Regioselective oxidative carbon-oxygen bond cleavage catalysed by copper(II) complexes: A relevant model study for lytic polysaccharides monooxygenases activity

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.

A newly-constructed bifunctional bacterial consortium for removing butyl xanthate and cadmium simultaneously from mineral processing wastewater: Experimental evaluation, degradation and biomineralization

Due to the technological limitations associated with beneficiation technology, large amounts of flotation reagents and heavy metals remain in mineral processing wastewater. Unfortunately, however, no treatment methods are available to mitigate the resulting pollution by them. In this study, a bacterial consortium SDMC (simultaneously degrade butyl xanthate and biomineralize cadmium) was constructed in an effort to simultaneously degrade butyl xanthate (BX) and biomineralize cadmium (Cd) by screening and domesticating two different bacterial species including Hypomicrobium and Sporosarcina. SDMC is efficient in removing the combined pollution due to BX and Cd with a 100% degradation rate for BX and 99% biomineralization rate for Cd within 4 h. Besides, SDMC can tolerate high concentrations of Fe(III) (0-40 mg/L). It has an excellent ability to utilize Fe(III) for enhanced removal of the combined pollutants. SDMC can effectively remove pollutants with a pH range of 6-9. Further, we discussed pathways for potential degradation and biomineralization: Cd(BX)₂-Cd²⁺, BX^-; BX^--CS₂, butyl perxanthate (BPX); Cd²⁺-(Ca0.67,Cd0.33)CO₃. The removal of the combined pollutants primarily entails decomposition, degradation, and biomineralization, C-O bond cleavage, and microbially induced carbonate precipitation (MICP). SDMC is a simple, efficient, and eco-friendly bifunctional bacterial consortium for effective treatment of BX-Cd combined pollution in mineral processing wastewater.

Advances in lytic polysaccharide monooxygenases with the cellulose-degrading auxiliary activity family 9 to facilitate cellulose degradation for biorefinery

One crucial step in processing the recalcitrant lignocellulosic biomass is the fast hydrolysis of natural cellulose to fermentable sugars that can be subsequently converted to biofuels and bio-based chemicals. Recent studies have shown that lytic polysaccharide monooxygenase (LPMOs) with auxiliary activity family 9 (AA9) are capable of efficiently depolymerizing the crystalline cellulose via regioselective oxidation reaction. Intriguingly, the catalysis by AA9 LPMOs requires reductant to provide electrons, and lignin and its phenolic derivatives can be oxidized, releasing reductant to activate the reaction. The activity of AA9 LPMOs can be enhanced by in-situ generation of H₂O₂ in the presence of O₂. Although scientific understanding of these enzymes remains somewhat unknown or controversial, structure modifications on AA9 LPMOs through protein engineering have emerged in recent years, which are prerequisite for their extensive applications in the development of cellulase-mediated lignocellulosic biorefinery processes. In this review, we critically comment on advances in studies for AA9 LPMOs, i.e., characteristic of AA9 LPMOs catalysis, external electron donors to AA9 LPMOs, especially the role of the oxidization of lignin and its derivatives, and AA9 LPMOs protein engineering as well as their extensive applications in the bioprocessing of lignocellulosic biomass. Perspectives are also highlighted for addressing the challenges.

Minimalist De Novo Design of an Artificial Enzyme

We employed a reductionist approach in designing the first heterochiral tripeptide that forms a robust heterogeneous short peptide catalyst similar to the "histidine brace" active site of lytic polysaccharide monooxygenases. The histidine brace is a conserved divalent copper ion-binding motif that comprises two histidine side chains and an amino group to create the T-shaped 3N geometry at the reaction center. The geometry parameters, including a large twist angle (73°) between the two imidazole rings of the model complex, are identical to those of native lytic polysaccharide monooxygenases (72.61°). The complex was synthesized and characterized as a structural and functional mimic of the histidine brace. UV-vis, vis-circular dichroism, Raman, and electron paramagnetic resonance spectroscopic analyses suggest a distorted square-pyramidal geometry with a 3N coordination at pH 7. Solution- and solid-state NMR results further confirm the 3N coordination in the copper center of the complex. The complex is pH-dependent and could catalyze the oxidation of benzyl alcohol in water to benzaldehyde with yields up to 82% in 3 h at pH 7 and above at 40 °C. The catalyst achieved 100% selectivity for benzaldehyde compared to conventional copper catalysis. The design of such a minimalist building block for functional soft materials with a pH switch can be a stepping stone in addressing needs for a cleaner and sustainable future catalyst.

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.

Fixation of atmospheric CO2 as C1-feedstock by nickel(ii) complexes

The development of molecular catalysts for the activation and conversion of atmospheric carbon dioxide (CO₂) into a value-added product is a great challenge. A series of nickel(ii) complexes, [Ni(L)(CH₃CN)₃](BPh₄)₂, 1-4 of diazepane based ligands, 4-methyl-1-[(pyridin-2-yl-methyl)]-1,4-diazepane (L1), 4-methyl-1-[2-(pyridine-2-yl)ethyl]-1,4-diazepane (L2), 4-methyl-1-[(quinoline-2-yl)-methyl]-1,4-diazepane (L3) and 1-[(4-methoxy-3,5-dimethyl-pyridin-2-yl)methyl]-4-methyl-1,4-diazepane (L4), have been synthesized and characterized as catalysts for the activation of atmospheric CO₂. The single-crystal X-ray structure of 1 shows a distorted octahedral geometry with a cis-β configuration around the NiN₆ coordination sphere. All the complexes are used as catalysts for the conversion of atmospheric CO₂ and epoxides into cyclic carbonates at 1 atmosphere (atm) pressure and in the presence of Et₃N. Catalyst 4 was found to be the most efficient catalyst and showed a 31% formation of cyclic carbonates with a TON of 620 under 1 atm air as the CO₂ source. This yield was enhanced to 94% with a TON of 1880 under 1 atm pure CO₂ gas and it is the highest catalytic efficiency known for nickel(ii)-based catalysts. Catalyst 4 enabled the transformation of a wide range of epoxides (eight examples) into corresponding cyclic carbonates with excellent selectivity (>99%) and yields of 59-94% and 11-31% under pure CO₂ and atmospheric CO₂, respectively. The catalytic efficiency is strongly influenced by the electronic nature of the complexes. The CO₂ fixation reactions without an epoxide substrate led to the formation of the carbonate bridged dinuclear nickel(ii) complexes [(LNi^II)₂CO₃](BPh₄)₂1a-4a, which are speculated as catalytically active intermediates. The formation of these species was accompanied by the formation of new absorption bands around 592-681 nm and was further confirmed by the ESI-MS and IR spectral studies. The molecular structures of these carbonate-bridged key intermediates were determined by X-ray analysis. The structures contain two Ni²⁺-centers bridged via a carbonate ion that originated from CO₂. Distorted square pyramidal geometries are adopted around each Ni(ii) center. All these results support that CO₂ fixation reactions occur via CO₂-bound nickel key intermediates.

Copper(II) complexes of tripodal ligand scaffold (N3O) as functional models for phenoxazinone synthase

The copper(II) complexes [Cu(L)NO₃] (1-9) of newer N₃O ligands (L1-L9) have been synthesized and characterized. The molecular structure of 1, 4, and 7 exhibited nearly a perfect square pyramidal geometry (τ, 0.04-0.11). The Cu-O_Phenolate bonds (~ 1.91 Å) are shorter than the Cu-N bonds (~ 2.06 Å) due to the stronger coordination of anionic phenolate oxygen. The Cu(II)/Cu(I) redox potentials of 1-9 appeared around -0.102 to -0.428 V versus Ag/Ag⁺ in water. The electronic spectra of the complexes showed the d-d transitions around 643-735 nm and axial EPR parameter (g_||, 2.243-2.270; A_||, 164-179 × 10^-4 cm^-1) that corresponds to square pyramidal geometry. The bonding parameters α², 0.760-0.825; β², 0.761-0.994; γ², 0.504-0.856 and K_||, 0.698-0.954 and K_⊥, 0.383-0.820 calculated from EPR spectra and energies of d-d transitions. The complexes catalyzed the conversion of substrate 2-aminophenol into 2-aminophenoxazine-3-one using molecular oxygen in the water and exhibited the yields of 41-61%. The formation of the product is accomplished by the appearance of a new absorption band at 430 nm and the rates of formation were calculated as 6.98-15.65 × 10^-3 s^-1 in water. The reaction follows Michaelis-Menten enzymatic reaction kinetics with turnover numbers (k_cat) 9.11 × 10⁵ h^-1 for 1 and 4.66 × 10⁵ h^-1 for 9 in water. The spectral, redox and kinetic studies were performed in water to mimic the enzymatic oxidation reaction conditions.

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).

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.