Solution Structure of the Apo and Copper(I)-Loaded Human Metallochaperone HAH1†

Copper Homeostasis in the Model Organism C. elegans

Cellular and organismic copper (Cu) homeostasis is regulated by Cu transporters and Cu chaperones to ensure the controlled uptake, distribution and export of Cu ions. Many of these processes have been extensively investigated in mammalian cell culture, as well as in humans and in mammalian model organisms. Most of the human genes encoding proteins involved in Cu homeostasis have orthologs in the model organism, Caenorhabditis elegans (C. elegans). Starting with a compilation of human Cu proteins and their orthologs, this review presents an overview of Cu homeostasis in C. elegans, comparing it to the human system, thereby establishing the basis for an assessment of the suitability of C. elegans as a model to answer mechanistic questions relating to human Cu homeostasis.

Interference between copper transport systems and platinum drugs

Cisplatin, or cis-diamminedichloridoplatinum(II) cis-[PtCl₂(NH₃)₂], is a platinum-based anticancer drug largely used for the treatment of various types of cancers, including testicular, ovarian and colorectal carcinomas, sarcomas, and lymphomas. Together with other platinum-based drugs, cisplatin triggers malignant cell death by binding to nuclear DNA, which appears to be the ultimate target. In addition to passive diffusion across the cell membrane, other transport systems, including endocytosis and some active or facilitated transport mechanisms, are currently proposed to play a pivotal role in the uptake of platinum-based drugs. In this review, an updated view of the current literature regarding the intracellular transport and processing of cisplatin will be presented, with special emphasis on the plasma membrane copper permease CTR1, the Cu-transporting ATPases, ATP7A and ATP7B, located in the trans-Golgi network, and the soluble copper chaperone ATOX1. Their role in eliciting cisplatin efficacy and their exploitation as pharmacological targets will be addressed.

Cu(I) Controls Conformational States in Human Atox1 Metallochaperone: An EPR and Multiscale Simulation Study

Atox1 is a human copper metallochaperone that is responsible for transferring copper ions from the main human copper transporter, hCtr1, to ATP7A/B in the Golgi apparatus. Atox1 interacts with the Ctr1 C-terminal domain as a dimer, although it transfers the copper ions to ATP7A/B in a monomeric form. The copper binding site in the Atox1 dimer involves Cys12 and Cys15, while Lys60 was also suggested to play a role in the copper binding. We recently showed that Atox1 can adopt various conformational states, depending on the interacting protein. In the current study, we apply EPR experiments together with hybrid quantum mechanics–molecular mechanics molecular dynamics simulations using a recently developed semiempirical density functional theory approach, to better understand the effect of Atox1’s conformational states on copper coordination. We propose that the flexibility of Atox1 occurs owing to protonation of one or more of the cysteine residues, and that Cys15 is an important residue for Atox1 dimerization, while Cys12 is a critical residue for Cu(I) binding. We also show that Lys60 electrostatically stabilizes the Cu(I)–Atox1 dimer.

Human glutaredoxin-1 can transfer copper to isolated metal binding domains of the P1B-type ATPase, ATP7B

Intracellular copper (Cu) in eukaryotic organisms is regulated by homeostatic systems, which rely on the activities of soluble metallochaperones that participate in Cu exchange through highly tuned protein-protein interactions. Recently, the human enzyme glutaredoxin-1 (hGrx1) has been shown to possess Cu metallochaperone activity. The aim of this study was to ascertain whether hGrx1 can act in Cu delivery to the metal binding domains (MBDs) of the P_1B-type ATPase ATP7B and to determine the thermodynamic factors that underpin this activity. hGrx1 can transfer Cu to the metallochaperone Atox1 and to the MBDs 5-6 of ATP7B (WLN5-6). This exchange is irreversible. In a mixture of the three proteins, Cu is delivered to the WLN5-6 preferentially, despite the presence of Atox1. This preferential Cu exchange appears to be driven by both the thermodynamics of the interactions between the proteins pairs and of the proteins with Cu(I). Crucially, protein-protein interactions between hGrx1, Atox1 and WLN5-6 were detected by NMR spectroscopy both in the presence and absence of Cu at a common interface. This study augments the possible activities of hGrx1 in intracellular Cu homeostasis and suggests a potential redundancy in this system, where hGrx1 has the potential to act under cellular conditions where the activity of Atox1 in Cu regulation is attenuated.

Interaction between Copper Chaperone Atox1 and Parkinson's Disease Protein α-Synuclein Includes Metal-Binding Sites and Occurs in Living Cells

Alterations in copper ion homeostasis appear coupled to neurodegenerative disorders, but mechanisms are unknown. The cytoplasmic copper chaperone Atox1 was recently found to inhibit amyloid formation in vitro of α-synuclein, the amyloidogenic protein in Parkinson's disease. As α-synuclein may have copper-dependent functions, and free copper ions promote α-synuclein amyloid formation, it is important to characterize the Atox1 interaction with α-synuclein on a molecular level. Here we applied solution-state nuclear magnetic resonance spectroscopy, with isotopically labeled α-synuclein and Atox1, to define interaction regions in both proteins. The α-synuclein interaction interface includes the whole N-terminal part up to Gln24; in Atox1, residues around the copper-binding cysteines (positions 11-16) are mostly perturbed, but additional effects are also found for residues elsewhere in both proteins. Because α-synuclein is N-terminally acetylated in vivo, we established that Atox1 also inhibits amyloid formation of this variant in vitro, and proximity ligation in human cell lines demonstrated α-synuclein-Atox1 interactions in situ. Thus, this interaction may provide the direct link between copper homeostasis and amyloid formation in vivo.

Oxidation of Human Copper Chaperone Atox1 and Disulfide Bond Cleavage by Cisplatin and Glutathione

Cancer cells cope with high oxidative stress levels, characterized by a shift toward the oxidized form (GSSG) of glutathione (GSH) in the redox couple GSSG/2GSH. Under these conditions, the cytosolic copper chaperone Atox1, which delivers Cu(I) to the secretory pathway, gets oxidized, i.e., a disulfide bond is formed between the cysteine residues of the Cu(I)-binding CxxC motif. Switching to the covalently-linked form, sulfur atoms are not able to bind the Cu(I) ion and Atox1 cannot play an antioxidant role. Atox1 has also been implicated in the resistance to platinum chemotherapy. In the presence of excess GSH, the anticancer drug cisplatin binds to Cu(I)-Atox1 but not to the reduced apoprotein. With the aim to investigate the interaction of cisplatin with the disulfide form of the protein, we performed a structural characterization in solution and in the solid state of oxidized human Atox1 and explored its ability to bind cisplatin under conditions mimicking an oxidizing environment. Cisplatin targets a methionine residue of oxidized Atox1; however, in the presence of GSH as reducing agent, the drug binds irreversibly to the protein with ammine ligands trans to Cys12 and Cys15. The results are discussed with reference to the available literature data and a mechanism is proposed connecting platinum drug processing to redox and copper homeostasis.

Copper chaperone ATOX1 is required for MAPK signaling and growth in BRAF mutation-positive melanoma

Copper (Cu) is a tightly regulated micronutrient that functions as a structural or catalytic cofactor for specific proteins essential for a diverse array of biological processes. While the study of the extremely rare genetic diseases, Menkes and Wilson, has highlighted the requirement for proper Cu acquisition and elimination in biological systems for cellular growth and proliferation, the importance of dedicated Cu transport systems, like the Cu chaperones ATOX1 and CCS, in the pathophysiology of cancer is not well defined. We found that ATOX1 was significantly overexpressed in human blood, breast, and skin cancer samples, while CCS was significantly altered in human brain, liver, ovarian, and prostate cancer when compared to normal tissue. Further analysis of genetic expression data in Cancer Cell Line Encyclopedia (CCLE) revealed that ATOX1 is highly expressed in melanoma cell lines over other cancer cell lines. We previously found that Cu is required for BRAF^V600E-driven MAPK signaling and melanomagenesis. Here we show that genetic loss of ATOX1 decreased BRAF^V600E-dependent growth and signaling in human melanoma cell lines. Pharmacological inhibition of ATOX1 with a small molecule, DCAC50, decreased the phosphorylation of ERK1/2 and reduced the growth of BRAF mutation-positive melanoma cell lines in a dose-dependent manner. Taken together, these results suggest that targeting the Cu chaperone ATOX1 as a novel therapeutic angle in BRAF^V600E-driven melanomas.

In-silico analysis of novel p.(Gly14Ser) variant of ATOX1 gene: plausible role in modulating ATOX1-ATP7B interaction

Clinical heterogeneity is commonly observed in Wilson disease (WD), including cases with identical ATP7B mutations. It is thought to be an outcome of impairment in other genes involved in cellular copper homeostasis in addition to the mutations in the ATP7B gene. ATOX1, a copper chaperone that delivers copper to ATP7B, is a potential genetic modifier of WD. In the present study, we analyzed the genetic variations in the ATOX1 gene in 50 WD patients and 60 controls. We identified four novel variants, of which, the coding region variant c.40G > A, p.(Gly14Ser) was observed in 2% alleles. Interestingly, p.(Gly14Ser) was seen with an early onset age, reduced serum ceruloplasmin level and manifestations of liver and brain in a WD patient unlike the other having identical ATP7B mutation but normal ATOX1 alleles. Further, computational analysis predicted that p.(Gly14Ser) substitution, in the critical copper binding motif (MXCXG₁₄C) of the protein, affects the protein-protein interaction involved in copper sharing and transfer between ATOX1 and ATP7B-MBD4. Our findings suggest that p.(Gly14Ser) variant of ATOX1 might play a role as a genetic modifier leading to phenotypic variation in WD.

Metal cofactors trafficking and assembly in the cell: a molecular view

Metal ions are essential cofactors required by the proteome of organisms from any kingdom of life to correctly exert their functions. Dedicated cellular import, transport and homeostasis systems assure that the needed metal ion is correctly delivered and inserted into the target proteins and avoid the presence of free metal ions in the cell, preventing oxidative damaging. Among metal ions, in eukaryotic organisms copper and iron are required by proteins involved in absolutely essential functions, such as respiration, oxidative stress protection, catalysis, gene expression regulation. Copper and iron binding proteins are localized in essentially all cellular compartments. Copper is physiologically present mainly as individual metal ion. Iron can be present both as individual metal ion or as part of cofactors, such as hemes and iron-sulfur (Fe-S) clusters. Both metal ions are characterized by the ability to cycle between different oxidation states, which enable them to catalyze redox reactions and to participate in electron transfer processes. Here we describe in detail the main processes responsible for the trafficking of copper and iron sulfur clusters, with particular interest for the structural aspects of the maturation of copper and iron-sulfur-binding proteins.

Platinum transfer from hCTR1 to Atox1 is dependent on the type of platinum complex

In spite of their wide application, the cellular uptake of platinum based anticancer drugs is still unclear. The copper transport protein, hCTR1, is proposed to facilitate the cellular uptake of cisplatin, whereas organic cation transport (OCT) is more important for oxaliplatin. It has been reported that both N-terminal and C-terminal metal binding motifs of hCTR1 are highly reactive to cisplatin, which is the initial step of protein assisted cellular uptake of cisplatin. It is still unknown how the platinum drugs in hCTR1 transfer to cytoplasmic media, and whether various platinum complexes possess different activities in this process. Herein, we investigated the reaction of the platinated C-terminal metal binding motif of hCTR1 (C8) with the down-stream protein Atox1. Results show that Atox1 is highly reactive to the platinated C8 adducts of cisplatin and transplatin, whereas the oxaliplatin/C8 adduct is much less reactive. The platinum transfer from C8 to Atox1 occurs in the reaction, which results in the protein unfolding of Atox1. These results demonstrated that the platinated intracellular-domain of hCTR1 is reactive to Atox1, and the reactivity is dependent on the ligand and the coordination structure of platinum complexes. The different reactivity is consistent with the hypothesis that hCTR1 is more significant in the transport of cisplatin than that of oxaliplatin.

Shared gene-network signatures between the human heavy metal proteome and neurological disorders and cancer types

In this work, for the first time, the human heavy metal proteome was predicted.

Open Boundary Simulations of Proteins and Their Hydration Shells by Hamiltonian Adaptive Resolution Scheme

The recently proposed Hamiltonian adaptive resolution scheme (H-AdResS) allows the performance of molecular simulations in an open boundary framework. It allows changing, on the fly, the resolution of specific subsets of molecules (usually the solvent), which are free to diffuse between the atomistic region and the coarse-grained reservoir. So far, the method has been successfully applied to pure liquids. Coupling the H-AdResS methodology to hybrid models of proteins, such as the molecular mechanics/coarse-grained (MM/CG) scheme, is a promising approach for rigorous calculations of ligand binding free energies in low-resolution protein models. Toward this goal, here we apply for the first time H-AdResS to two atomistic proteins in dual-resolution solvent, proving its ability to reproduce structural and dynamic properties of both the proteins and the solvent, as obtained from atomistic simulations.

Reduction potentials of protein disulfides and catalysis of glutathionylation and deglutathionylation by glutaredoxin enzymes

Glutaredoxins (Grxs) are a class of GSH (glutathione)-dependent thiol–disulfide oxidoreductase enzymes. They use the cellular redox buffer GSSG (glutathione disulfide)/GSH directly to catalyze these exchange reactions. Grxs feature dithiol active sites and can shuttle rapidly between three oxidation states, namely dithiol Grx(SH)2, mixed disulfide Grx(SH)(SSG) and oxidized disulfide Grx(SS). Each is characterized by a distinct standard reduction potential . The values for the redox couple Grx(SS)/Grx(SH)2 are available, but a recent estimate differs by over 100 mV from the literature values. No estimates are available for for the mixed disulfide couple Grx(SH)(SSG)/(Grx(SH)2 + GSH). This work determined both and for two representative Grx enzymes, Homo sapiens HsGrx1 and Escherichia coli EcGrx1. The empirical approaches were verified rigorously to overcome the sensitivity of these redox-labile enzymes to experimental conditions. The classic method of acid ‘quenching’ was demonstrated to shift the thiol–disulfide redox equilibria. Both enzymes exhibit an (vs. SHE) at a pH of 7.0. Their values (−213 and −230 mV for EcGrx1 and HsGrx1, respectively) are slightly less negative than that () of the redox buffer GSSG/2GSH. Both and vary with log [GSH], but the former more sensitively by a factor of 2. This confers dual catalytic functions to a Grx enzyme as either an oxidase at low [GSH] or as a reductase at high [GSH]. Consequently, these enzymes can participate efficiently in either glutathionylation or deglutathionylation. The catalysis is demonstrated to proceed via a monothiol ping-pong mechanism relying on a single Cys residue only in the dithiol active site.

The crystal structures of a copper-bound metallochaperone from Saccharomyces cerevisiae

Atx1 is a metallochaperone protein from the yeast Saccharomyces cerevisiae (yAtx1) that plays a major role in copper homeostasis in this organism. yAtx1 functions as a copper transfer protein by shuttling copper to the secretory pathway to control intracellular copper levels. Here we describe the first crystal structures of yAtx1 that have been determined in the presence of Cu(I). The structures from two different crystal forms have been solved and refined to resolutions of 1.65 and 1.93Å. In contrast to the previous metallated crystal structure of yAtx1 where a single Hg(II) atom was coordinated by one yAtx1 molecule, the Cu(I)-yAtx1 was crystallised as a dimer in both crystal forms, sharing one Cu(I) atom between two yAtx1 molecules. This is consistent with the crystal structure of the human homologue Cu(I)-hAtox1. Overall the structures in the two different crystal forms of Cu(I)-yAtx1 are remarkably similar to that of Cu(I)-hAtox1. However, subtle structural differences between Cu(I)-yCtr1 and Cu(I)-hAtox1 are observed in copper coordination geometries and in the conformations of Loop 2, with the latter potentially contributing to differential interactions and copper transfer mechanisms with membrane transport copper uptake systems.

The effects of metal ion contaminants on the double stranded DNA helix and diseases

Mineral metal ions are essential for the maintenance of the reactions that regulate homeostasis and the functions of our body. It is known that the regulation of the neurodegenerative system depends directly on life metal ions, such as Na, K, Mg, Ca, Fe, Mo, Cu, Co, Zn, Cr, Mn, while the toxic metals Cd, Pb, Hg, etc disturb homeostasis, leading to diseases. Particularly significant is the effect of toxic metals on the double stranded forms of DNA and conformations. It was found that the toxic metal ions by reacting specifically with the nucleic bases and electrostatically with the negatively phosphate groups of the DNA backbone cause changes in the structure of the DNA double helix, leading to breaks of single or double strands. Accumulation of these defects affects the protecting systems of the body and induces mutations, eventually leading to serious diseases. There are many metal ions, such as Cr, Al, Cd, Cu, Ni, which by binding directly to DNA molecule or by developing oxidative stress increase the instability of DNA, promoting epigenetic changes that lead to DNA damage. Toxic metal ions induce indirect DNA damage and influence the gene stability by inactivating encoding proteins or by changing the redox potential and the signaling of metalloenzymes.

Binding of Copper and Cisplatin to Atox1 Is Mediated by Glutathione through the Formation of Metal-Sulfur Clusters

Copper is an essential nutrient required for many biological processes involved in primary metabolism, but free copper is toxic due to its ability to catalyze formation of free radicals. To prevent toxic effects, in the cell copper is bound to proteins and low molecular weight compounds, such as glutathione, at all times. The widely used chemotherapy agent cisplatin is known to bind to copper-transporting proteins, including copper chaperone Atox1. Cisplatin interactions with Atox1 and other copper transporters are linked to cancer resistance to platinum-based chemotherapy. Here we analyze the binding of copper and cisplatin to Atox1 in the presence of glutathione under redox conditions that mimic intracellular environment. We show that copper(I) and glutathione form large polymers with a molecular mass of approximately 8 kDa, which can transfer copper to Atox1. Cisplatin also can form polymers with glutathione, albeit at a slower rate. Analysis of simultaneous binding of copper and cisplatin to Atox1 under physiological conditions shows that both metals are bound to the protein through copper-sulfur-platinum bridges.

The structural flexibility of the human copper chaperone Atox1: Insights from combined pulsed EPR studies and computations

Metallochaperones are responsible for shuttling metal ions to target proteins. Thus, a metallochaperone's structure must be sufficiently flexible both to hold onto its ion while traversing the cytoplasm and to transfer the ion to or from a partner protein. Here, we sought to shed light on the structure of Atox1, a metallochaperone involved in the human copper regulation system. Atox1 shuttles copper ions from the main copper transporter, Ctr1, to the ATP7b transporter in the Golgi apparatus. Conventional biophysical tools such as X-ray or NMR cannot always target the various conformational states of metallochaperones, owing to a requirement for crystallography or low sensitivity and resolution. Electron paramagnetic resonance (EPR) spectroscopy has recently emerged as a powerful tool for resolving biological reactions and mechanisms in solution. When coupled with computational methods, EPR with site-directed spin labeling and nanoscale distance measurements can provide structural information on a protein or protein complex in solution. We use these methods to show that Atox1 can accommodate at least four different conformations in the apo state (unbound to copper), and two different conformations in the holo state (bound to copper). We also demonstrate that the structure of Atox1 in the holo form is more compact than in the apo form. Our data provide insight regarding the structural mechanisms through which Atox1 can fulfill its dual role of copper binding and transfer.

Physicochemical code for quinary protein interactions in Escherichia coli

How proteins sense and navigate the cellular interior to find their functional partners remains poorly understood. An intriguing aspect of this search is that it relies on diffusive encounters with the crowded cellular background, made up of protein surfaces that are largely nonconserved. The question is then if/how this protein search is amenable to selection and biological control. To shed light on this issue, we examined the motions of three evolutionary divergent proteins in the Escherichia coli cytoplasm by in-cell NMR. The results show that the diffusive in-cell motions, after all, follow simplistic physical-chemical rules: The proteins reveal a common dependence on (i) net charge density, (ii) surface hydrophobicity, and (iii) the electric dipole moment. The bacterial protein is here biased to move relatively freely in the bacterial interior, whereas the human counterparts more easily stick. Even so, the in-cell motions respond predictably to surface mutation, allowing us to tune and intermix the protein's behavior at will. The findings show how evolution can swiftly optimize the diffuse background of protein encounter complexes by just single-point mutations, and provide a rational framework for adjusting the cytoplasmic motions of individual proteins, e.g., for rescuing poor in-cell NMR signals and for optimizing protein therapeutics.

Triplin, a small molecule, reveals copper ion transport in ethylene signaling from ATX1 to RAN1

Copper ions play an important role in ethylene receptor biogenesis and proper function. The copper transporter RESPONSIVE-TO-ANTAGONIST1 (RAN1) is essential for copper ion transport in Arabidopsis thaliana. However it is still unclear how copper ions are delivered to RAN1 and how copper ions affect ethylene receptors. There is not a specific copper chelator which could be used to explore these questions. Here, by chemical genetics, we identified a novel small molecule, triplin, which could cause a triple response phenotype on dark-grown Arabidopsis seedlings through ethylene signaling pathway. ran1-1 and ran1-2 are hypersensitive to triplin. Adding copper ions in growth medium could partially restore the phenotype on plant caused by triplin. Mass spectrometry analysis showed that triplin could bind copper ion. Compared to the known chelators, triplin acts more specifically to copper ion and it suppresses the toxic effects of excess copper ions on plant root growth. We further showed that mutants of ANTIOXIDANT PROTEIN1 (ATX1) are hypersensitive to tiplin, but with less sensitivity comparing with the ones of ran1-1 and ran1-2. Our study provided genetic evidence for the first time that, copper ions necessary for ethylene receptor biogenesis and signaling are transported from ATX1 to RAN1. Considering that triplin could chelate copper ions in Arabidopsis, and copper ions are essential for plant and animal, we believe that, triplin not only could be useful for studying copper ion transport of plants, but also could be useful for copper metabolism study in animal and human.

Computational modelling of the redistribution of copper isotopes by proteins in the liver

The copper isotopic composition of blood serum as the mass balance of the copper isotopic composition of serum proteins.

The C-Terminus of Human Copper Importer Ctr1 Acts as a Binding Site and Transfers Copper to Atox1

Uptake of copper (Cu) ions into human cells is mediated by the plasma membrane protein Ctr1 and is followed by Cu transfer to cytoplasmic Cu chaperones for delivery to Cu-dependent enzymes. The C-terminal cytoplasmic tail of Ctr1 is a 13-residue peptide harboring an HCH motif that is thought to interact with Cu. We here employ biophysical experiments under anaerobic conditions in peptide models of the Ctr1 C-terminus to deduce Cu-binding residues, Cu affinity, and the ability to release Cu to the cytoplasmic Cu chaperone Atox1. Based on NMR assignments and bicinchoninic acid competition experiments, we demonstrate that Cu interacts in a 1:1 stoichiometry with the HCH motif with an affinity, KD, of ∼10(-14) M. Removing either the Cys residue or the two His residues lowers the Cu-peptide affinity, but site specificity is retained. The C-terminal peptide and Atox1 do not interact in solution in the absence of Cu. However, as directly demonstrated at the residue level via NMR spectroscopy, Atox1 readily acquires Cu from the Cu-loaded peptide. We propose that Cu binding to the Ctr1 C-terminal tail regulates Cu transport into the cytoplasm such that the metal ion is only released to high-affinity Cu chaperones.

The Role of Copper Chaperone Atox1 in Coupling Redox Homeostasis to Intracellular Copper Distribution

Human antioxidant protein 1 (Atox1) is a small cytosolic protein with an essential role in copper homeostasis. Atox1 functions as a copper carrier facilitating copper transfer to the secretory pathway. This process is required for activation of copper dependent enzymes involved in neurotransmitter biosynthesis, iron efflux, neovascularization, wound healing, and regulation of blood pressure. Recently, new cellular roles for Atox1 have emerged. Changing levels of Atox1 were shown to modulate response to cancer therapies, contribute to inflammatory response, and protect cells against various oxidative stresses. It has also become apparent that the activity of Atox1 is tightly linked to the cellular redox status. In this review, we summarize biochemical information related to a dual role of Atox1 as a copper chaperone and an antioxidant. We discuss how these two activities could be linked and contribute to establishing the intracellular copper balance and functional identity of cells during differentiation.

Thioether Coordination Chemistry for Molecular Imaging of Copper in Biological Systems

Copper is an essential element in biological systems. Its potent redox activity renders it necessary for life, but at the same time, misregulation of its cellular pools can lead to oxidative stress implicated in aging and various disease states. Copper is commonly thought of as a static cofactor buried in protein active sites; however, evidence of a more loosely bound, labile pool of copper has emerged. To help identify and understand new roles for dynamic copper pools in biology, we have developed selective molecular imaging agents for this metal, drawing inspiration from both biological binding motifs and synthetic model complexes that reveal thioether coordination as a general design strategy for selective and sensitive copper recognition. In this review, we summarize some contributions, primarily from our own laboratory, on fluorescence- and magnetic resonance-based molecular-imaging probes for studying copper in living systems using thioether coordination chemistry.

Unbound position II in MXCXXC metallochaperone model peptides impacts metal binding mode and reactivity: Distinct similarities to whole proteins

The effect of position II in the binding sequence of copper metallochaperones, which varies between Thr and His, was investigated through structural analysis and affinity and oxidation kinetic studies of model peptides. A first Cys-Cu(I)-Cys model obtained for the His peptide at acidic and neutral pH, correlated with higher affinity and more rapid oxidation of its complex; in contrast, the Thr peptide with the Cys-Cu(I)-Met coordination under neutral conditions demonstrated weaker and pH dependent binding. Studies with human antioxidant protein 1 (Atox1) and three of its mutants where S residues were replaced with Ala suggested that (a) the binding affinity is influenced more by the binding sequence than by the protein fold (b) pH may play a role in binding reactivity, and (c) mutating the Met impacted the affinity and oxidation rate more drastically than did mutating one of the Cys, supporting its important role in protein function. Position II thus plays a dominant role in metal binding and transport.

Inhibition of human copper trafficking by a small molecule significantly attenuates cancer cell proliferation

Copper is a transition metal that plays critical roles in many life processes. Controlling the cellular concentration and trafficking of copper offers a route to disrupt these processes. Here we report small molecules that inhibit the human copper-trafficking proteins Atox1 and CCS, and so provide a selective approach to disrupt cellular copper transport. The knockdown of Atox1 and CCS or their inhibition leads to a significantly reduced proliferation of cancer cells, but not of normal cells, as well as to attenuated tumour growth in mouse models. We show that blocking copper trafficking induces cellular oxidative stress and reduces levels of cellular ATP. The reduced level of ATP results in activation of the AMP-activated protein kinase that leads to reduced lipogenesis. Both effects contribute to the inhibition of cancer cell proliferation. Our results establish copper chaperones as new targets for future developments in anticancer therapies.

Inhibition of human copper trafficking by a small molecule significantly attenuates cancer cell proliferation

Copper is a transition metal that plays critical roles in many life processes. Controlling the cellular concentration and trafficking of copper offers a route to disrupt these processes. Here we report small molecules that inhibit the human copper-trafficking proteins Atox1 and CCS, and so provide a selective approach to disrupt cellular copper transport. The knockdown of Atox1 and CCS or their inhibition leads to a significantly reduced proliferation of cancer cells, but not of normal cells, as well as to attenuated tumour growth in mouse models. We show that blocking copper trafficking induces cellular oxidative stress and reduces levels of cellular ATP. The reduced level of ATP results in activation of the AMP-activated protein kinase that leads to reduced lipogenesis. Both effects contribute to the inhibition of cancer cell proliferation. Our results establish copper chaperones as new targets for future developments in anticancer therapies.

Inhibition of human copper trafficking by a small molecule significantly attenuates cancer cell proliferation

Copper is a transition metal that plays critical roles in many life processes. Controlling the cellular concentration and trafficking of copper offers a route to disrupt these processes. Here we report small molecules that inhibit the human copper-trafficking proteins Atox1 and CCS, and so provide a selective approach to disrupt cellular copper transport. The knockdown of Atox1 and CCS or their inhibition leads to a significantly reduced proliferation of cancer cells, but not of normal cells, as well as to attenuated tumour growth in mouse models. We show that blocking copper trafficking induces cellular oxidative stress and reduces levels of cellular ATP. The reduced level of ATP results in activation of the AMP-activated protein kinase that leads to reduced lipogenesis. Both effects contribute to the inhibition of cancer cell proliferation. Our results establish copper chaperones as new targets for future developments in anticancer therapies.

Computational metallomics of the anticancer drug cisplatin

Cisplatin, cis-diamminedichlorido-platinum(II), is an important therapeutic tool in the struggle against different tumors, yet it is plagued with the emergence of resistance mechanisms after repeated administrations. This hampers greatly its efficacy. Overcoming resistance problems requires first and foremost an integrated and systematic understanding of the structural determinants and molecular recognition processes involving the drug and its cellular targets. Here we review a strategy that we have followed for the last few years, based on the combination of modern tools from computational chemistry with experimental biophysical methods. Using hybrid Quantum Mechanics/Molecular Mechanics (QM/MM) simulations, validated by spectroscopic experiments (including NMR, and CD), we have worked out for the first time at atomic level the structural determinants in solution of platinated cellular substrates. These include the copper homeostasis proteins Ctr1, Atox1, and ATP7A. All of these proteins have been suggested to influence the pre-target resistance mechanisms. Furthermore, coupling hybrid QM/MM simulations with classical Molecular Dynamics (MD) and free energy calculations, based on force field parameters refined by the so-called "Force Matching" procedure, we have characterized the structural modifications and the free energy landscape associated with the recognition between platinated DNA and the protein HMGB1, belonging to the chromosomal high-mobility group proteins HMGB that inhibit the repair of platinated DNA. This may alleviate issues relative to on-target resistance process. The elucidation of the mechanisms by which tumors are sensitive or refractory to cisplatin may lead to the discovery of prognostic biomarkers. The approach reviewed here could be straightforwardly extended to other metal-based drugs.

Conserved residue modulates copper-binding properties through structural dynamics in human copper chaperone Atox1

The human copper chaperone Atox1 plays a central role in the transport of copper in cells. It has been reported that the conserved residue Lys60 contributes to the heterocomplex stability of Atox1 with its target protein ATPase, and that the K60A mutation could diminish the copper transfer. In this work, we carried out the structure determination and dynamic analysis of Atox1 with the K60A mutation in order to elucidate the role of the conserved residue Lys60 in the copper transport. Results show that the K60A mutation results in crucial secondary structure rearrangements and side-chain orientation alteration of the metal-binding residues in Atox1. Protein dynamic studies reveal that the K60A mutation leads to increased overall flexibility, and a significant difference in dynamic properties of the metal-binding sites. The structure and dynamic changes cause a decrease in the copper-binding stability of the K60A mutant. In addition, Cu(i)-mediated hetero-protein interactions with ATP7A are present in the metal transfer of both Atox1 variants, although copper transfer is accompanied with smaller structural alteration in the K60A mutant. These results indicate that Lys60 is crucial in maintaining the structure and dynamic properties of Atox1.

Copper binding modulates the platination of human copper chaperone Atox1 by antitumor trans-platinum complexes

Cu(i) coordination enhances the reactivity of Atox1 towards antitumor-active trans-platinum complexes and promotes platinum transfer from the protein to dithiothreitol.

Interaction of classical platinum agents with the monomeric and dimeric Atox1 proteins: a molecular dynamics simulation study

We carried out molecular dynamics simulations and free energy calculations for a series of binary and ternary models of the cisplatin, transplatin and oxaliplatin agents binding to a monomeric Atox1 protein and a dimeric Atox1 protein to investigate their interaction mechanisms. All three platinum agents could respectively combine with the monomeric Atox1 protein and the dimeric Atox1 protein to form a stable binary and ternary complex due to the covalent interaction of the platinum center with the Atox1 protein. The results suggested that the extra interaction from the oxaliplatin ligand-Atox1 protein interface increases its affinity only for the OxaliPt + Atox1 model. The binding of the oxaliplatin agent to the Atox1 protein might cause larger deformation of the protein than those of the cisplatin and transplatin agents due to the larger size of the oxaliplatin ligand. However, the extra interactions to facilitate the stabilities of the ternary CisPt + 2Atox1 and OxaliPt + 2Atox1 models come from the α1 helices and α2-β4 loops of the Atox1 protein-Atox1 protein interface due to the cis conformation of the platinum agents. The combinations of two Atox1 proteins in an asymmetric way in the three ternary models were analyzed. These investigations might provide detailed information for understanding the interaction mechanism of the platinum agents binding to the Atox1 protein in the cytoplasm.

An expanding range of functions for the copper chaperone/antioxidant protein Atox1

SIGNIFICANCE

Antioxidant protein 1 (Atox1 in human cells) is a copper chaperone for the copper export pathway with an essential role in cellular copper distribution. In vitro, Atox1 binds and transfers copper to the copper-transporting ATPases, stimulating their catalytic activity. Inactivation of Atox1 in cells inhibits maturation of secreted cuproenzymes as well as copper export from cells.

RECENT ADVANCES

Accumulating data suggest that cellular functions of Atox1 are not limited to its copper-trafficking role and may include storage of labile copper, modulation of transcription, and antioxidant defense. The conserved metal binding site of Atox1, CxGC, differs from the metal-binding sites of copper-transporting ATPases and has a physiologically relevant redox potential that equilibrates with the GSH:GSSG pair.

CRITICAL ISSUES

Tight relationship appears to exist between intracellular copper levels and glutathione (GSH) homeostasis. The biochemical properties of Atox1 place it at the intersection of cellular networks that regulate copper distribution and cellular redox balance. Mechanisms through which Atox1 facilitates copper export and contributes to oxidative defense are not fully understood.

FUTURE DIRECTIONS

The current picture of cellular redox homeostasis and copper physiology will be enhanced by further mechanistic studies of functional interactions between the GSH:GSSG pair and copper-trafficking machinery.

Atox1 contains positive residues that mediate membrane association and aid subsequent copper loading

Copper chaperones bind intracellular copper and ensure proper trafficking to downstream targets via protein-protein interactions. In contrast to the mechanisms of copper binding and transfer to downstream targets, the mechanisms of initial copper loading of the chaperones are largely unknown. Here, we demonstrate that antioxidant protein 1 (Atox1 in human cells), the principal cellular copper chaperone responsible for delivery of copper to the secretory pathway, possesses the ability to interact with negatively charged lipid headgroups via distinct surface lysine residues. Moreover, loss of these residues lowers the efficiency of copper loading of Atox1 in vivo, suggesting that the membrane may play a scaffolding role in copper distribution to Atox1. These findings complement the recent discovery that the membrane also facilitates copper loading of the copper chaperone for superoxide dismutase 1 and provide further support for the emerging paradigm that the membrane bilayer plays a central role in cellular copper acquisition and distribution.

Determinants for simultaneous binding of copper and platinum to human chaperone Atox1: hitchhiking not hijacking

Cisplatin (CisPt) is an anticancer agent that has been used for decades to treat a variety of cancers. CisPt treatment causes many side effects due to interactions with proteins that detoxify the drug before reaching the DNA. One key player in CisPt resistance is the cellular copper-transport system involving the uptake protein Ctr1, the cytoplasmic chaperone Atox1 and the secretory path ATP7A/B proteins. CisPt has been shown to bind to ATP7B, resulting in vesicle sequestering of the drug. In addition, we and others showed that the apo-form of Atox1 could interact with CisPt in vitro and in vivo. Since the function of Atox1 is to transport copper (Cu) ions, it is important to assess how CisPt binding depends on Cu-loading of Atox1. Surprisingly, we recently found that CisPt interacted with Cu-loaded Atox1 in vitro at a position near the Cu site such that unique spectroscopic features appeared. Here, we identify the binding site for CisPt in the Cu-loaded form of Atox1 using strategic variants and a combination of spectroscopic and chromatographic methods. We directly prove that both metals can bind simultaneously and that the unique spectroscopic signals originate from an Atox1 monomer species. Both Cys in the Cu-site (Cys12, Cys15) are needed to form the di-metal complex, but not Cys41. Removing Met10 in the conserved metal-binding motif makes the loop more floppy and, despite metal binding, there are no metal-metal electronic transitions. In silico geometry minimizations provide an energetically favorable model of a tentative ternary Cu-Pt-Atox1 complex. Finally, we demonstrate that Atox1 can deliver CisPt to the fourth metal binding domain 4 of ATP7B (WD4), indicative of a possible drug detoxification mechanism.

Copper chaperones. The concept of conformational control in the metabolism of copper

Copper chaperones compose a specific class of proteins assuring safe handling and specific delivery of potentially harmful copper ions to a variety of essential copper proteins. Copper chaperones are structurally heterogeneous and can exist in multiple metal-loaded as well as oligomeric forms. Moreover, many copper chaperones can exist in various oxidative states and participate in redox catalysis, connected with their functioning. This review is focused on the analysis of the structural and functional properties of copper chaperones and their partners, which allowed us to define specific regulatory principles in copper metabolism connected with copper-induced conformational control of copper proteins.

Probing the coordination environment of the human copper chaperone HAH1: characterization of Hg(II)-bridged homodimeric species in solution

Although metal ion homeostasis in cells is often mediated through metallochaperones, there are opportunities for toxic metals to be sequestered through the existing transport apparatus. Proper trafficking of Cu(I) in human cells is partially achieved through complexation by HAH1, the human metallochaperone responsible for copper delivery to the Wilson and Menkes ATPase located in the trans-Golgi apparatus. In addition to binding copper, HAH1 strongly complexes Hg(II), with the X-ray structure of this complex previously described. It is important to clarify the solution behavior of these systems and, therefore, the binding of Hg(II) to HAH1 was probed over the pH range 7.5 to 9.4 using (199)Hg NMR, (199m)Hg PAC and UV-visible spectroscopies. The metal-dependent protein association over this pH range was examined using analytical gel-filtration. It can be concluded that at pH 7.5, Hg(II) is bound to a monomeric HAH1 as a two coordinate, linear complex (HgS2), like the Hg(II)-Atx1 X-ray structure (PDB ID: 1CC8). At pH 9.4, Hg(II) promotes HAH1 association, leading to formation of HgS3 and HgS4 complexes, which are in exchange on the μs-ns time scale. Thus, structures that may represent central intermediates in the process of metal ion transfer, as well as their exchange kinetics have been characterized.

Mechanistic aspects of hSOD1 maturation from the solution structure of Cu(I) -loaded hCCS domain 1 and analysis of disulfide-free hSOD1 mutants

Superoxide dismutase 1 (SOD1) maturation within the cell is mainly accomplished with the SOD1-specific chaperone, CCS, a dimeric protein with three distinct domains in each monomer. We recently showed that the first domain of human CCS (hCCSD1) is responsible for copper transfer to its protein partner, human SOD1 (hSOD1). The NMR solution structure of the copper(I)-loaded form of hCCSD1 reported here contributes further to characterization of the copper-transfer mechanism to hSOD1. NMR spectroscopy was also used to examine the hSOD1 mutants C57A, C146A, and C57A/C146A, which are unable to form the structurally conserved disulfide bond in SOD1, in order to investigate the role of these cysteines during hSOD1 copper acquisition. Together, the information on both hCCS and hSOD1, along with a sequence analysis of eukaryotic CCSD1, allows us to propose important mechanistic aspects regarding the copper-transfer process from hCCS to hSOD1.

Peptide models of Cu(I) and Zn(II) metallochaperones: the effect of pH on coordination and mechanistic implications

The first NMR structures of Cu(I) and Zn(II) peptide complexes as models of metallochaperones were derived with no predetermined binding mode. The cyclic peptide MDCSGCSRPG was reacted with Cu(I) and Zn(II) at low and moderate pH. This peptide features the conserved sequence of copper chaperones but with Asp at position 2 as appears in the zinc binding domain of ZntA. The structures were compared with those of the Cu(I) complexes of the wild-type sequence peptide MTCSGCSRPG. All analyses were conducted first with no metal-binding constraints to ensure accurate binding ligand assignment. Several structures included metal-Met binding, raising a possible role of Met in the metal transport mechanism. Both Cu(I) and Zn(II) gave different complexes when reacted with the peptide of the native-like sequence under different pH conditions, raising the possibility of pH-dependent transport mechanisms. Cu(I) bound the MTCSGCSRPG peptide through one Cys and the Met under acidic conditions and differently under basic conditions; Zn(II) bound the MDCSGCSRPG peptide through two Cys and the Met residues under acidic conditions and through one Cys and the Met under basic conditions, while Cu(I) bound the non-native Asp mutant peptide through the Asp and one Cys under both conditions, suggesting that Asp may inhibit pH-dependent binding for Cu(I). NOESY and ESI-HRMS supported the presence of an aqua ligand for Zn(II), which likely deprotonated under basic conditions to give a hydroxo group. Coordination similarities were detected among the model system and native proteins, which overall suggest that coordination flexibility is required for the function of metallochaperones.