Structure of the Reduced Copper Active Site in Preprocessed Galactose Oxidase: Ligand Tuning for One-Electron O2 Activation in Cofactor Biogenesis

Synthetic Copper-(Di)oxygen Complex Generation and Reactivity Relevant to Copper Protein O2-Processing

Synthetic copper-dioxygen complex design, generation and characterization, play a crucial role in elucidating the structure/function of copper-based metalloenzymes, including dopamine β-monooxygenase, lytic polysaccharide monooxygenases, particulate methane monooxygenase, tyrosinase, hemocyanin, and catechol oxidase. Designing suitable ligands to closely mimic the variable active sites found in these enzymes poses a challenging task for synthetic bioinorganic chemists. In this review, we have highlighted a few representative ligand systems capable of stabilizing various copper-dioxygen species such as CuII-(O2 •-)(superoxide), Cu2 II-(μ-η 1:η 1-O2 2-) (trans/cis-peroxide), Cu2 II-(μ-η 2:η 2-O2 2-)(side-on peroxide) and Cun II--OOH (hydroperoxide) species. Here, we discuss the ligand type utilized, syntheses, and spectroscopic characterization of these species. We also delineate reactivity patterns, particularly electrophilic arene hydroxylation by a side-on peroxo species which occurs via a "NIH shift" mechanism and thermodynamic-kinetic relationships among Cu2-(O2 •-)/O2 2-/-OOH moieties.

Non-standard amino acid incorporation into thiol dioxygenases

Thiol dioxygenases (TDOs) are non‑heme Fe(II)‑dependent enzymes that catalyze the O2-dependent oxidation of thiol substrates to their corresponding sulfinic acids. Six classes of TDOs have thus far been identified and two, cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO), are found in eukaryotes. All TDOs belong to the cupin superfamily of enzymes, which share a common β‑barrel fold and two cupin motifs: G(X)5HXH(X)3-6E(X)6G and G(X)5-7PXG(X)2H(X)3N. Crystal structures of TDOs revealed that these enzymes contain a relatively rare, neutral 3‑His iron‑binding facial triad. Despite this shared metal-binding site, TDOs vary greatly in their secondary coordination spheres. Site‑directed mutagenesis has been used extensively to explore the impact of changes in secondary sphere residues on substrate specificity and enzymatic efficiency. This chapter summarizes site-directed mutagenesis studies of eukaryotic TDOs, focusing on the tools and practicality of non‑standard amino acid incorporation.

Coordination Variations within Binuclear Copper Dioxygen-Derived (Hydro)Peroxo and Superoxo Species; Influences upon Thermodynamic and Electronic Properties

Copper ion is a versatile and ubiquitous facilitator of redox chemical and biochemical processes. These include the binding of molecular oxygen to copper(I) complexes where it undergoes stepwise reduction-protonation. A detailed understanding of thermodynamic relationships between such reduced/protonated states is key to elucidate the fundamentals of the chemical/biochemical processes involved. The dicopper(I) complex [CuI2(BPMPO-)]1+ {BPMPOH = 2,6-bis{[(bis(2-pyridylmethyl)amino]methyl}-4-methylphenol)} undergoes cryogenic dioxygen addition; further manipulations in 2-methyltetrahydrofuran generate dicopper(II) peroxo [CuII2(BPMPO-)(O22-)]1+, hydroperoxo [CuII2(BPMPO-)(-OOH)]2+, and superoxo [CuII2(BPMPO-)(O2•-)]2+ species, characterized by UV-vis, resonance Raman and electron paramagnetic resonance (EPR) spectroscopies, and cold spray ionization mass spectrometry. An unexpected EPR spectrum for [CuII2(BPMPO-)(O2•-)]2+ is explained by the analysis of its exchange-coupled three-spin frustrated system and DFT calculations. A redox equilibrium, [CuII2(BPMPO-)(O22-)]1+ ⇄ [CuII2(BPMPO-)(O2•-)]2+, is established utilizing Me8Fc+/Cr(η6-C6H6)2, allowing for [CuII2(BPMPO-)(O2•-)]2+/[CuII2(BPMPO-)(O22-)]1+ reduction potential calculation, E°' = -0.44 ± 0.01 V vs Fc+/0, also confirmed by cryoelectrochemical measurements (E°' = -0.40 ± 0.01 V). 2,6-Lutidinium triflate addition to [CuII2(BPMPO-)(O22-)]1+ produces [CuII2(BPMPO-)(-OOH)]2+; using a phosphazene base, an acid-base equilibrium was achieved, pKa = 22.3 ± 0.7 for [CuII2(BPMPO-)(-OOH)]2+. The BDFEOO-H = 80.3 ± 1.2 kcal/mol, as calculated for [CuII2(BPMPO-)(-OOH)]2+; this is further substantiated by H atom abstraction from O-H substrates by [CuII2(BPMPO-)(O2•-)]2+ forming [CuII2(BPMPO-)(-OOH)]2+. In comparison to known analogues, the thermodynamic and spectroscopic properties of [CuII2(BPMPO-)] O2-derived adducts can be accounted for based on chelate ring size variations built into the BPMPO- framework and the resulting enhanced CuII-ion Lewis acidity.

A frontier-orbital view of the initial steps of lytic polysaccharide monooxygenase reactions

Lytic polysaccharide monooxygenases (LPMOs) are copper enzymes that oxidatively cleave the strong C-H bonds in recalcitrant polysaccharide substrates, thereby playing a crucial role in biomass degradation. Recently, LPMOs have also been shown to be important for several pathogens. It is well established that the Cu(II) resting state of LPMOs is inactive, and the electronic structure of the active site needs to be altered to transform the enzyme into an active form. Whether this transformation occurs due to substrate binding or due to a unique priming reduction has remained speculative. Starting from four different crystal structures of the LPMO LsAA9A with well-defined oxidation states, we use a frontier molecular orbital approach to elucidate the initial steps of the LPMO reaction. We give an explanation for the requirement of the unique priming reduction and analyse electronic structure changes upon substrate binding. We further investigate how the presence of the substrate could facilitate an electron transfer from the copper active site to an H2O2 co-substrate. Our findings could help to control experimental LPMO reactions.

Mechanistic Insight into Peptidyl-Cysteine Oxidation by the Copper-Dependent Formylglycine-Generating Enzyme

The copper-dependent formylglycine-generating enzyme (FGE) catalyzes the oxygen-dependent oxidation of specific peptidyl-cysteine residues to formylglycine. Our QM/MM calculations provide a very likely mechanism for this transformation. The reaction starts with dioxygen binding to the tris-thiolate Cu^I center to form a triplet Cu^II -superoxide complex. The rate-determining hydrogen atom abstraction involves a triplet-singlet crossing to form a Cu^II -OOH species that couples with the substrate radical, leading to a Cu^I -alkylperoxo intermediate. This is accompanied by proton transfer from the hydroperoxide to the S atom of the substrate via a nearby water molecule. The subsequent O-O bond cleavage is coupled with the C-S bond breaking that generates the formylglycine and a Cu^II -oxyl complex. Moreover, our results suggest that the aldehyde oxygen of the final product originates from O₂ , which will be useful for future experimental work.

Copper radical oxidases: galactose oxidase, glyoxal oxidase, and beyond!

The copper radical oxidases (CROs) are an evolutionary and functionally diverse group of enzymes established by the historically significant galactose 6-oxidase and glyoxal oxidase from fungi. Inducted in 2013, CROs now constitute Auxiliary Activity Family 5 (AA5) in the Carbohydrate-Active Enzymes (CAZy) classification. CROs catalyse the two-electron oxidation of their substrates using oxygen as the final electron acceptor and are particularly distinguished by a cross-linked tyrosine-cysteine co-factor that is integral to radical stabilization. Recently, there has been a significant increase in the biochemically and structurally characterized CROs, which has revealed an expanded natural diversity of catalytic activities in the family. This review provides a brief historical introduction to CRO biochemistry and structural biology as a foundation for an update on current advances in CRO enzymology, biotechnology, and biology across kingdoms of life.

How Does Electronic Activity Drive Chemical Reactions? Insights from the Reaction Electronic Flux for the Conversion of Dopamine into Norepinephrine

Hydrogen atom transfer (HAT) is a crucial step in the physiological conversion of dopamine into norepinephrine catalyzed by dopamine β-monooxygenase. The way the reaction takes place is unclear, and a rational explanation on how the electronic activity drives the HAT seems to be necessary. In this work, we answer this question using the reaction electronic flux (REF), a DFT-based descriptor of electronic activity. Two reaction mechanisms will be analyzed using the REF's decomposition in polarization and electron transfer effects. Results show that both mechanisms proceed as follows: (1) polarization effects initiate the reactions producing structural distortions; (2) electron transfer processes take over near the transition states, triggering specific chemical events such as bond forming and breaking which are responsible to push the reactions toward the products; (3) after passing the transition state, polarization shows up again and drives the relaxation process toward the product. Similar polarization effects were observed in both reactions, but they present an opposite behavior of the electronic transfer flux disclosing the fact that electron transfer phenomena govern the reaction mechanisms.

Oxygen Activation by a Copper Complex with Sulfur-Only Coordination Relevant to the Formylglycine Generating Enzyme

Synthesizing hydrosulfido Cu thiolate complexes is quite challenging. In this report, two new and rare hydrosulfido Cu thiolate complexes, [Et₄N]₂[(mnt)Cu-SH] (2, mnt = maleonitrile dithiolene = S₂C₂(CN)₂) and [Et₄N]₃[(mnt)Cu-(μ-SH)-Cu(mnt)] (3), have been synthesized. Coordination sites and O₂ activation by complex 2 resemble the formylglycine generating enzyme (FGE), an enzyme recently crystallographically characterized with sulfur-only coordination around Cu (three thiolate ligands). The function of this enzyme (and complex 2) is surprising because vulnerable thiolates should not be well suited for O₂ activation rationally. Indeed, activation of oxygen by such an all-sulfur-coordinated Cu complex 2 is lacking in the literature. Aerial O₂ (ambient O₂ from the air) activation by complex 2 could proceed through a superoxide radical intermediate and a sulfur radical intermediate detected by resonance Raman (rR) spectroscopy and electron paramagnetic resonance (EPR) spectroscopy, respectively. The chemistry of 2 has been examined by its reactivity, crystal structure, and spectroscopic and cyclic voltammetric analyses. In addition, the results have been complemented with density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations.

Formation Mechanism of Cofactor Cys-Tyr in the Cysteine Dioxygenases (CDO and F2-CDO) and Its Influence on Catalysis: A QM/MM Study

Cysteine dioxygenase (CDO) is a nonheme mononuclear iron enzyme, which catalyzes the oxidation of cysteine to cysteine sulfinic acid. Crystal structure studies of mammalian CDO showed that there is a cross-linked cysteine-tyrosine (Cys-Tyr) cofactor in its active site. Moreover, the formation of the Cys-Tyr cofactor requires the metal cofactor (Fe²⁺) and O₂, and it was previously considered to substantially enhance the catalytic efficiency and half-life of CDO. Recently, a pure human CDO (F₂-CDO) without including the Cys-Tyr cofactor was crystalized by the site-directed mutagenesis approach in the anaerobic condition. In this work, to gain insights into the formation mechanism of the Cys-Tyr cofactor and whether it can really promote the catalytic reactivity of CDO, a series of computational models have been constructed, and quantum mechanical/molecular mechanical (QM/MM) calculations have been performed. Our calculation results reveal that WT-CDO and F₂-CDO follow different mechanisms for the formation of the Cys-Tyr cofactor. In F₂-CDO, the cofactor formation contains the H-abstraction, C-S bond formation, intramolecular F migration, and aromatization of the residue F₂Y157, in which the Fe-coordinate dioxygen can be recovered after the formation cofactor; however, in the WT-CDO, the cofactor formation shows some differences. During the reaction, hydrogen peroxide is generated, and the final aromatization requires the assistance of one water molecule. Furthermore, the overall barriers of cofactor formation are always higher than l-cysteine oxidation for both WT-CDO and F₂-CDO irrespective of the absence or presence of the cofactor. Thus, we can theoretically confirm that the Cys-Tyr cofactor is not essential for the oxidation activity of CDO, and cofactor formation is just an accompanying reaction but not a prerequisite for the oxidation reaction. These results may provide useful information for understanding the catalysis of CDO.

A Thioether-Ligated Cupric Superoxide Model with Hydrogen Atom Abstraction Reactivity

The central role of cupric superoxide intermediates proposed in hormone and neurotransmitter biosynthesis by noncoupled binuclear copper monooxygenases like dopamine-β-monooxygenase has drawn significant attention to the unusual methionine ligation of the Cu_M ("Cu_B") active site characteristic of this class of enzymes. The copper-sulfur interaction has proven critical for turnover, raising still-unresolved questions concerning Nature's selection of an oxidizable Met residue to facilitate C-H oxygenation. We describe herein a model for Cu_M, [(^TMGN₃S)Cu^I]⁺ ([1]⁺), and its O₂-bound analog [(^TMGN₃S)Cu^II(O₂^•-)]⁺ ([1·O₂]⁺). The latter is the first reported cupric superoxide with an experimentally proven Cu-S bond which also possesses demonstrated hydrogen atom abstraction (HAA) reactivity. Introduction of O₂ to a precooled solution of the cuprous precursor [1]B(C₆F₅)₄ (-135 °C, 2-methyltetrahydrofuran (2-MeTHF)) reversibly forms [1·O₂]B(C₆F₅)₄ (UV/vis spectroscopy: λ_max 442, 642, 742 nm). Resonance Raman studies (413 nm) using ¹⁶O₂ [¹⁸O₂] corroborated the identity of [1·O₂]⁺ by revealing Cu-O (446 [425] cm^-1) and O-O (1105 [1042] cm^-1) stretches, and extended X-ray absorption fine structure (EXAFS) spectroscopy showed a Cu-S interatomic distance of 2.55 Å. HAA reactivity between [1·O₂]⁺ and TEMPO-H proceeds rapidly (1.28 × 10^-1 M^-1 s^-1, -135 °C, 2-MeTHF) with a primary kinetic isotope effect of k_H/k_D = 5.4. Comparisons of the O₂-binding behavior and redox activity of [1]⁺ vs [2]⁺, the latter a close analog of [1]⁺ but with all N atom ligation (i.e., N₃S vs N₄), are presented.

Kβ X-ray Emission Spectroscopy as a Probe of Cu(I) Sites: Application to the Cu(I) Site in Preprocessed Galactose Oxidase

Cu(I) active sites in metalloproteins are involved in O₂ activation, but their O₂ reactivity is difficult to study due to the Cu(I) d¹⁰ closed shell which precludes the use of conventional spectroscopic methods. Kβ X-ray emission spectroscopy (XES) is a promising technique for investigating Cu(I) sites as it detects photons emitted by electronic transitions from occupied orbitals. Here, we demonstrate the utility of Kβ XES in probing Cu(I) sites in model complexes and a metalloprotein. Using Cu(I)Cl, emission features from double-ionization (DI) states are identified using varying incident X-ray photon energies, and a reasonable method to correct the data to remove DI contributions is presented. Kβ XES spectra of Cu(I) model complexes, having biologically relevant N/S ligands and different coordination numbers, are compared and analyzed, with the aid of density functional theory (DFT) calculations, to evaluate the sensitivity of the spectral features to the ligand environment. While the low-energy Kβ_2,5 emission feature reflects the ionization energy of ligand np valence orbitals, the high-energy Kβ_2,5 emission feature corresponds to transitions from molecular orbitals (MOs) having mainly Cu 3d character with the intensities determined by ligand-mediated d-p mixing. A Kβ XES spectrum of the Cu(I) site in preprocessed galactose oxidase (GO_pre) supports the 1Tyr/2His structural model that was determined by our previous X-ray absorption spectroscopy and DFT study. The high-energy Kβ_2,5 emission feature in the Cu(I)-GO_pre data has information about the MO containing mostly Cu 3d_x²-y² character that is the frontier molecular orbital (FMO) for O₂ activation, which shows the potential of Kβ XES in probing the Cu(I) FMO associated with small-molecule activation in metalloproteins.

Formation of Monofluorinated Radical Cofactor in Galactose Oxidase through Copper-Mediated C-F Bond Scission

Galactose oxidase (GAO) contains a Cu(II)-ligand radical cofactor. The cofactor, which is autocatalytically generated through the oxidation of the copper, consists of a cysteine-tyrosine radical (Cys-Tyr^•) as a copper ligand. The formation of the cross-linked thioether bond is accompanied by a C-H bond scission on Tyr272 with few details known thus far. Here, we report the genetic incorporation of 3,5-dichlorotyrosine (Cl₂-Tyr) and 3,5-difluorotyrosine (F₂-Tyr) to replace Tyr272 in the GAO^V previously optimized for expression through directed evolution. The proteins with an unnatural tyrosine residue are catalytically competent. We determined the high-resolution crystal structures of the GAO^V, Cl₂-Tyr272, and F₂-Tyr272 incorporated variants at 1.48, 1.23, and 1.80 Å resolution, respectively. The structural data showed only one halogen remained in the cofactor, indicating that an oxidative carbon-chlorine/fluorine bond scission has occurred during the autocatalytic process of cofactor biogenesis. Using hydroxyurea as a radical scavenger, the spin-coupled hidden Cu(II) was observed by EPR spectroscopy. Thus, the structurally defined catalytic center with genetic unnatural tyrosine substitution is in the radical containing form as in the wild-type, i.e., Cu(II)-(Cl-Tyr^•-Cys) or Cu(II)-(F-Tyr^•-Cys). These findings illustrate a previously unobserved C-F/C-Cl bond cleavage in biology mediated by a mononuclear copper center.

Monophenolase and catecholase activity of Aspergillus oryzae catechol oxidase: insights from hybrid QM/MM calculations

Catechol oxidase from Aspergillus oryzae (AoCO4) can not only catalyze oxidation of o-diphenols to o-quinones, but can also catalyze monooxygenation of small phenolics. To gain insight into the catecholase and monophenolase activities of AoCO4, the reaction mechanism of catechol oxidation was investigated by means of hybrid quantum mechanical/molecular mechanical (QM/MM) calculations. The oxy-form of AoCO4 was found to be a μ-η2:η2 side-on peroxo dicopper(ii) complex, which can undergo a proton coupled electron transfer from the substrate rather than a proton transfer from the nearby Ser302 residue to generate a hydroperoxide. The μ-1,1-OOH Cu2(i,ii) complex is thermodynamically more stable than the μ-η1:η2 hydroperoxide. Moreover, the cleavage of the O-O bond in the μ-1,1-OOH Cu2(i,ii) intermediate has a much lower barrier than that in the μ-η1:η2 hydroperoxide species. In both cases, the O-O bond cleavage is the rate-limiting step, generating the reactive (μ-O˙)(μ-OH) dicopper(ii) complex. In addition, our results demonstrated that the oxidation of catechol to quinone is much more preferred than the hydroxylation reaction. These findings may provide useful information for understanding the reactivity of the Cu2O2 active site of coupled binuclear copper enzymes.

Direct Sulfenylation of the Purine C8-H Bond with Thiophenols

The one-step copper-mediated regioselective formation of the C₈-S bond for purine derivatives with arylthiols was achieved using air as the green oxidant in the presence of 1.0 equiv of Na₂CO₃ and stoichiometric CuCl and 1,10-phenanthroline monohydrate. This method provides an economical, easy-to-handle, and effective method for the synthesis of 8-sulfenylpurine derivatives in moderate to excellent yields. The reaction is selective for C8 over C2 and C6. It also tolerates a free amine on the purine, and it has a wide substrate scope.

Harnessing the active site triad: merging hemilability, proton responsivity, and ligand-based redox-activity

Metalloenzymes catalyze many important reactions by managing the proton and electron flux at the enzyme active site. The motifs utilized to facilitate these transformations include hemilabile, redox-active, and so called proton responsive sites. Given the importance of incorporating and understanding these motifs in the area of coordination chemistry and catalysis, we highlight recent milestones in the field. Work incorporating the triad of hemilability, redox-activity, and proton responsivity into single ligand scaffolds will be described.

Determination of biocatalytic parameters of a copper radical oxidase using real-time reaction progress monitoring

VTNA is applied to reaction progress curves to glean key kinetic and mechanistic details for a copper radical oxidase.

Theory Demonstrated a "Coupled" Mechanism for O2 Activation and Substrate Hydroxylation by Binuclear Copper Monooxygenases

Multiscale simulations have been performed to address the longstanding issue of "dioxygen activation" by the binuclear copper monooxygenases (PHM and DβM), which have been traditionally classified as "noncoupled" binuclear copper enzymes. Our QM/MM calculations rule out that Cu_M(II)-O₂^• is an active species for H-abstraction from the substrate. In contrast, Cu_M(II)-O₂^• would abstract an H atom from the cosubstrate ascorbate to form a Cu_M(II)-OOH intermediate in PHM and DβM. Consistent with the recently reported structural features of DβM, the umbrella sampling shows that the "open" conformation of the Cu_M(II)-OOH intermediate could readily transform into the "closed" conformation in PHM, in which we located a mixed-valent μ-hydroperoxodicopper(I,II) intermediate, (μ-OOH)Cu(I)Cu(II). The subsequent O-O cleavage and OH moiety migration to Cu_H generate the unexpected species (μ-O^•)(μ-OH)Cu(II)Cu(II), which is revealed to be the reactive intermediate responsible for substrate hydroxylation. We also demonstrate that the flexible Met ligand is favorable for O-O cleavage reactions, while the replacement of Met with the strongly bound His ligand would inhibit the O-O cleavage reactivity. As such, the study not only demonstrates a "coupled" mechanism for O₂ activation by binuclear copper monooxygenases but also deciphers the full catalytic cycle of PHM and DβM in accord with the available experimental data. These findings of O₂ activation and substrate hydroxylation by binuclear copper monooxygenases could expand our understanding of the reactivities of the synthetic monocopper complexes.

Stabilizing a NiII-aqua complex via intramolecular hydrogen bonds: synthesis, structure, and redox properties

Hydrogen bonds within the secondary coordination sphere are effective in controlling the chemistry of synthetic metal complexes. Coupling the capacity of hydrogen bonds with those of redox-active ligands offers a promising approach to enhance the functional properties of transition metal complexes. These qualities were successfully illustrated with the [NNN]3-pincer ligand N,N' -(azanediylbis(2,l-phenylene))bis(2,4,6-triisopropyl-benzene-sulfonamido ([ibaps]3-) through the preparation of the NiII-OH2 complex, [NiII(ibaps)(OH2)]-. The [ibaps]3- ligand contains two appended sulfonamido groups that support the formation of intramolecular hydrogen bonds. The bulky 2,4,6-triisopropylphenyl rings are necessary to ensure that only one ligand binds to a single metal ion. The molecular structure of the complex shows a square planar N3O primary coordination sphere and two intramolecular hydrogen bonds involving the aqua ligand. Electrochemical measurements in acetonitrile revealed two oxidation events at potentials below that of the ferrocenium/ferrocene couple. Oxidation with 1 equiv of ferrocenium produced the one-electron oxidized species, [Ni(ibaps)(OH2)]. Experimental and computational studies support this assignment.

Probing the Cys-Tyr Cofactor Biogenesis in Cysteine Dioxygenase by the Genetic Incorporation of Fluorotyrosine

Cysteine dioxygenase (CDO) is a nonheme iron enzyme that adds two oxygen atoms from dioxygen to the sulfur atom of l-cysteine. Adjacent to the iron site of mammalian CDO, there is a post-translationally generated Cys-Tyr cofactor, whose presence substantially enhances the oxygenase activity. The formation of the Cys-Tyr cofactor in CDO is an autocatalytic process, and it is challenging to study by traditional techniques because the cross-linking reaction is a side, uncoupled, single-turnover oxidation buried among multiple turnovers of l-cysteine oxygenation. Here, we take advantage of our recent success in obtaining a purely uncross-linked human CDO due to site-specific incorporation of 3,5-difluoro-l-tyrosine (F₂-Tyr) at the cross-linking site through the genetic code expansion strategy. Using EPR spectroscopy, we show that nitric oxide (^•NO), an oxygen surrogate, similarly binds to uncross-linked F₂-Tyr157 CDO as in wild-type human CDO. We determined X-ray crystal structures of uncross-linked F₂-Tyr157 CDO and mature wild-type CDO in complex with both l-cysteine and ^•NO. These structural data reveal that the active site cysteine (Cys93 in the human enzyme), rather than the generally expected tyrosine (i.e., Tyr157), is well-aligned to be oxidized should the normal oxidation reaction uncouple. This structure-based understanding is further supported by a computational study with models built on the uncross-linked ternary complex structure. Together, these results strongly suggest that the first target to oxidize during the iron-assisted Cys-Tyr cofactor biogenesis is Cys93. Based on these data, a plausible reaction mechanism implementing a cysteine radical involved in the cross-link formation is proposed.

[FeII(L•)2][TCNQF4•−]2: A Redox-Active Double Radical Salt

The reaction of [FeII(L•)2][BF4]2 with LiTCNQF4 results in the formation of [FeII(L•)2][TCNQF4•−]2·2CH3CN (1) (L• is the neutral aminoxyl radical ligand 4,4-dimethyl-2,2-di(2-pyridyl)oxazolidine-N-oxide; TCNQF4 is 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane). Single-crystal X-ray diffraction; Raman, Fourier-transform infrared (FTIR) and ultraviolet–visible spectroscopies; and electrochemical studies are all consistent with the presence of a low-spin FeII ion, the neutral radical form (L•) of the ligand, and the radical anion TCNQF4•−. 1 is largely diamagnetic and the electrochemistry shows five well-resolved, diffusion-controlled, reversible one-electron processes.

Intramolecular Hydrogen Bonding Enhances Stability and Reactivity of Mononuclear Cupric Superoxide Complexes

[(L)CuII(O2•-)]+ (i.e., cupric-superoxo) complexes, as the first and/or key reactive intermediates in (bio)chemical Cu-oxidative processes, including in the monooxygenases PHM and DβM, have been systematically stabilized by intramolecular hydrogen bonding within a TMPA ligand-based framework. Also, gradual strengthening of ligand-derived H-bonding dramatically enhances the [(L)CuII(O2•-)]+ reactivity toward hydrogen-atom abstraction (HAA) of phenolic O-H bonds. Spectroscopic properties of the superoxo complexes and their azido analogues, [(L)CuII(N3-)]+, also systematically change as a function of ligand H-bonding capability.

Cleavage of a carbon-fluorine bond by an engineered cysteine dioxygenase

Cysteine dioxygenase (CDO) plays an essential role in sulfur metabolism by regulating homeostatic levels of cysteine. Human CDO contains a post-translationally generated Cys93-Tyr157 cross-linked cofactor. Here, we investigated this Cys-Tyr cross-linking by incorporating unnatural tyrosines in place of Tyr157 via a genetic method. The catalytically active variants were obtained with a thioether bond between Cys93 and the halogen-substituted Tyr157, and we determined the crystal structures of both wild-type and engineered CDO variants in the purely uncross-linked form and with a mature cofactor. Along with mass spectrometry and ¹⁹F NMR, these data indicated that the enzyme could catalyze oxidative C-F or C-Cl bond cleavage, resulting in a substantial conformational change of both Cys93 and Tyr157 during cofactor assembly. These findings provide insights into the mechanism of Cys-Tyr cofactor biogenesis and may aid the development of bioinspired aromatic carbon-halogen bond activation.

Oxygen Activation by Cu LPMOs in Recalcitrant Carbohydrate Polysaccharide Conversion to Monomer Sugars

Natural carbohydrate polymers such as starch, cellulose, and chitin provide renewable alternatives to fossil fuels as a source for fuels and materials. As such, there is considerable interest in their conversion for industrial purposes, which is evidenced by the established and emerging markets for products derived from these natural polymers. In many cases, this is achieved via industrial processes that use enzymes to break down carbohydrates to monomer sugars. One of the major challenges facing large-scale industrial applications utilizing natural carbohydrate polymers is rooted in the fact that naturally occurring forms of starch, cellulose, and chitin can have tightly packed organizations of polymer chains with low hydration levels, giving rise to crystalline structures that are highly recalcitrant to enzymatic degradation. The topic of this review is oxidative cleavage of carbohydrate polymers by lytic polysaccharide mono-oxygenases (LPMOs). LPMOs are copper-dependent enzymes (EC 1.14.99.53-56) that, with glycoside hydrolases, participate in the degradation of recalcitrant carbohydrate polymers. Their activity and structural underpinnings provide insights into biological mechanisms of polysaccharide degradation.

Protein oxidation involved in Cys-Tyr post-translational modification

Some post-translationally modified tyrosines can perform reversible redox chemistry similar to metal cofactors. The most studied of these tyrosine modifications is the intramolecular thioether-crosslinked 3'-(S-cysteinyl)-tyrosine (Cys-Tyr) in galactose oxidase. This Cu-mediated tyrosine modification in galactose oxidase involves direct electron transfer (inner-sphere) to the coordinated tyrosine. Mammalian cysteine dioxygenase enzymes also contain a Cys-Tyr that is formed, presumably, through outer-sphere electron transfer from a non-heme iron center ~6Å away from the parent residues. An orphan protein (BF4112), amenable to UV spectroscopic characterization, has also been shown to form Cys-Tyr between Tyr 52 and Cys 98 by an adjacent Cu²⁺ ion-loaded, mononuclear metal ion binding site. Native Cys-Tyr fluorescence under denaturing conditions provides a more robust methodology for Cys-Tyr yield determination. Cys-Tyr specificity, relative to 3,3'-dityrosine, was provided in this fluorescence assay by guanidinium chloride. Replacing Tyr 52 with Phe or the Cu²⁺ ion with a Zn²⁺ ion abolished Cys-Tyr formation. The Cys-Tyr fluorescence-based yields were decreased but not completely removed by surface Tyr mutations to Phe (Y4F/Y109F, 50%) and Cys 98 to Ser (25%). The small absorbance and fluorescence emission intensities for C98S BF4112 were surprising until a significantly red-shifted emission was observed. The red-shifted emission spectrum and monomer to dimer shift seen by reducing, denaturing SDS-PAGE demonstrate a surface tyrosyl radical product (dityrosine) when Cys 98 is replaced with Ser. These results demonstrate surface tyrosine oxidation in BF4112 during Cys-Tyr formation and that protein oxidation can be a significant side reaction in forming protein derived cofactors.

Identification of key structural features of the elusive Cu-Aβ complex that generates ROS in Alzheimer's disease

Oxidative stress is linked to the etiology of Alzheimer's disease (AD), the most common cause of dementia in the elderly. Redox active metal ions such as copper catalyze the production of Reactive Oxygen Species (ROS) when bound to the amyloid-β (Aβ) peptide encountered in AD. We propose that this reaction proceeds through a low-populated Cu-Aβ state, denoted the "catalytic in-between state" (CIBS), which is in equilibrium with the resting state (RS) of both Cu(i)-Aβ and Cu(ii)-Aβ. The nature of this CIBS is investigated in the present work. We report the use of complementary spectroscopic methods (X-ray absorption spectroscopy, EPR and NMR) to characterize the binding of Cu to a wide series of modified peptides in the RS. ROS production by the resulting Cu-peptide complexes was evaluated using fluorescence and UV-vis based methods and led to the identification of the amino acid residues involved in the Cu-Aβ CIBS species. In addition, a possible mechanism by which the ROS are produced is also proposed. These two main results are expected to affect the current vision of the ROS production mechanism by Cu-Aβ but also in other diseases involving amyloidogenic peptides with weakly structured copper binding sites.

Activation of dioxygen by copper metalloproteins and insights from model complexes

Nature uses dioxygen as a key oxidant in the transformation of biomolecules. Among the enzymes that are utilized for these reactions are copper-containing metalloenzymes, which are responsible for important biological functions such as the regulation of neurotransmitters, dioxygen transport, and cellular respiration. Enzymatic and model system studies work in tandem in order to gain an understanding of the fundamental reductive activation of dioxygen by copper complexes. This review covers the most recent advancements in the structures, spectroscopy, and reaction mechanisms for dioxygen-activating copper proteins and relevant synthetic models thereof. An emphasis has also been placed on cofactor biogenesis, a fundamentally important process whereby biomolecules are post-translationally modified by the pro-enzyme active site to generate cofactors which are essential for the catalytic enzymatic reaction. Significant questions remaining in copper-ion-mediated O2-activation in copper proteins are addressed.

Hybrid compounds assembled from copper-triazole complexes and phosphomolybdic acid as advanced catalysts for the oxidation of olefins with oxygen

Hybrid compounds of [CuI4(3atrz)₄][PMoVI11Mo^VO₄₀] (1) and [CuI6(3atrz)₆][PMo₁₂O₄₀]₂ (2) are active catalysts for olefin oxidation.