validated DyP structures have provided the first comprehensive kinetic and structural framework for how DyPs can modulate their distal heme pocket Asp/Arg dyad to use either the Asp or the Arg to facilitate proton transfer and rate enhancement of peroxide heterolysis.

XFEL Crystal Structures of Peroxidase Compound II

Oxygen activation in all heme enzymes requires the formation of high oxidation states of iron, usually referred to as ferryl heme. There are two known intermediates: Compound I and Compound II. The nature of the ferryl heme-and whether it is an FeIV =O or FeIV -OH species-is important for controlling reactivity across groups of heme enzymes. The most recent evidence for Compound I indicates that the ferryl heme is an unprotonated FeIV =O species. For Compound II, the nature of the ferryl heme is not unambiguously established. Here, we report 1.06 Å and 1.50 Å crystal structures for Compound II intermediates in cytochrome c peroxidase (CcP) and ascorbate peroxidase (APX), collected using the X-ray free electron laser at SACLA. The structures reveal differences between the two peroxidases. The iron-oxygen bond length in CcP (1.76 Å) is notably shorter than in APX (1.87 Å). The results indicate that the ferryl species is finely tuned across Compound I and Compound II species in closely related peroxidase enzymes. We propose that this fine-tuning is linked to the functional need for proton delivery to the heme.

XFEL Crystal Structures of Peroxidase Compound II

Oxygen activation in all heme enzymes requires the formation of high oxidation states of iron, usually referred to as ferryl heme. There are two known intermediates: Compound I and Compound II. The nature of the ferryl heme-and whether it is an FeIV=O or FeIV-OH species-is important for controlling reactivity across groups of heme enzymes. The most recent evidence for Compound I indicates that the ferryl heme is an unprotonated FeIV=O species. For Compound II, the nature of the ferryl heme is not unambiguously established. Here, we report 1.06 Å and 1.50 Å crystal structures for Compound II intermediates in cytochrome c peroxidase (CcP) and ascorbate peroxidase (APX), collected using the X-ray free electron laser at SACLA. The structures reveal differences between the two peroxidases. The iron-oxygen bond length in CcP (1.76 Å) is notably shorter than in APX (1.87 Å). The results indicate that the ferryl species is finely tuned across Compound I and Compound II species in closely related peroxidase enzymes. We propose that this fine-tuning is linked to the functional need for proton delivery to the heme.

Local Electric Fields as a Natural Switch of Heme-Iron Protein Reactivity

Heme-iron oxidoreductases operating through the high-valent Fe^IVO intermediates perform crucial and complicated transformations, such as oxidations of unreactive saturated hydrocarbons. These enzymes share the same Fe coordination, only differing by the axial ligation, e.g., Cys in P450 oxygenases, Tyr in catalases, and His in peroxidases. By examining ~200 heme-iron proteins, we show that the protein hosts exert highly specific intramolecular electric fields on the active sites, and there is a strong correlation between the direction and magnitude of this field and the protein function. In all heme proteins, the field is preferentially aligned with the Fe-O bond ( F_z ). The Cys-ligated P450 oxygenases have the highest average F_z of 28.5 MV cm^-1, i.e., most enhancing the oxyl-radical character of the oxo group, and consistent with the ability of these proteins to activate strong C-H bonds. In contrast, in Tyr-ligated proteins, the average F_z is only 3.0 MV cm^-1, apparently suppressing single-electron off-pathway oxidations, and in His-ligated proteins, F_z is -8.7 MV cm^-1. The operational field range is given by the trade-off between the low reactivity of the Fe^IVO Compound I at the more negative F_z , and the low selectivity at the more positive F_z . Consequently, a heme-iron site placed in the field characteristic of another heme-iron protein class loses its canonical function, and gains an adverse one. Thus, electric fields produced by the protein scaffolds, together with the nature of the axial ligand, control all heme-iron chemistry.