Chloride Control of the Mechanism of Human Serum Ceruloplasmin (Cp) Catalysis

Metal-Protein Hybrid Materials: Unlocking New Frontiers in Biomedical Applications

Metal-protein hybrid materials represent a novel class of functional materials that exhibit exceptional physicochemical properties and tunable structures, rendering them remarkable applications in diverse fields, including materials engineering, biocatalysis, biosensing, and biomedicine. The design and development of multifunctional and biocompatible metal-protein hybrid materials have been the subject of extensive research and a key aspiration for practical applications in clinical settings. This review provides a comprehensive analysis of the design strategies, intrinsic properties, and biomedical applications of these hybrid materials, with a specific emphasis on their potential in cancer therapy, drug and vaccine delivery, antibacterial treatments, and tissue regeneration. Through rational design, stable metal-protein hybrid materials can be synthesized using straightforward methods, enabling them with therapeutic, delivery, immunomodulatory, and other desired functionalities. Finally, the review outlines the existing limitations and challenges associated with metal-protein hybrid materials and evaluates their potential for clinical translation, providing insights into their practical implementation within biomedical applications.

The Ferroxidase-Permease System for Transport of Iron Across Membranes: From Yeast to Humans

Transport of iron across the cell membrane is a tightly controlled process carried out by specific proteins in all living cells. In yeast and in mammals, a system formed by an enzyme with ferroxidase activity coupled to a membrane transporter supports iron uptake or iron efflux, respectively. Ferroxidase belongs to the family of blue multicopper oxidases, enzymes able to couple the one-electron oxidation of substrate(s) to full reduction of molecular oxygen to water. On the other hand, the permeases are widely different and are specific to Fe3+ and Fe2+ in yeast and multicellular organisms, respectively. This review will describe the yeast and human ferroxidase-permease systems, highlighting similarities and differences in structure, function and regulation of the respective protein components.

Controllable multi-halogenation of a non-native substrate by SyrB2 iron halogenase

Geminal, multi-halogenated functional groups are widespread in natural products and pharmaceuticals, yet no synthetic methodologies exist that enable selective multi-halogenation of unactivated C-H bonds. Biocatalysts are powerful tools for late-stage C-H functionalization, as they operate with high degrees of regio-, chemo-, and stereoselectivity. 2-oxoglutarate (2OG)-dependent non-heme iron halogenases chlorinate and brominate aliphatic C-H bonds offering a solution for achieving these challenging transformations. Here, we describe the ability of a non-heme iron halogenase, SyrB2, to controllably halogenate non-native substrate alpha-aminobutyric acid (Aba) to yield mono-chlorinated, di-chlorinated, and tri-chlorinated products. These chemoselective outcomes are achieved by controlling the loading of 2OG cofactor and SyrB2 biocatalyst. By using a ferredoxin-based biological reductant for electron transfer to the catalytic center of SyrB2, we demonstrate order-of-magnitude enhancement in the yield of tri-chlorinated product that were previously inaccessible using any single halogenase enzyme. We also apply these strategies to broaden SyrB2's reactivity scope to include multi-bromination and demonstrate chemoenzymatic conversion of the ethyl side chain in Aba to an ethylyne functional group. We show how steric hindrance induced by the successive addition of halogen atoms on Aba's C4 carbon dictates the degree of multi-halogenation by hampering C3-C4 bond rotation within SyrB2's catalytic pocket. Overall, our work showcases the synthetic potential of iron halogenases to facilitate multi-C-H functionalization chemistry.

Degradation of Amyloid-β Species by Multi-Copper Oxidases

Background

Reduction of the production of amyloid-β (Aβ) species has been intensively investigated as potential therapeutic approaches for Alzheimer's disease (AD). However, the degradation of Aβ species, another potential beneficial approach, has been far less explored.

Objective

To investigate the potential of multi-copper oxidases (MCOs) in degrading Aβ peptides and their potential benefits for AD treatment.

Methods

We investigated the degradation efficiency of MCOs by using electrophoresis and validated the ceruloplasmin (CP)-Aβ interaction using total internal reflection fluorescence microscopy, fluorescence photometer, and fluorescence polarization measurement. We also investigated the therapeutic effect of ascorbate oxidase (AO) by using induced pluripotent stem (iPS) neuron cells and electrophysiological analysis with brain slices.

Results

We discovered that CP, an important MCO in human blood, could degrade Aβ peptides. We also found that other MCOs could induce Aβ degradation as well. Remarkably, we revealed that AO had the strongest degrading effect among the tested MCOs. Using iPS neuron cells, we observed that AO could rescue neuron toxicity which induced by Aβ oligomers. In addition, our electrophysiological analysis with brain slices suggested that AO could prevent an Aβ-induced deficit in synaptic transmission in the hippocampus.

Conclusions

To the best of our knowledge, our report is the first to demonstrate that MCOs have a degrading function for peptides/proteins. Further investigations are warranted to explore the possible benefits of MCOs for future AD treatment.

Degradation of amyloid beta species by multi-copper oxidases

Reduction of the production of amyloid beta (Aβ) species has been intensively investigated as potential therapeutic approaches for Alzheimer's disease (AD). However, the degradation of Aβ species, another potential beneficial approach, has been far less explored. In this study, we discovered that ceruloplasmin (CP), an important multi-copper oxidase (MCO) in human blood, could degrade Aβ peptides. We also found that the presence of Vitamin C could enhance the degrading effect in a concentration-dependent manner. We then validated the CP-Aβ interaction using total internal reflection fluorescence (TIRF) microscopy, fluorescence photometer, and fluorescence polarization measurement. Based on the above discovery, we hypothesized that other MCOs had similar Aβ-degrading functions. Indeed, we found that other MCOs could induce Aβ degradation as well. Remarkably, we revealed that ascorbate oxidase (AO) had the strongest degrading effect among the tested MCOs. Using induced pluripotent stem (iPS) neuron cells, we observed that AO could rescue neuron toxicity which induced by Aβ oligomers. In addition, our electrophysiological analysis with brain slices suggested that AO could prevent an Ab-induced deficit in synaptic transmission in the hippocampus. To the best of our knowledge, our report is the first to demonstrate that MCOs have a degrading function for peptides/proteins. Further investigations are warranted to explore the possible benefits of MCOs for future AD treatment.

Tuning the Type 1 Reduction Potential of Multicopper Oxidases: Uncoupling the Effects of Electrostatics and H-Bonding to Histidine Ligands

In multicopper oxidases (MCOs), the type 1 (T1) Cu accepts electrons from the substrate and transfers these to the trinuclear Cu cluster (TNC) where O₂ is reduced to H₂O. The T1 potential in MCOs varies from 340 to 780 mV, a range not explained by the existing literature. This study focused on the ∼350 mV difference in potential of the T1 center in Fet3p and Trametes versicolor laccase (TvL) that have the same 2His1Cys ligand set. A range of spectroscopies performed on the oxidized and reduced T1 sites in these MCOs shows that they have equivalent geometric and electronic structures. However, the two His ligands of the T1 Cu in Fet3p are H-bonded to carboxylate residues, while in TvL they are H-bonded to noncharged groups. Electron spin echo envelope modulation spectroscopy shows that there are significant differences in the second-sphere H-bonding interactions in the two T1 centers. Redox titrations on type 2-depleted derivatives of Fet3p and its D409A and E185A variants reveal that the two carboxylates (D409 and E185) lower the T1 potential by 110 and 255-285 mV, respectively. Density functional theory calculations uncouple the effects of the charge of the carboxylates and their difference in H-bonding interactions with the His ligands on the T1 potential, indicating 90-150 mV for anionic charge and ∼100 mV for a strong H-bond. Finally, this study provides an explanation for the generally low potentials of metallooxidases relative to the wide range of potentials of the organic oxidases in terms of different oxidized states of their TNCs involved in catalytic turnover.

Molecular Functions of Ceruloplasmin in Metabolic Disease Pathology

Ceruloplasmin (CP) is a multicopper oxidase and antioxidant that is mainly produced in the liver. CP not only plays a crucial role in the metabolic balance of copper and iron through its oxidase function but also exhibits antioxidant activity. In addition, CP is an acute-phase protein. In addition to being associated with aceruloplasminemia and neurodegenerative diseases such as Wilson's disease, Alzheimer's disease, and Parkinson's disease, CP also plays an important role in metabolic diseases, which are caused by metabolic disorders and vigorous metabolism, mainly including diabetes, obesity, hyperlipidemia, etc. Based on the physiological functions of CP, we provide an overview of the association of type 2 diabetes, obesity, hyperlipidemia, coronary heart disease, CP oxidative stress, inflammation, and metabolism of copper and iron. Studies have shown that metabolic diseases are closely related to systemic inflammation, oxidative stress, and disorders of copper and iron metabolism. Therefore, we conclude that CP, which can reduce the formation of free radicals in tissues, can be induced during inflammation and infection, and can correct the metabolic disorder of copper and iron, has protective and diagnostic effects on metabolic diseases.

Multiple Proton-Coupled Electron Transfers at a Tricopper Cluster: Modeling the Reductive Regeneration Process in Multicopper Oxidases

Metal clusters in enzymes carry out the life-sustaining reactions by accumulating multiple redox equivalents in a narrow potential range. This redox potential leveling effect commonly observed in Nature has yet to be reproduced with synthetic metal clusters. Herein, we employ a fully encapsulated synthetic tricopper complex to model the three-electron two-proton reductive regeneration of fully reduced trinuclear copper cluster Cu^ICu^ICu^I(μ₂-OH₂) (FR) from native intermediate Cu^IICu^IICu^II(μ₃-O) (NI) in multicopper oxidases (MCOs). The tricopper cluster can access four oxidation states (I,I,I to II,II,II) and four protonation states ([Cu₃(μ₃-O)]LH, [Cu₃(μ₃-OH)]L, [Cu₃(μ₃-OH)]LH, and [Cu₃(μ₃-OH₂)]L, where LH denotes the protonated ligand), allowing mechanistic investigation of proton-coupled electron transfer (PCET) relevant to MCOs. Seven tricopper complexes with discrete oxidation and protonation states were characterized with spectroscopy or X-ray single-crystal diffraction. A stepwise electron transfer-proton transfer (ET-PT) mechanism is established for the reduction of Cu^IICu^IICu^II(μ₃-O)LH to Cu^IICu^IICu^I(μ₃-OH)L, while a stepwise PT-ET mechanism is determined for the reduction of Cu^IICu^ICu^I(μ₃-OH)LH to Cu^ICu^ICu^I(μ₂-OH₂)L. The switch-over from ET-PT to PT-ET mechanism showcases that the tricopper complex can adopt different PCET mechanisms to circumvent high-barrier proton transfer steps. Overall, three-electron two-proton reduction occurs within a narrow potential range of 170 mV, exemplifying the redox potential leveling effect of secondary proton relays in delivering multiple redox equivalents at metal clusters.

Electron Transfer to the Trinuclear Copper Cluster in Electrocatalysis by the Multicopper Oxidases

High-potential multicopper oxidases (MCOs) are excellent catalysts able to perform the oxygen reduction reaction (ORR) at remarkably low overpotentials. Moreover, MCOs are able to interact directly with the electrode surfaces via direct electron transfer (DET), that makes them the most commonly used electrocatalysts for oxygen reduction in biofuel cells. The central question in MCO electrocatalysis is whether the type 1 (T1) Cu is the primary electron acceptor site from the electrode, or whether electrons can be transferred directly to the trinuclear copper cluster (TNC), bypassing the rate-limiting intramolecular electron transfer step from the T1 site. Here, using site-directed mutagenesis and electrochemical methods combined with data modeling of electrode kinetics, we have found that there is no preferential superexchange pathway for DET to the T1 site. However, due to the high reorganization energy of the fully oxidized TNC, electron transfer from the electrode to the TNC does occur primarily through the T1 site. We have further demonstrated that the lower reorganization energy of the TNC in its two-electron reduced, alternative resting, form enables DET to the TNC, but this only occurs in the first turnover. This study provides insight into the factors that control the kinetics of electrocatalysis by the MCOs and a guide for the design of more efficient biocathodes for the ORR.

A spore-based portable kit for on-site detection of fluoride ions

The excess residues of fluoride ions cause serious human health problems, making their detection highly valuable. In this work, a whole-cell-based biosensor was presented for the detection of fluoride ions, which can inhibit the color reaction of 3,3',5,5',-tetramethylbenzidine (TMB) catalyzed by the CotA-laccase of spore surface. This reaction for the detection of fluoride ions could be read out through UV-vis spectrophotometer, smartphone, or standard colorimetric card within 10 min. Under optimum conditions, a linear range of 1-600 μmol L^-1 with a detection limit of 0.12 μmol L^-1 (3σ/k) was achieved for fluoride ions detection by using UV-vis spectrophotometer. The biosensor coupling with smartphone had a good linear response to fluoride ions concentration in the range of 5-600 μmol L^-1 with LOD of 0.90 μmol L^-1 (3σ/k). The standard colorimetric card can be directly used for recognizing the fluoride ions level via naked-eyes. A portable kit based on a colorimetric card and smartphone was developed and has been successfully applied for fluoride ions monitoring in surface waters and groundwater. This developed method has several advantages such as rapid, outstanding selectivity and anti-interference, low-cost, ease of operation and storage, and eco-friendliness, meeting the demands of point-of-care testing of fluoride ions and disease prevention.

Serum Ceruloplasmin Is the Candidate Predictive Biomarker for Acute Aortic Dissection and Is Related to Thrombosed False Lumen: a Propensity Score-Matched Observational Case-Control Study

Acute aortic dissection (AAD), one of the fatal diseases observed at the department of vascular surgery, is associated with a great mortality rate at the early stage. Ceruloplasmin (CP) is the plasma protein that functions as a copper transporter. The current retrospective research was carried out to assess CP contents and to examine the possible part in diagnosing patients with AAD. In addition, propensity score matching (PSM) was also utilized for reducing the bias in case screening as well as the clinical confounders. Using PSM, this study included 85 pairs of AAD cases (Stanford A and B dissection) and matched controls, and their CP levels were also detected through enzyme-linked immunosorbent assay (ELISA). Additionally, the relative clinical data were extracted from participants included in this study. After PSM adjustment for clinical variables, including gender, age, body mass index (BMI), heart ratio (HR), smoking, hypertension, diabetes mellitus, coronary heart disease (CHD), and stroke, the serum CP contents among AAD cases were remarkably increased compared with those among the normal subjects. Besides, the CP contents showed independent association with the AAD risk. Typically, the CP level was significantly positively correlated with platelet (R = 0.329) or C-reactive protein (R = 0.340) level. Meanwhile, the area under the receiver operating characteristic (ROC) curve (AUC) was 0.929 when CP was used to diagnose AAD, and the best threshold value was 36.82mg/dL. Serum CP content significantly increased in cases with thrombosed false lumen (FL) relative to those in patent FL cases. Results of logistic regression analysis suggested that a greater CP content indicated an increased thrombosed FL risk (OR = 1.11; 95% CI: 1.01-1.23; P = 0.040). Findings in this study suggest that serum ceruloplasmin contents evidently increased among acute aortic dissection cases. CP shows close correlation with the inflammatory factors among AAD cases. Further, CP may serve as the candidate biomarker to diagnose AAD and to identify an increased risk of thrombosed false lumen.

Encapsulation of tricopper cluster in a synthetic cryptand enables facile redox processes from CuICuICuI to CuIICuIICuII states

One-pot reaction of tris(2-aminoethyl)amine (TREN), [Cu^I(MeCN)₄]PF₆, and paraformaldehyde affords a mixed-valent [TREN4Cu^IICu^ICu^I(μ₃-OH)](PF₆)₃ complex. The macrocyclic azacryptand TREN4 contains four TREN motifs, three of which provide a bowl-shape binding pocket for the [Cu₃(μ₃-OH)]³⁺ core. The fourth TREN caps on top of the tricopper cluster to form a cryptand, imposing conformational constraints and preventing solvent interaction. Contrasting the limited redox capability of synthetic tricopper complexes reported so far, [TREN4Cu^IICu^ICu^I(μ₃-OH)](PF₆)₃ exhibits several reversible single-electron redox events. The distinct electrochemical behaviors of [TREN4Cu^IICu^ICu^I(μ₃-OH)](PF₆)₃ and its solvent-exposed analog [TREN3Cu^IICu^IICu^II(μ₃-O)](PF₆)₄ suggest that isolation of tricopper core in a cryptand enables facile electron transfer, allowing potential application of synthetic tricopper complexes as redox catalysts. Indeed, the fully reduced [TREN4Cu^ICu^ICu^I(μ₃-OH)](PF₆)₂ can reduce O₂ under acidic conditions. The geometric constraints provided by the cryptand are reminiscent of Nature's multicopper oxidases (MCOs). For the first time, a synthetic tricopper cluster was isolated and fully characterized at Cu^ICu^ICu^I (4a), Cu^IICu^ICu^I (4b), and Cu^IICu^IICu^I (4c) states, providing structural and spectroscopic models for many intermediates in MCOs. Fast electron transfer rates (10⁵ to 10⁶ M^-1 s^-1) were observed for both Cu^ICu^ICu^I/Cu^IICu^ICu^I and Cu^IICu^ICu^I/Cu^IICu^IICu^I redox couples, approaching the rapid electron transfer rates of copper sites in MCO.

Role of a Tyrosine Radical in Human Ceruloplasmin Catalysis

Multicopper oxidases (MCOs) are a large family of diverse enzymes found in both eukaryotes and prokaryotes that couple one-electron oxidations of various substrates to the four-electron reduction of O2 to H2O, functioning through a set of metallocofactors consisting of one type 1 copper (T1 Cu) and one trinuclear copper cluster (TNC). Human serum ceruloplasmin (Cp) is a unique member of MCOs composed of six cupredoxin domains and harbors six Cu ions arranged as three T1 Cu and one TNC. The native substrate of Cp is Fe2+. It is an essential ferroxidase critical for iron homeostasis and is closely associated with metal-mediated diseases and metal neurotoxicity. In human serum, Cp operates under substrate-limiting low [Fe2+] but high [O2] conditions, implying the possible involvement of partially reduced intermediates in Cp catalysis. In this work, we studied for the first time Cp reactivities at defined partially reduced states and discovered a tyrosine radical weakly magnetically coupled to the native intermediate (NI) of the TNC via a hydrogen bond. Our results lead to a new hypothesis that human iron transport is regulated as the paired transfer of iron from ferroportin to Cp to transferrin, and the tyrosine residue in Cp acts as a gate to avoid reactive oxygen species (ROS) formation when Fe2+ delivery is dysregulated.

Rapid Decay of the Native Intermediate in the Metallooxidase Fet3p Enables Controlled FeII Oxidation for Efficient Metabolism

The multicopper oxidases (MCOs) couple four 1e^- oxidations of substrate to the 4e^- reduction of O₂ to H₂O. These divide into two groups: those that oxidize organic substrates with high turnover frequencies (TOFs) up to 560 s^-1 and those that oxidize metal ions with low TOFs, ∼1 s^-1 or less. The catalytic mechanism of the organic oxidases has been elucidated, and the high TOF is achieved through rapid intramolecular electron transfer (IET) to the native intermediate (NI), which only slowly decays to the resting form. Here, we uncover the factors that govern the low TOF in Fet3p, a prototypical metallooxidase, in the context of the MCO mechanism. We determine that the NI decays rapidly under optimal turnover conditions, and the mechanism thereby becomes rate-limited by slow IET to the resting enzyme. Development of a catalytic model leads to the important conclusions that proton delivery to the NI controls the mechanism and enables the slow turnover in Fet3p that is functionally significant in Fe metabolism enabling efficient ferroxidase activity while avoiding ROS generation.

Inhibition in multicopper oxidases: a critical review

This review critiques the literature on inhibition of O₂-reduction catalysis in multicopper oxidases like laccase and bilirubin oxidase and provide recommendations for best practice when carrying out experiments and interpreting published data.