Integrating a Copper-Histidine Brace in a Mimetic Nanozyme Streamlines the Tyrosinase Recognition Moiety to Achieve Chiral Differentiation

Designing artificial mimetic enzymes with high activity/selectivity to replace chiral bioenzymes is of great interest in the development of chiral materials consisting of molecules, enantiomers, that exist in two forms as mirror images of one another but cannot be superimposed. In this study, the chiral catalytic structural unit was streamlined from tyrosinase to integrate a mimetic nanozyme. The chiral amino acid l-histidine, as the chiral binding/recognition site, and the active metal site Cu were coupled (Cu@l-His) to create a copper-histidine brace with enantioselective catalytic ability to tyrosinol enantiomers. Results of kinetic parameters and activation energies confirmed the excellent peroxidase-like activity with a preference of Cu@l-His to l-tyrosinol. Such a preference could be attributed to the structurally oriented copper-histidine brace with a stronger affinity and catalytic activity to l-tyrosinol. By accurately evaluating chiral recognition units derived from bioenzymes, stable and superior chiral mimetic nanoenzymes could be constructed in a more straightforward and simplified manner, and they could also be extended to the reconstruction of diverse chiral enzymes.

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.

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 Copper Cage‐Complex as Mimic of the pMMO Cu C Site

The active site of particulate methane monooxygenase (pMMO) and its mechanism of action are not known. Recently, the Cu_C site emerged as a potential active site, but to date it lacks any study on biomimetic resemblance of the coordination environment provided by the enzyme. Here, the synthesis of a cage ligand providing such an environment is reported. Copper is incorporated, and coordination occurs by the two imidazole and one carboxylate group offered by the ligand. Depending on the oxidation state, it can adopt different coordination modes, as evidenced by the solid‐state structures and computational investigation. The copper(I) state readily reacts with dioxygen and thereby undergoes CH activation. Moreover, the catalytic aerobic oxidation of hydroquinones as ubiquinol mimics is shown. Clean one‐electron oxidation occurs under mild conditions and EPR analysis of the copper(II) state in the presence of water reveals striking similarities to the data obtained from pMMO.

Refining details of the structural and electronic properties of the CuB site in pMMO enzyme through sequential molecular dynamics/CPKS-EPR calculations

This work investigated the structural and electronic properties of the copper mononuclear site of the PmoB part of the pMMO enzyme at the molecular level. We propose that the Cu_B catalytic site in the soluble portion of pMMO at room temperature and under physiological conditions is a mononuclear copper complex in a distorted octahedral arrangement with the residues His³³, His¹³⁷, and His¹³⁹ on the equatorial base and two water molecules on the axial axis. Our view was based on the molecular dynamics results and DFT calculations of the electronic paramagnetic resonance parameters and comparisons with experimental EPR data. This new proposed model for the Cu_B site brings additional support concerning the recent experimental evidence, which pointed out that a saturated coordination sphere of the copper ion in the Cu_B center is an essential factor that makes it less efficient than the Cu_C site in the methane oxidation. Therefore, according to the Cu_B site model proposed here, an additional step involving a displacement of at least one water molecule of the copper coordination sphere by the O₂ molecule prior to its activation must be necessary. This scenario is less likely to occur in the Cu_C center once this one is buried in the alpha-helices, which are part of the pMMO structure bound to the membrane wall, and consequently located in a less solvent-exposed region. In addition, we also present a simple and efficient sequential S-MD/CPKS protocol to compute EPR parameters that can, in principle, be expanded for the study of other copper-containing proteins.

A computational mechanistic study of CH hydroxylation with mononuclear copper–oxygen complexes

A computational study of methane hydroxylation by oxygen-bound monocopper complexes.

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.

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.

Copper binding and reactivity at the histidine brace motif: insights from mutational analysis of the Pseudomonas fluorescens copper chaperone CopC

The histidine brace (His-brace) is a copper-binding motif that is associated with both oxidative enzymes and proteinaceous copper chaperones. Here, we used biochemical and structural methods to characterize mutants of a His-brace-containing copper chaperone from Pseudomonas fluorescens (PfCopC). A total of 15 amino acid variants in primary and second-sphere residues were produced and characterized in terms of their copper binding and redox properties. PfCopC has a very high affinity for Cu(II) and also binds Cu(I). A high reorganization barrier likely prevents redox cycling and, thus, catalysis. In contrast, mutations in the conserved second-sphere Glu27 enable slow oxidation of ascorbate. The crystal structure of the variant E27A confirmed copper binding at the His-brace. Unexpectedly, Asp83 at the equatorial position was shown to be indispensable for Cu(II) binding in the His-brace of PfCopC. A PfCopC mutant that was designed to mimic the His-brace from lytic polysaccharide monooxygenase-like family X325 did not bind Cu(II), but was still able to bind Cu(I). These results highlight the importance of the proteinaceous environment around the copper His-brace for reactivity and, thus, the difference between enzyme and chaperone.