Glutathione limits aquacopper(I) to sub-femtomolar concentrations through cooperative assembly of a tetranuclear cluster

Interplay between Copper, Phosphatidylserine, and α-Synuclein Suggests a Link between Copper Homeostasis and Synaptic Vesicle Cycling

Copper homeostasis is critical to the functioning of the brain, and its breakdown is linked with many brain diseases. Copper is also known to interact with the negatively charged lipid, phosphatidylserine (PS), as well as α-synuclein, an aggregation-prone protein enriched in the synapse, which plays a role in synaptic vesicle docking and fusion. However, the interplay between copper, PS lipid, and α-synuclein is not known. Herein, we report a detailed and predominantly kinetic study of the interactions among these three components pertinent to copper homeostasis and neurotransmission. We found that synaptic vesicle-mimicking small unilamellar vesicles (SUVs) can sequester any excess free Cu2+ within milliseconds, and bound Cu2+ on SUVs can be reduced to Cu+ by GSH at a nearly constant rate under physiological conditions. Moreover, we revealed that SUV-bound Cu2+ does not affect the binding between wild-type α-synuclein and SUVs but affect that between N-terminal acetylated α-synuclein and SUVs. In contrast, Cu2+ can effectively displace both types of α-synuclein from the vesicles. Our results suggest that synaptic vesicles may mediate copper transfer in the brain, while copper could participate in synaptic vesicle docking to the plasma membrane via its regulation of the interaction between α-synuclein and synaptic vesicle.

Protein-Labeling Reagents Selectively Activated by Copper(I)

Copper is an essential trace element that participates in many biological processes through its unique redox cycling between cuprous (Cu+) and cupric (Cu2+) oxidation states. To elucidate the biological functions of copper, chemical biology tools that enable selective visualization and detection of copper ions and proteins in copper-rich environments are required. Herein, we describe the design of Cu+-responsive reagents based on a conditional protein labeling strategy. Upon binding Cu+, the probes generated quinone methide via oxidative bond cleavage, which allowed covalent labeling of surrounding proteins with high Cu+ selectivity. Using gel- and imaging-based analyses, the best-performing probe successfully detected changes in the concentration of labile Cu+ in living cells. Moreover, conditional proteomics analysis suggested intramitochondrial Cu+ accumulation in cells undergoing cuproptosis. Our results highlight the power of Cu+-responsive protein labeling in providing insights into the molecular mechanisms of Cu+ metabolism and homeostasis.

Cu(I)-Glutathione Assembly Supported on ZIF-8 as Robust and Efficient Catalyst for Mild CO2 Conversions

The Cu-glutathione (GSH) redox system, essential in biology, is designed here as a supramacromolecular assembly in which the tetrahedral 18e Cu(I) center loses a thiol ligand upon adsorption onto ZIF-8, as shown by EXAFS and DFT calculation, to generate a very robust 16e planar trigonal single-atom Cu(I) catalyst. Synergy between Cu(I) and ZIF-8, revealed by catalytic experiments and DFT calculation, affords CO2 conversion into high-value-added chemicals with a wide scope of substrates by reaction with terminal alkynes or propargyl amines in excellent yields under mild conditions and reuse at least 10 times without significant decrease in catalytic efficiency.

Glutathione Protects other Cellular Thiols against Oxidation by CuII-Dp44mT

Cu-thiosemicarbazones have been intensively investigated for their application in cancer therapy or as antimicrobials. Copper(II)-di-2-pyridylketone-4,4-dimethyl-thiosemicarbazone (CuII-Dp44mT) showed anticancer activity in the submicromolar concentration range in cell culture. The interaction of CuII-Dp44mT with thiols leading to their depletion or inhibition was proposed to be involved in this activity. Indeed, CuII-Dp44mT can catalyze the oxidation of thiols although with slow kinetics. The present work aims to obtain insights into the catalytic activity and selectivity of CuII-Dp44mT toward the oxidation of different biologically relevant thiols. Reduced glutathione (GSH), L-cysteine (Cys), N-acetylcysteine (NAC), D-penicillamine (D-Pen), and the two model proteins glutaredoxin (Grx) and thioredoxin (Trx) were investigated. CuII-Dp44mT catalyzed the oxidation of these thiols with different kinetics, with rates in the following order D-Pen>Cys≫NAC>GSH and Trx>Grx. CuII-Dp44mT was more efficient than CuII chloride for the oxidation of NAC and GSH, but not D-Pen and Cys. In mixtures of biologically relevant concentrations of GSH and either Cys, Trx, or Grx, the oxidation kinetics and spectral properties were similar to that of GSH alone, indicating that the interaction of these thiols with CuII-Dp44mT is dominated by GSH. Hence GSH could protect other thiols against potential deleterious oxidation by CuII-Dp44mT.

Resisting death by metal: metabolism and Cu/Zn homeostasis in bacteria

Metal ions such as zinc and copper play important roles in host-microbe interactions and their availability can drastically affect the survival of pathogenic bacteria in a host niche. Mechanisms of metal homeostasis protect bacteria from starvation, or intoxication, defined as when metals are limiting, or in excess, respectively. In this mini-review, we summarise current knowledge on the mechanisms of resistance to metal stress in bacteria, focussing specifically on the homeostasis of cellular copper and zinc. This includes a summary of the factors that subvert metal stress in bacteria, which are independent of metal efflux systems, and commentary on the role of small molecules and metabolic systems as important mediators of metal resistance.

Glutathione Mediates Control of Dual Differential Bio-orthogonal Labelling of Biomolecules

Traditional approaches to bio-orthogonal reaction discovery have focused on developing reagent pairs that react with each other faster than they are metabolically degraded. Glutathione (GSH) is typically responsible for the deactivation of most bio-orthogonal reagents. Here we demonstrate that GSH promotes a Cu-catalysed (3+2) cycloaddition reaction between an ynamine and an azide. We show that GSH acts as a redox modulator to control the Cu oxidation state in these cycloadditions. Rate enhancement of this reaction is specific for ynamine substrates and is tuneable by the Cu:GSH ratio. This unique GSH-mediated reactivity gradient is then utilised in the dual sequential bio-orthogonal labelling of peptides and oligonucleotides via two distinct chemoselective (3+2) cycloadditions.

Glutathione Mediates Control of Dual Differential Bio-orthogonal Labelling of Biomolecules

Traditional approaches to bio-orthogonal reaction discovery have focused on developing reagent pairs that react with each other faster than they are metabolically degraded. Glutathione (GSH) is typically responsible for the deactivation of most bio-orthogonal reagents. Here we demonstrate that GSH promotes a Cu-catalysed (3+2) cycloaddition reaction between an ynamine and an azide. We show that GSH acts as a redox modulator to control the Cu oxidation state in these cycloadditions. Rate enhancement of this reaction is specific for ynamine substrates and is tuneable by the Cu:GSH ratio. This unique GSH-mediated reactivity gradient is then utilised in the dual sequential bio-orthogonal labelling of peptides and oligonucleotides via two distinct chemoselective (3+2) cycloadditions.

Discovery of an unusual copper homeostatic mechanism in yeast cells respiring on minimal medium and an unexpectedly diverse labile copper pool

Copper is essential for all eukaryotic cells but many details of how it is trafficked within the cell and how it is homeostatically regulated remain uncertain. Here, we characterized the copper content of cytosol and mitochondria using liquid chromatography with ICP-MS detection. Chromatograms of cytosol exhibited over two dozen peaks due to copper proteins and coordination complexes. Yeast cells respiring on minimal media did not regulate copper import as media copper concentration increased; rather, they imported copper at increasing rates while simultaneously increasing the expression of metallothionein CUP1 which then sequestered most of the excessive imported copper. Peak intensities due to superoxide dismutase SOD1, other copper proteins, and numerous coordination complexes also increased, but not as drastically. The labile copper pool was unexpectedly diverse and divided into two groups. One group approximately comigrated with copper-glutathione, -cysteine, and -histidine standards; the other developed only at high media copper concentrations and at greater elution volumes. Most cytosolic copper arose from copper-bound proteins, especially CUP1. Cytosol contained an unexpectedly high percentage of apo-copper proteins and apo-coordination complexes. Copper-bound forms of non-CUP1 proteins and complexes coexisted with apo-CUP1 and with the chelator BCS. Both experiments suggest unexpectedly stable-binding copper proteins and coordination complexes in cytosol. COX17Δ cytosol chromatograms were like those of WT cells. Chromatograms of soluble mitochondrial extracts were obtained, and mitoplasting helped distinguish copper species in the intermembrane space versus in the matrix/inner membrane. Issues involving the yeast copperome, copper homeostasis, labile copper pool, and copper trafficking are discussed.

ROS-producing nanomaterial engineered from Cu(I) complexes with P2N2-ligands for cancer cells treating

The work presents core-shell nanoparticles (NPs) built from the novel Cu(I) complexes with cyclic P2N2-ligands (1,5-diaza-3,7-diphosphacyclooctanes) that can visualize their entry into cancer and normal cells using a luminescent signal and treat cells by self-enhancing generation of reactive oxygen species (ROS). Variation of P- and N-substituents in the series of P2N2-ligands allows structure optimization of the Cu(I) complexes for the formation of the luminescent NPs with high chemical stability. The non-covalent modification of the NPs with triblock copolymer F-127 provides their high colloidal stability, followed by efficient cell internalization of the NPs visualized by their blue (⁓450 nm) luminescence. The cytotoxic effects of the NPs toward the normal and some of cancer cells are significantly lower than those of the corresponding molecular complexes, which correlates with the chemical stability of the NPs in the solutions. The ability of the NPs to self-enhanced and H2O2-induced ROS generation is demonstrated in solutions and intracellular space by means of the standard electron spin resonance (ESR) and fluorescence techniques correspondingly. The anticancer specificity of the NPs toward HuTu 80 cancer cells and the apoptotic cell death pathway correlate with the intracellular level of ROS, which agrees well with the self-enhancing ROS generation of the NPs. The enhanced level of ROS revealed in HuTu 80 cells incubated with the NPs can be associated with the significant level of their mitochondrial localization.

SNAP-tagging live cells via chelation-assisted copper-catalyzed azide-alkyne cycloaddition

SNAP-tag is a single-turnover enzyme that has become a powerful tool, hence a popular choice, of targeted cellular protein labeling. Three SNAP-tag substrates that carry the copper-chelating 2-picolyl azide moiety are prepared, one of which has an unconventional 5-pyridylmethyl-substituted guanine structure, rather than the usual benzylguanine that is optimized to be accepted by SNAP-tag. All three substrates are effective in transferring a 2-picolyl azide moiety to SNAP-tag in live cells under conventional labeling conditions (30-minute incubation of cells with labeling reagents at 37 °C under 5% CO2). Live cells that are decorated with chelating azido groups on the extracellular side of membranes undergo copper-catalyzed azide-alkyne cycloaddition (CuAAC) with an ethynyl-functionalized fluorophore to accomplish membrane protein labeling by a fluorescent dye. The chelation-assisted CuAAC labeling step is rapid (<1 minute) with a relatively low dose of the copper catalyst (20 μM), and consequently exerts no ill effect on the labeled cells. A SNAP-tag substrate that carries a non-chelating azide moiety, on the other hand, fails to produce satisfactory labeling under the same constraints. The rapid, live cell-compatible SNAP-tag/chelation-assisted CuAAC two-step method expands the utility of SNAP-tag in protein labeling applications.

Cu(I) binds to Zn7-MT2 via two parallel pathways

Metallothionein proteins are essential for Cu(I) and Zn(II) homeostasis as well as heavy metal detoxification. The metallation properties of MT2 are of great interest due to their wide patterns of expression and correlation with multiple diseases including cancers, neurological disorders, and respiratory diseases. Use of isotopically pure 63Cu(I) and 68Zn(II) eliminates the complexity of the Cu, Zn-MT2 mass spectral peaks due to significant overlap of naturally abundant isotopes. This allows for the resolution of the precise Cu(I) and Zn(II) stoichiometries when both Cu(I) and Zn(II) are bound to MT2 at physiological pH as expected in vivo. Exact Cu: Zn ratios were determined from mass spectral simulations carried out for every point in the titration. We report that Cu(I) metallation of Zn7-MT2 can only be understood in terms of two pathways occurring in parallel with pathway ① resulting in Cu5Zn5-MT2 and Cu9Zn3-MT2. Pathway ② results in Cu6Zn4-MT2 and Cu10Zn2-MT2, which are the major products of the reaction. From the electrospray ionization (ESI)-mass spectral data we report a series of formation constants (KF) for species starting from Zn7-MT2 up to Cu11Zn2-MT2. Room temperature phosphorescence and circular dichroism (CD) spectra were measured in parallel with the ESI-mass spectrometry data allowing for the assignment of specific species to specific spectral bands. Through analysis of the CD spectral bands, we propose that Cu(I) binds to the β domain first to form a Cu5Zn1 cluster or Cu6 cluster with emission at 670 and 750 nm, respectively, leaving the Zn4 cluster in the α domain.

Metallothionein-3: 63 Cu(I) binds to human 68 Zn7 -βα MT3 with no preference for Cu4 -β cluster formation

Human metallothioneins (MTs) are involved in binding the essential elements, Cu(I) and Zn(II), and the toxic element, Cd(II), in metal-thiolate clusters using 20 reduced cysteines. The brain-specific MT3 binds a mixture of Cu(I) and Zn(II) in vivo. Its metallation properties are critically important because of potential connections between Cu, Zn and neurodegenerative diseases. We report that the use of isotopically pure 63 Cu(I) and 68 Zn(II) greatly enhances the element resolution in the ESI-mass spectral data revealing species with differing Cu:Zn ratios but the same total number of metals. Room temperature phosphorescence and circular dichroism spectral data measured in parallel with ESI-mass spectral data identified the presence of specific Cu(I)-thiolate clusters in the presence of Zn(II). A series of Cu(I)-thiolate clusters form following Cu(I) addition to apo MT3: the two main clusters that form are a Cu6 cluster in the β domain followed by a Cu4 cluster in the α domain. 63 Cu(I) addition to 68 Zn7 -MT3 results in multiple species, including clustered Cu5 Zn5 -MT3 and Cu9 Zn3 -MT3. We assign the domain location of the metals for Cu5 Zn5 -MT3 as a Cu5 Zn1 -β cluster and a Zn4 -α cluster and for Cu9 Zn3 -MT3 as a Cu6 -β cluster and a Cu3 Zn3 -α cluster. While many reports of the average MT3 metal content exist, determining the exact Cu,Zn stoichiometry has proven very difficult even with native ESI-MS. The work in this paper solves the ambiguity introduced by the overlap of the naturally abundant Cu(I) and Zn(II) isotopes. Contrary to other reports, there is no indication of a major fraction of Cu4 -β-Znn -α-MT3 forming.

Derivatization of the Peptidic Xxx-Zzz-His Motif toward a Ligand with Attomolar CuII Affinity under Maintaining High Selectivity and Fast Redox Silencing

Cu chelation in biological systems is of interest as a tool to study the metabolism of this essential metal or for applications in the case of diseases with a systemic or local Cu overload, such as Wilson's or Alzheimer's disease. The choice of the chelating agent must meet several criteria. Among others, affinities and kinetics of metal binding and related metal selectivity are important parameters of the chelators to consider. Here, we report on the synthesis and characterization of Cu-binding properties of two ligands, L1 and L2, derivatives of the well-known peptidic Cu^II-binding motif Xxx-Zzz-His (also called ATCUN), where Cu^II is bound to the N-terminal amine, two amidates, and the imidazole. In either L, the N-terminal amine was replaced with a pyridine, and for L2, one amide was replaced with an amine compared to Xxx-Zzz-His. In particular, L2 showed several interesting features, including a Cu^II-binding affinity with a log K_D^app = -16.0 similar to that of EDTA and stronger than all reported ATCUN peptides. L2 showed high selectivity for Cu^II over Zn^II and other essential metal ions, even under the challenging conditions of the presence of human serum albumin. Further, L2 showed fast and efficient Cu^II redox silencing qualities and Cu^II-L2 was stable in the presence of mM GSH concentrations. Benefitting the fact that L2 can be easily elongated on its peptide part by standard SPPS to add other functions, L2 has attractive properties as a Cu^II chelator for application in biological systems.

Luminescent Water-Dispersible Nanoparticles Engineered from Copper(I) Halide Cluster Core and P,N-Ligand with an Optimal Balance between Stability and ROS Generation

The present work introduces the solvent exchange procedure as a route for conversion of the Cu4I4L2 complex, where the Cu4I4 cluster core is coordinated with two P,N-ligands (L), into an aqueous colloid. The analysis of both colloidal and supernatant phases revealed some losses in CuI going from the initial Cu4I4L2 complex to Cu2I2L3-based nanoparticles. The comparative analysis of IR, 31P NMR spectroscopy, ESI mass-spectrometry and luminescence data argued for a contribution of the “butterfly”-like structures of the Cu2I2 cluster core to Cu2I2L3-based nanoparticles, although the amorphous nature of the latter restricted structure evaluation from the PXRD data. The green luminescence of the colloids revealed their chemical stability under pH variations in the solutions of some amino acids and peptides, and to specify the temperature and concentration conditions triggering the oxidative degradation of the nanoparticles. The spin trap-facilitated ESR study indicated that the oxidative transformations were followed by the generation of reactive oxygen species (ROS). The physiological temperature level (310 K) enhanced the ROS generation by nanoparticles, but the ROS level was suppressed in the solution of GSH at pH = 7.0. The cytotoxicity of nanoparticles was evaluated in the M-HeLa cell line and is discussed in correlation with their cell internalization and intracellular oxidative transformations.

Dual Role of Glutathione as a Reducing Agent and Cu-Ligand Governs the ROS Production by Anticancer Cu-Thiosemicarbazone Complexes

α-Pyridyl thiosemicarbazones (TSC) such as Triapine (3AP) and Dp44mT are a promising class of anticancer agents. Contrary to Triapine, Dp44mT showed a pronounced synergism with Cu^II, which may be due to the generation of reactive oxygen species (ROS) by Dp44mT-bound Cu^II ions. However, in the intracellular environment, Cu^II complexes have to cope with glutathione (GSH), a relevant Cu^II reductant and Cu^I-chelator. Here, aiming at rationalizing the different biological activity of Triapine and Dp44mT, we first evaluated the ROS production by their Cu^II-complexes in the presence of GSH, showing that Cu^II-Dp44mT is a better catalyst than Cu^II-3AP. Furthermore, we performed density functional theory (DFT) calculations, which suggest that a different hard/soft character of the complexes could account for their different reactivity with GSH.

Optochemical control of Cu(I) homeostasis in mammalian cells

Copper can act as a double-edged sword as it can cause fatal diseases when in excess or shortage. Precise control of copper homeostasis is maintained by a complex machinery inside cells. To overcome imbalances in copper concentration, we have developed a simple system to control the cellular copper concentration by using a photocaged chelator and light. This photocaged chelator allowed us to control cellular copper concentration in a spatiotemporal manner.

Analysis of copper-induced protein precipitation across the E. coli proteome

Metal cations have been exploited for their precipitation properties in a wide variety of studies, ranging from differentiating proteins from serum and blood to identifying the protein targets of drugs. Despite widespread recognition of this phenomenon, the mechanisms of metal-induced protein aggregation have not been fully elucidated. Recent studies have suggested that copper's (Cu) ability to induce protein aggregation may be a main contributor to Cu-induced cell death. Here, we provide the first proteome-wide analysis of the relative sensitivities of proteins across the Escherichia coli proteome to Cu-induced aggregation. We utilize a metal-induced protein precipitation (MiPP) methodology that relies on quantitative bottom-up proteomics to define the metal concentration-dependent precipitation properties of proteins on a proteomic scale. Our results establish that Cu far surpasses other metals in promoting protein aggregation and that the protein aggregation is reversible upon metal chelation. The bulk of the Cu bound in the protein aggregates is Cu1+, regardless of the Cu2+ source. Analysis of our MiPP data allows us to investigate underlying biophysical characteristics that determine a protein's sensitivity to Cu-induced aggregation, which is independent of the relative concentration of protein in the lysate. Overall, this analysis provides new insights into the mechanism behind Cu cytotoxicity, as well as metal cation-induced protein aggregation.

63Cu(I) binding to human kidney 68Zn7-βα MT1A: determination of Cu(I)-thiolate cluster domain specificity from ESI-MS and room temperature phosphorescence spectroscopy

Mammalian metallothioneins (MTs) are important proteins in Zn(II) and Cu(I) homeostasis with the Zn(II) and Cu(I) binding to the 20 cysteines in metal-thiolate clusters. Previous electrospray ionization (ESI) mass spectrometric (MS) analyses of Cu(I) binding to Zn7-MT were complicated by significant overlap of the natural abundance isotopic patterns for Zn(II) and Cu(I) leading to impossibly ambiguous stoichiometries. In this paper, isotopically pure 63Cu(I) and 68Zn(II) allowed determination of the specific stoichiometries in the 68 Zn,63Cu-βα MT1A species formed following the stepwise addition of 63Cu(I) to 68Zn7-βα MT1A. These species were characterized by ESI-MS and room temperature emission spectroscopy. The key species that form and their emission band centres are Zn5Cu5-βα MT1A (λ = 684 nm), Zn4Cu6-βα MT1A (λ = 750 nm), Zn3Cu9-βα MT1A (λ = 750 nm), Zn2Cu10-βα MT1A (λ = 750 nm), and Zn1Cu14-βα MT1A (λ = 634 nm). The specific domain stoichiometry of each species was determined by assessing the species forming following 63Cu(I) addition to the 68Zn3-β MT1A and 68Zn4-α MT1A domain fragments. The domain fragment emission suggests that Zn5Cu5-βα MT1A contains a Zn1Cu5-β cluster and the Zn4Cu6-βα MT1A, Zn3Cu9-βα MT1A, and Zn2Cu10-βα MT1A each contain a Cu6-β cluster. The species forming with >10 mol. eq. of 63Cu(I) in βα-MT1A exhibit emission from the Cu6-β cluster and an α domain cluster. This high emission intensity is seen at the end of the titrations of 68Zn7-βα MT1A and the 68Zn4-α MT1A domain fragment suggesting that the initial presence of the Zn(II) results in clustered Cu(I) binding in the α domain.

Protein metalation in a nutshell

Metalation, the acquisition of metals by proteins, must avoid mis-metalation with tighter binding metals. This is illustrated by four selected proteins that require different metals: all show similar ranked orders of affinity for bioavailable metals, as described in a universal affinity series (the Irving-Williams series). Crucially, cellular protein metalation occurs in competition with other metal binding sites. The strength of this competition defines the intracellular availability of each metal: its magnitude has been estimated by calibrating a cells' set of DNA-binding, metal-sensing, transcriptional regulators. This has established that metal availabilities (as free energies for forming metal complexes) are maintained to the inverse of the universal series. The tightest binding metals are least available. With these availabilities, correct metalation is achieved.

In-cell NMR: Why and how?

NMR spectroscopy has been applied to cells and tissues analysis since its beginnings, as early as 1950. We have attempted to gather here in a didactic fashion the broad diversity of data and ideas that emerged from NMR investigations on living cells. Covering a large proportion of the periodic table, NMR spectroscopy permits scrutiny of a great variety of atomic nuclei in all living organisms non-invasively. It has thus provided quantitative information on cellular atoms and their chemical environment, dynamics, or interactions. We will show that NMR studies have generated valuable knowledge on a vast array of cellular molecules and events, from water, salts, metabolites, cell walls, proteins, nucleic acids, drugs and drug targets, to pH, redox equilibria and chemical reactions. The characterization of such a multitude of objects at the atomic scale has thus shaped our mental representation of cellular life at multiple levels, together with major techniques like mass-spectrometry or microscopies. NMR studies on cells has accompanied the developments of MRI and metabolomics, and various subfields have flourished, coined with appealing names: fluxomics, foodomics, MRI and MRS (i.e. imaging and localized spectroscopy of living tissues, respectively), whole-cell NMR, on-cell ligand-based NMR, systems NMR, cellular structural biology, in-cell NMR… All these have not grown separately, but rather by reinforcing each other like a braided trunk. Hence, we try here to provide an analytical account of a large ensemble of intricately linked approaches, whose integration has been and will be key to their success. We present extensive overviews, firstly on the various types of information provided by NMR in a cellular environment (the "why", oriented towards a broad readership), and secondly on the employed NMR techniques and setups (the "how", where we discuss the past, current and future methods). Each subsection is constructed as a historical anthology, showing how the intrinsic properties of NMR spectroscopy and its developments structured the accessible knowledge on cellular phenomena. Using this systematic approach, we sought i) to make this review accessible to the broadest audience and ii) to highlight some early techniques that may find renewed interest. Finally, we present a brief discussion on what may be potential and desirable developments in the context of integrative studies in biology.

Copper-Catalyzed Glutathione Oxidation is Accelerated by the Anticancer Thiosemicarbazone Dp44mT and Further Boosted at Lower pH

Glutathione (GSH) is the most abundant thiol in mammalian cells and plays a crucial role in maintaining redox cellular homeostasis. The thiols of two GSH molecules can be oxidized to the disulfide GSSG. The cytosolic GSH/GSSG ratio is very high (>100), and its reduction can lead to apoptosis or necrosis, which are of interest in cancer research. Cu^II ions are very efficient oxidants of thiols, but with an excess of GSH, Cu^I_n(GS)_m clusters are formed, in which Cu^I is very slowly reoxidized by O₂ at pH 7.4 and even more slowly at lower pH. Here, the aerobic oxidation of GSH by Cu^II was investigated at different pH values in the presence of the anticancer thiosemicarbazone Dp44mT, which accumulates in lysosomes and induces lysosomal membrane permeabilization in a Cu-dependent manner. The results showed that Cu^II-Dp44mT catalyzes GSH oxidation faster than Cu^II alone at pH 7.4 and hence accelerates the production of very reactive hydroxyl radicals. Moreover, GSH oxidation and hydroxyl radical production by Cu^II-Dp44mT were accelerated at the acidic pH found in lysosomes. To decipher this unusually faster thiol oxidation at lower pH, density functional theory (DFT) calculations, electrochemical and spectroscopic studies were performed. The results suggest that the acceleration is due to the protonation of Cu^II-Dp44mT on the hydrazinic nitrogen, which favors the rate-limiting reduction step without subsequent dissociation of the Cu^I intermediate. Furthermore, preliminary biological studies in cell culture using the proton pump inhibitor bafilomycin A1 indicated that the lysosomal pH plays a role in the activity of Cu^II-Dp44mT.

Unveiling the interplay between homogeneous and heterogeneous catalytic mechanisms in copper-iron nanoparticles working under chemically relevant tumour conditions

The present work sheds light on a generally overlooked issue in the emerging field of bio-orthogonal catalysis within tumour microenvironments (TMEs): the interplay between homogeneous and heterogeneous catalytic processes. In most cases, previous works dealing with nanoparticle-based catalysis in the TME focus on the effects obtained (e.g. tumour cell death) and attribute the results to heterogeneous processes alone. The specific mechanisms are rarely substantiated and, furthermore, the possibility of a significant contribution of homogeneous processes by leached species - and the complexes that they may form with biomolecules - is neither contemplated nor pursued. Herein, we have designed a bimetallic catalyst nanoparticle containing Cu and Fe species and we have been able to describe the whole picture in a more complex scenario where both homogeneous and heterogeneous processes are coupled and fostered under TME relevant chemical conditions. We investigate the preferential leaching of Cu ions in the presence of a TME overexpressed biomolecule such as glutathione (GSH). We demonstrate that these homogeneous processes initiated by the released by Cu-GSH interactions are in fact responsible for the greater part of the cell death effects found (GSH, a scavenger of reactive oxygen species, is depleted and highly active superoxide anions are generated in the same catalytic cycle). The remaining solid CuFe nanoparticle becomes an active catalyst to supply oxygen from oxygen reduced species, such as superoxide anions (by-product from GSH oxidation) and hydrogen peroxide, another species that is enriched in the TME. This activity is essential to sustain the homogeneous catalytic cycle in the oxygen-deprived tumour microenvironment. The combined heterogeneous-homogeneous mechanisms revealed themselves as highly efficient in selectively killing cancer cells, due to their higher GSH levels compared to healthy cell lines.

Unique underlying principles shaping copper homeostasis networks

Abstract

Copper is essential in cells as a cofactor for key redox enzymes. Bacteria have acquired molecular components that sense, uptake, distribute, and expel copper ensuring that cuproenzymes are metallated and steady-state metal levels are maintained. Toward preventing deleterious reactions, proteins bind copper ions with high affinities and transfer the metal via ligand exchange, warranting that copper ions are always complexed. Consequently, the directional copper distribution within cell compartments and across cell membranes requires specific dynamic interactions and metal exchange between cognate holo-apo protein partners. These metal exchange reactions are determined by thermodynamic and kinetics parameters and influenced by mass action. Then, copper distribution can be conceptualized as a molecular system of singular interacting elements that maintain a physiological copper homeostasis. This review focuses on the impact of copper high-affinity binding and exchange reactions on the homeostatic mechanisms, the conceptual models to describe the cell as a homeostatic system, the various molecule functions that contribute to copper homeostasis, and the alternative system architectures responsible for copper homeostasis in model bacteria.

Graphical Abstract

Reaction of N-Acetylcysteine with Cu2+: Appearance of Intermediates with High Free Radical Scavenging Activity: Implications for Anti-/Pro-Oxidant Properties of Thiols

We studied the kinetics of the reaction of N-acetyl-l-cysteine (NAC or RSH) with cupric ions at an equimolar ratio of the reactants in aqueous acid solution (pH 1.4−2) using UV/Vis absorption and circular dichroism (CD) spectroscopies. Cu2+ showed a strong catalytic effect on the 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) radical (ABTSr) consumption and autoxidation of NAC. Difference spectra revealed the formation of intermediates with absorption maxima at 233 and 302 nm (ε302/Cu > 8 × 103 M−1 cm−1) and two positive Cotton effects centered at 284 and 302 nm. These intermediates accumulate during the first, O2-independent, phase of the NAC autoxidation. The autocatalytic production of another chiral intermediate, characterized by two positive Cotton effects at 280 and 333 nm and an intense negative one at 305 nm, was observed in the second reaction phase. The intermediates are rapidly oxidized by added ABTSr; otherwise, they are stable for hours in the reaction solution, undergoing a slow pH- and O2-dependent photosensitive decay. The kinetic and spectral data are consistent with proposed structures of the intermediates as disulfide-bridged dicopper(I) complexes of types cis-/trans-CuI2(RS)2(RSSR) and CuI2(RSSR)2. The electronic transitions observed in the UV/Vis and CD spectra are tentatively attributed to Cu(I) → disulfide charge transfer with an interaction of the transition dipole moments (exciton coupling). The catalytic activity of the intermediates as potential O2 activators via Cu(II) peroxo-complexes is discussed. A mechanism for autocatalytic oxidation of Cu(I)−thiolates promoted by a growing electronically coupled −[CuI2(RSSR)]n− polymer is suggested. The obtained results are in line with other reported observations regarding copper-catalyzed autoxidation of thiols and provide new insight into these complicated, not yet fully understood systems. The proposed hypotheses point to the importance of the Cu(I)−disulfide interaction, which may have a profound impact on biological systems.

Copper Induces Protein Aggregation, a Toxic Process Compensated by Molecular Chaperones

Copper is well known for its antimicrobial and antiviral properties. Under aerobic conditions, copper toxicity relies in part on the production of reactive oxygen species (ROS), especially in the periplasmic compartment. However, copper is significantly more toxic under anaerobic conditions, in which ROS cannot be produced. This toxicity has been proposed to arise from the inactivation of proteins through mismetallations. Here, using the bacterium Escherichia coli, we discovered that copper treatment under anaerobic conditions leads to a significant increase in protein aggregation. In vitro experiments using E. coli lysates and tightly controlled redox conditions confirmed that treatment with Cu+ under anaerobic conditions leads to severe ROS-independent protein aggregation. Proteomic analysis of aggregated proteins revealed an enrichment of cysteine- and histidine-containing proteins in the Cu+-treated samples, suggesting that nonspecific interactions of Cu+ with these residues are likely responsible for the observed protein aggregation. In addition, E. coli strains lacking the cytosolic chaperone DnaK or trigger factor are highly sensitive to copper stress. These results reveal that bacteria rely on these chaperone systems to protect themselves against Cu-mediated protein aggregation and further support our finding that Cu toxicity is related to Cu-induced protein aggregation. Overall, our work provides new insights into the mechanism of Cu toxicity and the defense mechanisms that bacteria employ to survive. IMPORTANCE With the increase of antibiotic drug resistance, alternative antibacterial treatment strategies are needed. Copper is a well-known antimicrobial and antiviral agent; however, the underlying molecular mechanisms by which copper causes cell death are not yet fully understood. Herein, we report the finding that Cu+, the physiologically relevant copper species in bacteria, causes widespread protein aggregation. We demonstrate that the molecular chaperones DnaK and trigger factor protect bacteria against Cu-induced cell death, highlighting, for the first time, the central role of these chaperones under Cu+ stress. Our studies reveal Cu-induced protein aggregation to be a central mechanism of Cu toxicity, a finding that will serve to guide future mechanistic studies and drug development.

Transition Metals Induce Quenching of Monomeric Near-Infrared Fluorescent Proteins

Transition metals such as zinc and copper are essential in numerous life processes, and both deficiency and toxic overload of these metals are associated with various diseases. Fluorescent metal sensors are powerful tools for studying the roles of metal ions in the physiology and pathology of biological systems. Green fluorescent protein (GFP) and its derivatives are highly utilized for protein-based sensor design, but application to anaerobic systems is limited because these proteins require oxygen to become fluorescent. Bacteriophytochrome-based monomeric near-infrared fluorescent proteins (miRFPs) covalently bind a bilin cofactor, which can be added exogenously for anaerobic cells. miRFPs can also have emission wavelengths extending to >700 nm, which is valuable for imaging applications. Here, we evaluated the suitability of miRFP670 and miRFP709 as platforms for single fluorescent protein metal ion sensors. We found that divalent metal ions like Zn²⁺, Co²⁺, Ni²⁺, and Cu²⁺ can quench from ∼6-20% (Zn²⁺, Co²⁺, and Ni²⁺) and up to nearly 90% (Cu²⁺) of the fluorescence intensity of pure miRFPs and have similar impacts in live Escherichia coli cells expressing miRFPs. The presence of a 6× histidine tag for purification influences metal quenching, but significant Cu²⁺-induced quenching and a picomolar binding affinity are retained in the absence of the His₆ tag in both cuvettes and live bacterial cells. By comparing the Cu²⁺ and Cu⁺-induced quenching results for miRFP670 and miRFP709 and through examining absorption spectra and previously reported crystal structures, we propose a surface metal binding site near the biliverdin IXα chromophore.

Do glutathione and copper interact to modify Alzheimer's disease pathogenesis?

Alzheimer's disease (AD) is a progressive neurodegenerative disorder first described in 1906 that is currently estimated to impact ∼40 million people worldwide. Extensive research activities have led to a wealth of information on the pathogenesis, hallmarks, and risk factors of AD; however, therapeutic options remain extremely limited. The large number of pathogenic factors that have been reported to potentially contribute to AD include copper dyshomeostasis as well as increased oxidative stress, which is related to alterations to molecular antioxidants like glutathione (GSH). While the individual roles of GSH and copper in AD have been studied by many research groups, their interactions have received relatively little attention, although they appear to interact and affect each other's regulation. Existing knowledge on how GSH-copper interactions may affect AD is sparse and lacks focus. This review first highlights the most relevant individual roles that GSH and copper play in physiology and AD, and then collects and assesses research concerning their interactions, in an effort to provide a more accessible and understandable picture of the role of GSH, copper, and their interactions in AD.

Evidence for a Long-Lived, Cu-Coupled and Oxygen-Inert Disulfide Radical Anion in the Assembly of Metallothionein-3 Cu(I)4-Thiolate Cluster

The human copper-binding protein metallothionein-3 (MT-3) can reduce Cu(II) to Cu(I) and form a polynuclear Cu(I)₄-Cys_5-6 cluster concomitant with intramolecular disulfide bonds formation, but the cluster is unusually inert toward O₂ and redox-cycling. We utilized a combined array of rapid-mixing spectroscopic techniques to identify and characterize the transient radical intermediates formed in the reaction between Zn₇MT-3 and Cu(II) to form Cu(I)₄Zn(II)₄MT-3. Stopped-flow electronic absorption spectroscopy reveals the rapid formation of transient species with absorption centered at 430-450 nm and consistent with the generation of disulfide radical anions (DRAs) upon reduction of Cu(II) by MT-3 cysteine thiolates. These DRAs are oxygen-stable and unusually long-lived, with lifetimes in the seconds regime. Subsequent DRAs reduction by Cu(II) leads to the formation of a redox-inert Cu(I)₄-Cys₅ cluster with short Cu-Cu distances (<2.8 Å), as revealed by low-temperature (77 K) luminescence spectroscopy. Rapid freeze-quench Raman and electron paramagnetic resonance (EPR) spectroscopy characterization of the intermediates confirmed the DRA nature of the sulfur-centered radicals and their subsequent oxidation to disulfide bonds upon Cu(II) reduction, generating the final Cu(I)₄-thiolate cluster. EPR simulation analysis of the radical g- and A-values indicate that the DRAs are directly coupled to Cu(I), potentially explaining the observed DRA stability in the presence of O₂. We thus provide evidence that the MT-3 Cu(I)₄-Cys₅ cluster assembly process involves the controlled formation of novel long-lived, copper-coupled, and oxygen-stable disulfide radical anion transient intermediates.

Development of affinity bead-based in vitro metal-ligand binding assay reveals dominant cadmium affinity of thiol-rich small peptides phytochelatins beyond glutathione

For a better understanding of metal-ligand interaction and its function in cells, we developed an easy, sensitive, and high-throughput method to quantify ligand-metal(loid) binding affinity under physiological conditions by combining ligand-attached affinity beads and inductively coupled plasma-optical emission spectrometry (ICP-OES). Glutathione (GSH) and two phytochelatins (PC2 and PC3, small peptides with different numbers of free thiols) were employed as model ligands and attached to hydrophilic beads. The principle of the assay resembles that of affinity purification of proteins in biochemistry: metals binding to the ligand on the beads and the rest in the buffer are separated by a spin column and quantified by ICP-OES. The binding assay using the GSH-attached beads and various metal(loid)s suggested the different affinity of the metal-GSH interactions, in accordance with the order of the Irving-Williams series and the reported stability constants. The binding assay using PC2 or PC3-attached beads suggested positive binding between PCs and Ni(II), Cu(II), Zn(II), Cd(II), and As(III) in accordance with the number of thiols in PC2 and PC3. We then conducted the competition assay using Cd(II), Mn(II), Fe(II), Cu(II), and Zn(II), and the results suggested a better binding affinity of PC2 with Cd(II) than with the essential metals. Another competition assay using PC2 and GSH suggested a robust binding affinity between PCs and Cd(II) compared to GSH and Cd(II). These results suggested the dominance of PC-Cd complex formation in vitro, supporting the physiological importance of PCs for the detoxification of cadmium in vivo. We also discuss the potential application of the assay.

Copper Homeostatic Mechanisms and Their Role in the Virulence of Escherichia coli and Salmonella enterica

Copper is an essential micronutrient that also exerts toxic effects at high concentrations. This review summarizes the current state of knowledge on copper handling and homeostasis systems in Escherichia coli and Salmonella enterica. We describe the mechanisms by which transcriptional regulators, efflux pumps, detoxification enzymes, metallochaperones, and ancillary copper response systems orchestrate cellular response to copper stress. E. coli and S. enterica are important pathogens of humans and animals. We discuss the critical role of copper during killing of these pathogens by macrophages and in nutritional immunity at the bacterial-pathogen-host interface. In closing, we identify opportunities to advance our understanding of the biological roles of copper in these model enteric bacterial pathogens.

The Bioinorganic Chemistry of Mammalian Metallothioneins

The functions, purposes, and roles of metallothioneins have been the subject of speculations since the discovery of the protein over 60 years ago. This article guides through the history of investigations and resolves multiple contentions by providing new interpretations of the structure-stability-function relationship. It challenges the dogma that the biologically relevant structure of the mammalian proteins is only the one determined by X-ray diffraction and NMR spectroscopy. The terms metallothionein and thionein are ambiguous and insufficient to understand biological function. The proteins need to be seen in their biological context, which limits and defines the chemistry possible. They exist in multiple forms with different degrees of metalation and types of metal ions. The homoleptic thiolate coordination of mammalian metallothioneins is important for their molecular mechanism. It endows the proteins with redox activity and a specific pH dependence of their metal affinities. The proteins, therefore, also exist in different redox states of the sulfur donor ligands. Their coordination dynamics allows a vast conformational landscape for interactions with other proteins and ligands. Many fundamental signal transduction pathways regulate the expression of the dozen of human metallothionein genes. Recent advances in understanding the control of cellular zinc and copper homeostasis are the foundation for suggesting that mammalian metallothioneins provide a highly dynamic, regulated, and uniquely biological metal buffer to control the availability, fluctuations, and signaling transients of the most competitive Zn(II) and Cu(I) ions in cellular space and time.

Intermediate Cu(II)-Thiolate Species in the Reduction of Cu(II)GHK by Glutathione: A Handy Chelate for Biological Cu(II) Reduction

Gly-His-Lys (GHK) is a tripeptide present in the human bloodstream that exhibits a number of biological functions. Its activity is attributed to the copper-complexed form, Cu(II)GHK. Little is known, however, about the molecular aspects of the mechanism of its action. Here, we examined the reaction of Cu(II)GHK with reduced glutathione (GSH), which is the strongest reductant naturally occurring in human plasma. Spectroscopic techniques (UV–vis, CD, EPR, and NMR) and cyclic voltammetry helped unravel the reaction mechanism. The impact of temperature, GSH concentration, oxygen access, and the presence of ternary ligands on the reaction were explored. The transient GSH-Cu(II)GHK complex was found to be an important reaction intermediate. The kinetic and redox properties of this complex, including tuning of the reduction rate by ternary ligands, suggest that it may provide a missing link in copper trafficking as a precursor of Cu(I) ions, for example, for their acquisition by the CTR1 cellular copper transporter.

Gly-His-Lys (GHK) is a human bioactive tripeptide thought to be activated by Cu(II) binding, but little is known about the molecular aspects of its action. UV−vis, circular dichroism (CD), EPR, and NMR spectroscopies, and cyclic voltammetry were used to examine the reduction of Cu(II)GHK with glutathione (GSH), the most abundant biological thiol. A semistable GSH-Cu(II)GHK reaction intermediate was discovered, with properties suitable for delivering Cu(I) to biological transport proteins.

X1INH, an improved next-generation affinity-optimized hydrazonic ligand, attenuates abnormal copper(I)/copper(II)-α-Syn interactions and affects protein aggregation in a cellular model of synucleinopathy

Although normal aging presents an accumulation of copper and iron in the brain, this becomes more relevant in neurodegeneration. α-Synuclein (α-Syn) misfolding has long been linked with the development of Parkinson's disease (PD). Copper binding promotes aggregation of α-Syn, as well as generalized oxidative stress. In this sense, the use of therapies that target metal dyshomeostasis has been in focus in the past years. Metal-Protein Attenuating Compounds (MPACs) are moderate chelators that aim at disrupting specific, abnormal metal-protein interactions. Our research group has now established that N-acylhydrazones compose a set of truly encouraging MPACs for the bioinorganic management of metal-enhanced aggregopathies. In the present work, a novel ligand, namely 1-methyl-1H-imidazole-2-carboxaldehyde isonicotinoyl hydrazone (X1INH), is reported. We describe solution studies on the interaction and affinity of this compound for copper(ii) ions showing that a fine tuning of metal-affinity was achieved. A series of in vitro biophysical NMR experiments were performed in order to assess the X1INH ability to compete with α-Syn monomers for the binding of both copper(i) and copper(ii) ions, which are central in PD pathology. A preference for copper(i) has been observed. X1INH is less toxic to human neuroglioma (H4) cells in comparison to structure-related compounds. Finally, we show that treatment with X1INH results in a higher number of smaller, less compact inclusions in a well-established model of α-Syn aggregation. Thus, X1INH constitutes a promising MPAC for the treatment of Parkinson's disease.

Tripodal scaffolds with three appended imidazole thiones for Cu(I) chelation and protection from Cu-mediated oxidative stress

Imidazole thiones appear as interesting building blocks for Cu(I) chelation and protection against Cu-mediated oxidative stress. Therefore, a series of tripodal molecules derived from nitrilotriacetic acid appended with three imidazole thiones belonging either to histamine-like or histidine-like moieties were synthesized. These tripods demonstrate intermediate affinity between that previously measured for tripodal analogues bearing three thiol moieties such as cysteine and those grafted with three thioethers, like methionines, consistently with the thione group in the imidazole thione moiety existing as a tautomer between a thiol and a thione. The two non-alkylated tripods derived from thioimidazole, T^H and T^H* demonstrated three orders of magnitude larger affinity for Cu(I) (logK^{pH 7.4} = 14.3) than their analogues derived from N,N'-dialkylated thioimidazole T^Me and T^Et (logK^{pH 7.4} = 11-11.6). Their efficiency to inhibit Cu-mediated oxidative stress is demonstrated by several assays involving ascorbate consumption or biomolecule damages and correlates with their ability to chelate Cu(I), related to their conditional complexation constants at pH 7.4. The two non-alkylated tripods derived from thioimidazole, T^H and T^H* are significantly more powerful in reducing Cu-mediated oxidative stress than their analogues derived from N,N'-dialkylated thioimidazole T^Me and T^Et.

The requirement for cobalt in vitamin B12: A paradigm for protein metalation

Vitamin B12, cobalamin, is a cobalt-containing ring-contracted modified tetrapyrrole that represents one of the most complex small molecules made by nature. In prokaryotes it is utilised as a cofactor, coenzyme, light sensor and gene regulator yet has a restricted role in assisting only two enzymes within specific eukaryotes including mammals. This deployment disparity is reflected in another unique attribute of vitamin B12 in that its biosynthesis is limited to only certain prokaryotes, with synthesisers pivotal in establishing mutualistic microbial communities. The core component of cobalamin is the corrin macrocycle that acts as the main ligand for the cobalt. Within this review we investigate why cobalt is paired specifically with the corrin ring, how cobalt is inserted during the biosynthetic process, how cobalt is made available within the cell and explore the cellular control of cobalt and cobalamin levels. The partitioning of cobalt for cobalamin biosynthesis exemplifies how cells assist metalation.

Silver in biology and medicine: opportunities for metallomics researchers

The antibacterial properties of silver have been known for centuries and the threat of antibiotic-resistant bacteria has led to renewed focus on the noble metal. Silver is now commonly included in a range of household and medical items to imbue them with bactericidal properties. Despite this, the chemical fate of the metal in biological systems is poorly understood. Silver(I) is a soft metal with high affinity for soft donor atoms and displays much similarity to the chemistry of Cu(I). In bacteria, interaction of silver with the cell wall/membrane, DNA, and proteins and enzymes can lead to cell death. Additionally, the intracellular generation of reactive oxygen species by silver is posited to be a significant antimicrobial action. While the antibacterial action of silver is well known, bacteria found in silver mines display resistance against it through use of a protein ensemble thought to have been specifically developed for the metal, highlighting the need for judicious use. In mammals, ∼10-20% of ingested silver is retained by the body and thought to predominantly localize in the liver or kidneys. Chronic exposure can result in argyria, a condition characterized by blue staining of the skin, resulting from subdermal deposition of silver [as Ag(0)/sulfides], but more insidious side effects, such as inclusions in the brain, seizures, liver/kidney damage, and immunosuppression, have also been reported. Here, we hope to highlight the current understanding of the biological chemistry of silver and the necessity for continued study of these systems to fill existing gaps in knowledge.

Role of Glutathione in Buffering Excess Intracellular Copper in Streptococcus pyogenes

Copper (Cu) is an essential metal for bacterial physiology but in excess it is bacteriotoxic. To limit Cu levels in the cytoplasm, most bacteria possess a transcriptionally responsive system for Cu export. In the Gram-positive human pathogen Streptococcus pyogenes (group A Streptococcus [GAS]), this system is encoded by the copYAZ operon. This study demonstrates that although the site of GAS infection represents a Cu-rich environment, inactivation of the copA Cu efflux gene does not reduce virulence in a mouse model of invasive disease. In vitro, Cu treatment leads to multiple observable phenotypes, including defects in growth and viability, decreased fermentation, inhibition of glyceraldehyde-3-phosphate dehydrogenase (GapA) activity, and misregulation of metal homeostasis, likely as a consequence of mismetalation of noncognate metal-binding sites by Cu. Surprisingly, the onset of these effects is delayed by ∼4 h even though expression of copZ is upregulated immediately upon exposure to Cu. Further biochemical investigations show that the onset of all phenotypes coincides with depletion of intracellular glutathione (GSH). Supplementation with extracellular GSH replenishes the intracellular pool of this thiol and suppresses all the observable effects of Cu treatment. These results indicate that GSH buffers excess intracellular Cu when the transcriptionally responsive Cu export system is overwhelmed. Thus, while the copYAZ operon is responsible for Cu homeostasis, GSH has a role in Cu tolerance and allows bacteria to maintain metabolism even in the presence of an excess of this metal ion.IMPORTANCE The control of intracellular metal availability is fundamental to bacterial physiology. In the case of copper (Cu), it has been established that rising intracellular Cu levels eventually fill the metal-sensing site of the endogenous Cu-sensing transcriptional regulator, which in turn induces transcription of a copper export pump. This response caps intracellular Cu availability below a well-defined threshold and prevents Cu toxicity. Glutathione, abundant in many bacteria, is known to bind Cu and has long been assumed to contribute to bacterial Cu handling. However, there is some ambiguity since neither its biosynthesis nor uptake is Cu-regulated. Furthermore, there is little experimental support for this physiological role of glutathione beyond measuring growth of glutathione-deficient mutants in the presence of Cu. Our work with group A Streptococcus provides new evidence that glutathione increases the threshold of intracellular Cu availability that can be tolerated by bacteria and thus advances fundamental understanding of bacterial Cu handling.

L-Cysteine-mediated modulation of copper trafficking in prostate cancer cells: an in vitro and in vivo investigation with 64Cu and 64Cu-PET

Copper imbalance is implicated in many diseases, including cancer. Copper in blood is mainly transported by carrier proteins but a small fraction is bound to low molecular weight species, possibly amino acids. Their roles in cellular copper delivery are unknown. Our aim was to test whether accumulation of 64Cu into cancer-derived cells can be influenced by copper-binding serum amino acids. In vitro cellular accumulation of 64Cu was measured in Hank's Balanced Salt Solution in the presence of 100 μM l-histidine, l-methionine, l-cysteine and l-threonine. l-Cysteine markedly increased 64Cu accumulation and retention in DU145, PC3 and SK-OV-3 cells, while some other cell lines did not show an effect. This effect was not due to 64Cu delivery in the form of a 64Cu-cysteine complex, nor to reduction of 64Cu(ii) to 64Cu(i) by l-cysteine. Pre-incubation of cells with l-cysteine increased 64Cu accumulation, even if l-cysteine was removed from HBSS before 64Cu was added. The effect of l-cysteine on 64Cu accumulation was not mediated by increased glutathione synthesis. Despite the demonstrable in vitro effect, pre-injection of l-cysteine precursor N-acetyl-cysteine (NAC) in vivo did not enhance 64Cu delivery to DU145 xenografts in mice. Instead, it decreased 64Cu accumulation in the DU145 tumour and in brain, as assessed by PET imaging. We conclude that 64Cu is not delivered to DU145 cancer cells in vitro as a complex with amino acids but its cellular accumulation is enhanced by l-cysteine or NAC influx to cells. The latter effect was not demonstrable in vivo in the DU145 xenograft.

Cu Homeostasis in Bacteria: The Ins and Outs

Copper (Cu) is an essential trace element for all living organisms and used as cofactor in key enzymes of important biological processes, such as aerobic respiration or superoxide dismutation. However, due to its toxicity, cells have developed elaborate mechanisms for Cu homeostasis, which balance Cu supply for cuproprotein biogenesis with the need to remove excess Cu. This review summarizes our current knowledge on bacterial Cu homeostasis with a focus on Gram-negative bacteria and describes the multiple strategies that bacteria use for uptake, storage and export of Cu. We furthermore describe general mechanistic principles that aid the bacterial response to toxic Cu concentrations and illustrate dedicated Cu relay systems that facilitate Cu delivery for cuproenzyme biogenesis. Progress in understanding how bacteria avoid Cu poisoning while maintaining a certain Cu quota for cell proliferation is of particular importance for microbial pathogens because Cu is utilized by the host immune system for attenuating pathogen survival in host cells.

The Glutathione/Metallothionein System Challenges the Design of Efficient O2 -Activating Copper Complexes

Copper complexes are of medicinal and biological interest, including as anticancer drugs designed to cleave intracellular biomolecules by O2 activation. To exhibit such activity, the copper complex must be redox active and resistant to dissociation. Metallothioneins (MTs) and glutathione (GSH) are abundant in the cytosol and nucleus. Because they are thiol-rich reducing molecules with high CuI affinity, they are potential competitors for a copper ion bound in a copper drug. Herein, we report the investigation of a panel of CuI /CuII complexes often used as drugs, with diverse coordination chemistries and redox potentials. We evaluated their catalytic activity in ascorbate oxidation based on redox cycling between CuI and CuII , as well as their resistance to dissociation or inactivation under cytosolically relevant concentrations of GSH and MT. O2 -activating CuI /CuII complexes for cytosolic/nuclear targets are generally not stable against the GSH/MT system, which creates a challenge for their future design.

Copper Transporters? Glutathione Reactivity of Products of Cu-Aβ Digestion by Neprilysin

Aβ_4–42 is the major subspecies of Aβ peptides characterized by avid Cu(II) binding via the ATCUN/NTS motif. It is thought to be produced in vivo proteolytically by neprilysin, but in vitro experiments in the presence of Cu(II) ions indicated preferable formation of C-terminally truncated ATCUN/NTS species including Cu^IIAβ_4–16, Cu^IIAβ_4–9, and also Cu^IIAβ_12–16, all with nearly femtomolar affinities at neutral pH. Such small complexes may serve as shuttles for copper clearance from extracellular brain spaces, on condition they could survive intracellular conditions upon crossing biological barriers. In order to ascertain such possibility, we studied the reactions of Cu^IIAβ_4–16, Cu^IIAβ_4–9, Cu^IIAβ_12–16, and Cu^IIAβ_1–16 with reduced glutathione (GSH) under aerobic and anaerobic conditions using absorption spectroscopy and mass spectrometry. We found Cu^IIAβ_4–16 and Cu^IIAβ_4–9 to be strongly resistant to reduction and concomitant formation of Cu(I)–GSH complexes, with reaction times ∼10 h, while Cu^IIAβ_12–16 was reduced within minutes and Cu^IIAβ_1–16 within seconds of incubation. Upon GSH exhaustion by molecular oxygen, the Cu^IIAβ complexes were reformed with no concomitant oxidative damage to peptides. These finding reinforce the concept of Aβ_4–x peptides as physiological trafficking partners of brain copper.

Aβ₄₋₁₆, Aβ₄₋₉, and Aβ₁₂₋₁₆, oligopeptide products of β-amyloid degradation by neprilysin, bind Cu^II ions very tightly and are considered as possible Cu^II carriers in the brain. We demonstrated that Cu^II(Aβ_4−x) complexes, but not Cu^II(Aβ₁₂₋₁₆), are kinetically resistant to reduction by glutathione. No covalent Aβ peptide modifications were observed during the copper reduction and reoxidation by ambient oxygen, yielding the original complexes. These features suggest that Cu^II(Aβ_4−x) complexes might be able to cross the blood−brain barrier.

Preorganized PSP Ligands Yield Monomeric Cu(I) Complexes with Subzeptomolar Cu(I) Dissociation Constants

Unraveling the function of biological copper (Cu) requires tools that can selectively recognize and manipulate this trace nutrient within the complex chemical environment of biological systems. Increasing evidence suggests that cells maintain an exchangeable pool of Cu(I) that is buffered in the high zeptomolar to low attomolar range. While mixed amine-thioether donors have been commonly employed for the design of Cu(I)-selective ligands and probes, their dissociation constants are limited to the pico- to femtomolar range. To address this challenge, we combined our previously devised phosphine sulfide-stabilized phosphine donor motifs with a rigid 1,2-phenylene or 1,8-naphthylene ligand backbone. The resulting ligands, phenPS and naphPS, bind Cu(I) with a 1:1 complex stoichiometry and offer dissociation constants of 0.6 and 0.8 zM, respectively. Concluding from the crystal structures of the free and Cu(I)-bound ligands, the 1,2-phenylene-bridged ligand phenPS provides a high degree of structural preorganization to accommodate the Cu(I) center without large conformational changes, while the 1,8-naphthylene-bridged ligand revealed significant out-of-plane distortions in both the free and Cu(I)-bound states. Both ligands were accessed by palladium-catalyzed cross-coupling reactions from the corresponding arylhalides under mild conditions, an approach that could be readily expanded toward the design of other ligands and probes.

Copper-induced cell death and the protective role of glutathione: the implication of impaired protein folding rather than oxidative stress

Copper (Cu) is a bioelement essential for a myriad of enzymatic reactions, which when present in high concentration leads to cytotoxicity. Whereas Cu toxicity is usually assumed to originate from the metal's ability to enhance lipid peroxidation, the role of oxidative stress has remained uncertain since no antioxidant therapy has ever been effective. Here we show that Cu overload induces cell death independently of the metal's ability to oxidize the intracellular milieu. In fact, cells neither lose control of their thiol homeostasis until briefly before the onset of cell death, nor trigger a consistent antioxidant response. As expected, glutathione (GSH) protects the cell from Cu-mediated cytotoxicity but, surprisingly, fully independent of its reactive thiol. Moreover, the oxidation state of extracellular Cu is irrelevant as cells accumulate the metal as cuprous ions. We provide evidence that cell death is driven by the interaction of cuprous ions with proteins which impairs protein folding and promotes aggregation. Consequently, cells mostly react to Cu by mounting a heat shock response and trying to restore protein homeostasis. The protective role of GSH is based on the binding of cuprous ions, thus preventing the metal interaction with proteins. Due to the high intracellular content of GSH, it is depleted near the Cu entry site, and hence Cu can interact with proteins and cause aggregation and cytotoxicity immediately below the plasma membrane.

Activity-based ratiometric FRET probe reveals oncogene-driven changes in labile copper pools induced by altered glutathione metabolism

Copper is essential for life, and beyond its well-established ability to serve as a tightly bound, redox-active active site cofactor for enzyme function, emerging data suggest that cellular copper also exists in labile pools, defined as loosely bound to low-molecular-weight ligands, which can regulate diverse transition metal signaling processes spanning neural communication and olfaction, lipolysis, rest-activity cycles, and kinase pathways critical for oncogenic signaling. To help decipher this growing biology, we report a first-generation ratiometric fluorescence resonance energy transfer (FRET) copper probe, FCP-1, for activity-based sensing of labile Cu(I) pools in live cells. FCP-1 links fluorescein and rhodamine dyes through a Tris[(2-pyridyl)methyl]amine bridge. Bioinspired Cu(I)-induced oxidative cleavage decreases FRET between fluorescein donor and rhodamine acceptor. FCP-1 responds to Cu(I) with high metal selectivity and oxidation-state specificity and facilitates ratiometric measurements that minimize potential interferences arising from variations in sample thickness, dye concentration, and light intensity. FCP-1 enables imaging of dynamic changes in labile Cu(I) pools in live cells in response to copper supplementation/depletion, differential expression of the copper importer CTR1, and redox stress induced by manipulating intracellular glutathione levels and reduced/oxidized glutathione (GSH/GSSG) ratios. FCP-1 imaging reveals a labile Cu(I) deficiency induced by oncogene-driven cellular transformation that promotes fluctuations in glutathione metabolism, where lower GSH/GSSG ratios decrease labile Cu(I) availability without affecting total copper levels. By connecting copper dysregulation and glutathione stress in cancer, this work provides a valuable starting point to study broader cross-talk between metal and redox pathways in health and disease with activity-based probes.

Cytosolic Copper Binding by a Bacterial Storage Protein and Interplay with Copper Efflux

Escherichia coli has a well-characterized copper (Cu) transporting ATPase (CopA) that removes this potentially toxic metal ion from the cytosol. Growth of the strain lacking CopA (ΔcopA) is inhibited above 0.5 mM Cu, whilst a similar effect does not occur in wild type (WT) E. coli until over 2.5 mM Cu. Limited expression of CopA can restore growth to WT levels in ΔcopAE. coli in the presence of Cu. To study the influence of a bacterial cytosolic Cu storage protein (Csp3) on how E. coli handles Cu, the protein from Bacillus subtilis (BsCsp3) has been overexpressed in the WT and ΔcopA strains. BsCsp3 can protect both strains from Cu toxicity, promoting growth at up to ~1.5 and ~3.5 mM Cu, respectively. Higher levels of Csp3 expression are needed to provide resistance to Cu toxicity in ΔcopAE. coli. At 1.5 mM Cu, BsCsp3 purified from ΔcopAE. coli binds up to approximately four equivalents of Cu(I) per monomer. A similar number of Cu(I) equivalents can be bound by BsCsp3 purified from WT E. coli also grown at 1.5 mM Cu, a concentration that does not cause toxicity in this strain. Much lower amounts of BsCsp3 are produced in WT E. coli grown in the presence of 3.4 mM Cu, but the protein still counteracts toxicity and is almost half loaded with Cu(I). Csp3s can protect E. coli from Cu toxicity by sequestering cuprous ions in the cytosol. This appears to include an ability to acquire and withhold Cu(I) from the main efflux system in a heterologous host.