Copper Complexes as Bioinspired Models for Lytic Polysaccharide Monooxygenases

Copper-oxygen adducts: new trends in characterization and properties towards C-H activation

This review summarizes the latest discoveries in the field of C-H activation by copper monoxygenases and more particularly by their bioinspired systems. This work first describes the recent background on copper-containing enzymes along with additional interpretations about the nature of the active copper-oxygen intermediates. It then focuses on relevant examples of bioinorganic synthetic copper-oxygen intermediates according to their nuclearity (mono to polynuclear). This includes a detailed description of the spectroscopic features of these adducts as well as their reactivity towards the oxidation of recalcitrant Csp3 -H bonds. The last part is devoted to the significant expansion of heterogeneous catalytic systems based on copper-oxygen cores (i.e. within zeolite frameworks).

Enabling a bioinspired N,N,N-copper coordination motif through spatial control in UiO-67: synthesis and reactivity

Metal-organic frameworks (MOFs) featuring zirconium-based clusters are widely used for the development of functionalized materials due to their exceptional stability. In this study, we report the synthesis of a novel N,N,N-ligand compatible with a biphenyl dicarboxylic acid-based MOF. However, the resulting copper(I) complex exhibited unexpected coordination behaviour, lacking the intended trifold coordination motif. Herein, we demonstrate the successful immobilization of a bioinspired ligand within the MOF, which preserved its crystalline and porous nature while generating a well-defined copper site. Comprehensive spectroscopic analyses, including X-ray absorption, UV/Vis, and infrared spectroscopy, were conducted to investigate the copper site and its thermal behaviour. The immobilized ligand exhibited the desired tridentate coordination to copper, providing access to a coordination motif otherwise unattainable. Notably, water molecules were also found to coordinate to copper. Upon heating, the copper centre within the MOF exhibited reversible dehydration, suggesting facile creation of open coordination sites. Furthermore, the copper site displayed reduction at elevated temperatures and subsequent susceptibility to oxidation by molecular oxygen. Lastly, both the molecular complexes and the MOF were evaluated as catalysts for the oxidation of cyclohexane using hydrogen peroxide. This work highlights the successful immobilization of a bioinspired ligand in a zirconium-based MOF, shedding light on the structural features, thermal behaviour, and catalytic potential of the resulting copper sites.

Expanding the catalytic landscape of metalloenzymes with lytic polysaccharide monooxygenases

Lytic polysaccharide monooxygenases (LPMOs) have an essential role in global carbon cycle, industrial biomass processing and microbial pathogenicity by catalysing the oxidative cleavage of recalcitrant polysaccharides. Despite initially being considered monooxygenases, experimental and theoretical studies show that LPMOs are essentially peroxygenases, using a single copper ion and H2O2 for C-H bond oxygenation. Here, we examine LPMO catalysis, emphasizing key studies that have shaped our comprehension of their function, and address side and competing reactions that have partially obscured our understanding. Then, we compare this novel copper-peroxygenase reaction with reactions catalysed by haem iron enzymes, highlighting the different chemistries at play. We conclude by addressing some open questions surrounding LPMO catalysis, including the importance of peroxygenase and monooxygenase reactions in biological contexts, how LPMOs modulate copper site reactivity and potential protective mechanisms against oxidative damage.

Chemical protein synthesis enabled engineering of saccharide oxidative cleavage activity in artificial metalloenzymes

Chemical protein (semi-)synthesis is a powerful technique allowing the incorporation of unnatural functionalities at any desired protein site. Herein we describe a facile one-pot semi-synthetic strategy for the construction of a type 2 copper center in the active site of azurin, which is achieved by substitution of Met121 with unnatural amino acid residues bearing a strong ligand N,N-bis(pyridylmethyl)amine (DPA) to mimic the function of typical histidine brace-bearing copper monooxygenases, such as lytic polysaccharide monooxygenases (LPMOs) involved in polysaccharide breakdown. The semi-synthetic proteins were routinely obtained in over 10-mg scales to allow for spectroscopic measurements (UV-Vis, CD, and EPR), which provides structural evidences for the CuII-DPA-modified azurins. 4-nitrophenyl-β-D-glucopyranoside (PNPG) was used as a model substrate for the H2O2-driven oxidative cleavage reaction facilitated by semi-synthetic azurins, and the CuII-6 complex showed a highest activity (TTN 253). Interestingly, our semi-synthetic azurins were able to tolerate high H2O2 concentrations (up to 4000-fold of the enzyme), making them promising for practical applications. Collectively, we establish that chemical protein synthesis can be exploited as a reliable technology in affording large quantities of artificial metalloproteins to facilitate the transformation of challenging chemical reactions.

A designed Copper Histidine-brace enzyme for oxidative depolymerization of polysaccharides as a model of lytic polysaccharide monooxygenase

The "Histidine-brace" (His-brace) copper-binding site, composed of Cu(His)2 with a backbone amine, is found in metalloproteins with diverse functions. A primary example is lytic polysaccharide monooxygenase (LPMO), a class of enzymes that catalyze the oxidative depolymerization of polysaccharides, providing not only an energy source for native microorganisms but also a route to more effective industrial biomass conversion. Despite its importance, how the Cu His-brace site performs this unique and challenging oxidative depolymerization reaction remains to be understood. To answer this question, we have designed a biosynthetic model of LPMO by incorporating the Cu His-brace motif into azurin, an electron transfer protein. Spectroscopic studies, including ultraviolet-visible (UV-Vis) absorption and electron paramagnetic resonance, confirm copper binding at the designed His-brace site. Moreover, the designed protein is catalytically active towards both cellulose and starch, the native substrates of LPMO, generating degraded oligosaccharides with multiturnovers by C1 oxidation. It also performs oxidative cleavage of the model substrate 4-nitrophenyl-D-glucopyranoside, achieving a turnover number ~9% of that of a native LPMO assayed under identical conditions. This work presents a rationally designed artificial metalloenzyme that acts as a structural and functional mimic of LPMO, which provides a promising system for understanding the role of the Cu His-brace site in LPMO activity and potential application in polysaccharide degradation.

Molecular Mechanism of Substrate Oxidation in Lytic Polysaccharide Monooxygenases: Insight from Theoretical Investigations

Lytic polysaccharide monooxygenases (LPMOs) are copper enzymes that today comprise a large enzyme superfamily, grouped into the distinct members AA9-AA17 (with AA12 exempted). The LPMOs have the potential to facilitate the upcycling of biomass waste products by boosting the breakdown of cellulose and other recalcitrant polysaccharides. The cellulose biopolymer is the main component of biomass waste and thus comprises a large, unexploited resource. The LPMOs work through a catalytic, oxidative reaction whose mechanism is still controversial. For instance, the nature of the intermediate performing the oxidative reaction is an open question, and the same holds for the employed co-substrate. Here we review theoretical investigations addressing these questions. The applied theoretical methods are usually based on quantum mechanics (QM), often combined with molecular mechanics (QM/MM). We discuss advantages and disadvantages of the employed theoretical methods and comment on the interplay between theoretical and experimental results.

Design and directed evolution of noncanonical β-stereoselective metalloglycosidases

Metallohydrolases are ubiquitous in nearly all subclasses of hydrolases, utilizing metal elements to activate a water molecule and facilitate its subsequent dissociation of diverse chemical bonds. However, such a catalytic role of metal ions is rarely found with glycosidases that hydrolyze the glycosidic bonds in sugars. Herein, we design metalloglycosidases by constructing a hydrolytically active Zn-binding site within a barrel-shaped outer membrane protein OmpF. Structure- and mechanism-based redesign and directed evolution have led to the emergence of Zn-dependent glycosidases with catalytic proficiency of 2.8 × 10⁹ and high β-stereoselectivity. Biochemical characterizations suggest that the Zn-binding site constitutes a key catalytic motif along with at least one adjacent acidic residue. This work demonstrates that unprecedented metalloenzymes can be tailor-made, expanding the scope of inorganic reactivities in proteinaceous environments, resetting the structural and functional diversity of metalloenzymes, and providing the potential molecular basis of unidentified metallohydrolases and novel whole-cell biocatalysts.

Generation and Aerobic Oxidative Catalysis of a Cu(II) Superoxo Complex Supported by a Redox-Active Ligand

Cu systems feature prominently in aerobic oxidative catalysis in both biology and synthetic chemistry. Metal ligand cooperativity is a common theme in both areas as exemplified by galactose oxidase and by aminoxyl radicals in alcohol oxidations. This has motivated investigations into the aerobic chemistry of Cu and specifically the isolation and study of Cu-superoxo species that are invoked as key catalytic intermediates. While several examples of complexes that model biologically relevant Cu(II) superoxo intermediates have been reported, they are not typically competent aerobic catalysts. Here, we report a new Cu complex of the redox-active ligand ^tBu,TolDHP (2,5-bis((2-t-butylhydrazono)(p-tolyl)methyl)-pyrrole) that activates O₂ to generate a catalytically active Cu(II)-superoxo complex via ligand-based electron transfer. Characterization using ultraviolet (UV)-visible spectroscopy, Raman isotope labeling studies, and Cu extended X-ray absorption fine structure (EXAFS) analysis confirms the assignment of an end-on κ¹ superoxo complex. This Cu-O₂ complex engages in a range of aerobic catalytic oxidations with substrates including alcohols and aldehydes. These results demonstrate that bioinspired Cu systems can not only model important bioinorganic intermediates but can also mediate and provide mechanistic insight into aerobic oxidative transformations.

Oxidative Cleavage of Glycosidic Bonds by Synthetic Mimics of Lytic Polysaccharide Monooxygenases

Lytic polysaccharide monooxygenases (LPMOs) cleave polysaccharides through copper-bound oxyl radicals. We report a synthetic mimic of LPMO that uses micelle-stabilized hydrogen bonds to bind a glycan and two molecularly imprinted hydrophobic pockets to accommodate the alkyl tail of the glycoside and a copper cofactor, respectively. Cleavage of alkyl glycosides and oligosaccharides with hydrogen peroxide occurs at room temperature in aqueous solution, with selectivities for both the glycan and the alkyl aglycon.

Peptide-based chemical models for lytic polysaccharide monooxygenases

The Cu(ii)–HPH-NH2 and Cu(ii)–HPHPY-NH2 systems were proved to be relevant functional models of LPMOs, even at neutral pH.

Structural Elucidation, Aggregation, and Dynamic Behaviour of N,N,N,N-Copper(I) Schiff Base Complexes in Solid and in Solution: A Combined NMR, X-ray Spectroscopic and Crystallographic Investigation

A series of Cu(I) complexes of bidentate or tetradentate Schiff base ligands bearing either 1-H-imidazole or pyridine moieties were synthesized. The complexes were studied by a combination of NMR and X-ray spectroscopic techniques. The differences between the imidazole- and pyridine-based ligands were examined by 1H, 13C and 15N NMR spectroscopy. The magnitude of the 15Nimine coordination shifts was found to be strongly affected by the nature of the heterocycle in the complexes. These trends showed good correlation with the obtained Cu-Nimine bond lengths from single-crystal X-ray diffraction measurements. Variable-temperature NMR experiments, in combination with diffusion ordered spectroscopy (DOSY) revealed that one of the complexes underwent a temperature-dependent interconversion between a monomer, a dimer and a higher aggregate. The complexes bearing tetradentate imidazole ligands were further studied using Cu K-edge XAS and VtC XES, where DFT-assisted assignment of spectral features suggested that these complexes may form polynuclear oligomers in solid state. Additionally, the Cu(II) analogue of one of the complexes was incorporated into a metal-organic framework (MOF) as a way to obtain discrete, mononuclear complexes in the solid state.

Metalloglycosidase Mimics: Oxidative Cleavage of Saccharides Promoted by Multinuclear Copper Complexes under Physiological Conditions

Degradation of saccharides is relevant to the design of catalytic therapeutics, the production of biofuels, inhibition of biofilms, as well as other applications in chemical biology. Herein, we report the design of multinuclear Cu complexes that enable cleavage of saccharides under physiological conditions. Reactivity studies with para-nitrophenyl (pNP)-conjugated carbohydrates show that dinuclear Cu complexes exhibit a synergistic effect and promote faster and more robust cleavage of saccharide substrates, relative to the mononuclear Cu complex, while no further enhancement is observed for the tetranuclear Cu complex. The use of scavengers for reactive oxygen species confirms that saccharide cleavage is promoted by the formation of superoxide and hydroxyl radicals through Cu^II/I redox chemistry, similar to that observed for native copper-containing lytic polysaccharide monooxygenases (LMPOs). Differences in selectivity for di- and tetranuclear Cu complexes are modest. However, these are the first reported small multinuclear Cu complexes that show selectivity and reactivity against mono- and disaccharide substrates and form a basis for further development of metalloglycosidases for applications in chemical biology.

Activity-Based Sensing of Ascorbate by Using Copper-Mediated Oxidative Bond Cleavage

Ascorbate is an important biological reductant and enzyme cofactor. Although direct detection through ascorbate-mediated reduction is possible, this approach suffers from poor selectivity due to the wide range of cellular reducing agents. To overcome this limitation, we leverage reduction potential of ascorbate to mediate a copper-mediated oxidative bond cleavage of ether-caged fluorophores. The copper(II) complexes supported by a {bis(2-pyridylmethyl)}benzylamine or a {bis(2-pyridylmethyl)}(2-methoxybenzyl)amine ligand were identified as an ascorbate responsive unit and their reaction with ascorbate yields a copper-based oxidant that enables rapid benzylic oxidation and the release of an ether-caged dye (coumarin or fluorescein). The copper-mediated bond cleavage is specific to ascorbate and the trigger can be readily derivatized for tuning photophysical properties of the probes. The probes were successfully applied for the fluorometric detection of ascorbate in commercial food samples, human plasma, and serum, and within live cells by using confocal microscopy and flow cytometry.

Modelling a ‘histidine brace’ motif in mononuclear copper monooxygenases

A mononuclear copper complex bearing a ‘histidine brace’ is synthesised and characterised as an active-site model of mononuclear copper monooxygenases such as lytic polysaccharide monooxygenases (LPMOs) and particulate methane monooxygenase (pMMO).

An unusual partial occupancy of labile chloride and aqua ligands in cocrystallized isomers of a nickel(II) complex bearing a tripodal N4-donor ligand

A novel Ni²⁺ complex with the N₄-donor tripodal ligand bis[(1-methyl-1H-imidazol-2-yl)methyl][2-(pyridin-2-yl)ethyl]amine (L), namely, aqua{bis[(1-methyl-1H-imidazol-2-yl-κN³)methyl][2-(pyridin-2-yl-κN)ethyl]amine-κN}chloridonickel(II) perchlorate, [NiCl(C₁₇H₂₂N₆)(H₂O)]ClO₄ or [NiCl(H₂O)(L)Cl]ClO₄ (1), was synthesized and characterized by spectroscopic and spectrometric methods. The crystal structure of 1 reveals an interesting and unusual cocrystallization of isomeric complexes, which are crystallographically disordered with partial occupancy of the labile cis aqua and chloride ligands. The Ni²⁺ centre exhibits a distorted octahedral environment, with similar bond lengths for the two Ni-N(imidazole) bonds. The bond length increases for Ni-N(pyridine) and Ni-N(amine), which is in agreement with literature examples. The bond lengths of the disordered labile sites are also in the expected range and the Ni-Cl and Ni-O bond lengths are comparable with similar compounds. The electronic, redox and solution stability behaviour of 1 were also evaluated, and the data obtained suggest the maintenance of structural integrity, with no sign of demetalation or decomposition under the studied conditions.

Dimethylanilinic N-Oxides and Their Oxygen Surrogacy Role in the Formation of a Putative High-Valent Copper-Oxygen Species

The reaction of p-cyano-N,N-dimethylaniline N-oxide, an O-atom donor, with different copper(I) complexes (at room temperature and in acetone) indicates the formation via O-atom transfer of a high-valent copper oxyl species, CuII-O•, a putative key intermediate in the catalytic cycle of copper-containing monooxygenases. The formation of p-cyano-N-hydroxymethyl-N-methylaniline and p-cyano-N-methylaniline as the main products of the reaction highlight the capability of this species to hydroxylate strong C-H bonds (bond dissociation energy ∼ 90 kcal/mol). A plausible mechanism for the reactivity of this catalytic system is proposed.

Oxidative Cleavage of Cellobiose by Lytic Polysaccharide Monooxygenase (LPMO)-Inspired Copper Complexes

The potentially tridentate ligand bis[(1-methyl-2-benzimidazolyl)ethyl]amine (2BB) was employed to prepare copper complexes [(2BB)Cu^I]OTf and [(2BB)Cu^II(H₂O)₂](OTf)₂ as bioinspired models of lytic polysaccharide copper-dependent monooxygenase (LPMO) enzymes. Solid-state characterization of [(2BB)Cu^I]OTf revealed a Cu(I) center with a T-shaped coordination environment and metric parameters in the range of those observed in reduced LPMOs. Solution characterization of [(2BB)Cu^II(H₂O)₂](OTf)₂ indicates that [(2BB)Cu^II(H₂O)₂]²⁺ is the main species from pH 4 to 7.5; above pH 7.5, the hydroxo-bridged species [{(2BB)Cu^II(H₂O) _x }₂(μ-OH)₂]²⁺ is also present, on the basis of cyclic voltammetry and mass spectrometry. These observations imply that deprotonation of the central amine of Cu(II)-coordinated 2BB is precluded, and by extension, amine deprotonation in the histidine brace of LPMOs appears unlikely at neutral pH. The complexes [(2BB)Cu^I]OTf and [(2BB)Cu^II(H₂O)₂](OTf)₂ act as precursors for the oxidative degradation of cellobiose as a cellulose model substrate. Spectroscopic and reactivity studies indicate that a dicopper(II) side-on peroxide complex generated from [(2BB)Cu^I]OTf/O₂ or [(2BB)Cu^II(H₂O)₂](OTf)₂/H₂O₂/NEt₃ oxidizes cellobiose both in acetonitrile and aqueous phosphate buffer solutions, as evidenced from product analysis by high-performance liquid chromatography-mass spectrometry. The mixture of [(2BB)Cu^II(H₂O)₂](OTf)₂/H₂O₂/NEt₃ results in more extensive cellobiose degradation. Likewise, the use of both [(2BB)Cu^I]OTf and [(2BB)Cu^II(H₂O)₂](OTf)₂ with KO₂ afforded cellobiose oxidation products. In all cases, a common Cu(II) complex formulated as [(2BB)Cu^II(OH)(H₂O)]⁺ was detected by mass spectrometry as the final form of the complex.

Copper-Promoted Functionalization of Organic Molecules: from Biologically Relevant Cu/O2 Model Systems to Organometallic Transformations

Copper is one of the most abundant and less toxic transition metals. Nature takes advantage of the bioavailability and rich redox chemistry of Cu to carry out oxygenase and oxidase organic transformations using O₂ (or H₂O₂) as oxidant. Inspired by the reactivity of these Cu-dependent metalloenzymes, chemists have developed synthetic protocols to functionalize organic molecules under enviormentally benign conditions. Copper also promotes other transformations usually catalyzed by 4d and 5d transition metals (Pd, Pt, Rh, etc.) such as nitrene insertions or C-C and C-heteroatom coupling reactions. In this review, we summarized the most relevant research in which copper promotes or catalyzes the functionalization of organic molecules, including biological catalysis, bioinspired model systems, and organometallic reactivity. The reaction mechanisms by which these processes take place are discussed in detail.

Exclusive imidazole ligation to CuIII2O2 and CuIIICuII2O2 cores

Direct oxygenation of imidazole-ligated Cu(i) generates dinuclear and trinuclear Cu(iii) species with exclusive imidazole ligation.

Recent insights into lytic polysaccharide monooxygenases (LPMOs)

Lytic polysaccharide monooxygenases (LPMOs) are copper enzymes discovered within the last 10 years. By degrading recalcitrant substrates oxidatively, these enzymes are major contributors to the recycling of carbon in nature and are being used in the biorefinery industry. Recently, two new families of LPMOs have been defined and structurally characterized, AA14 and AA15, sharing many of previously found structural features. However, unlike most LPMOs to date, AA14 degrades xylan in the context of complex substrates, while AA15 is particularly interesting because they expand the presence of LPMOs from the predominantly microbial to the animal kingdom. The first two neutron crystallography structures have been determined, which, together with high-resolution room temperature X-ray structures, have putatively identified oxygen species at or near the active site of LPMOs. Many recent computational and experimental studies have also investigated the mechanism of action and substrate-binding mode of LPMOs. Perhaps, the most significant recent advance is the increasing structural and biochemical evidence, suggesting that LPMOs follow different mechanistic pathways with different substrates, co-substrates and reductants, by behaving as monooxygenases or peroxygenases with molecular oxygen or hydrogen peroxide as a co-substrate, respectively.

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.