Copper metallochaperones

Radical-induced DNA damage by cytotoxic square-planar copper(II) complexes incorporating o-phthalate and 1,10-phenanthroline or 2,2'-dipyridyl

DNA-targeting copper(II) reagents have emerged as suitable drug candidates owing to the clinical success of the copper-activated, natural chemotherapeutic drug bleomycin. This agent and the synthetic chemical nuclease copper(II) bis-1,10-phenanthroline represent important templates for inorganic drug design owing to their ability to initiate free radical DNA scission. Herein, we report the synthesis and biological properties of 1:1:1 square-planar copper(II) complexes incorporating the dicarboxylate o-phthalate and 1,10-phenanthroline (1) or 2,2'-dipyridyl (2) ligands. Their broad-spectrum chemotherapeutic potential has been assessed at 24- and 96-h intervals, along with that of the clinical agent cisplatin, using breast (MCF-7), prostate (DU145), colon (HT29), and intrinsically cisplatin-resistant ovarian (SK-OV-3) human cancer cells. 1 represents a potent cytotoxic agent with IC(50) values ranging from 5.6 to 3.4μM across all cell lines, including SK-OV-3. The production of endogenous reactive oxygen species within SK-OV-3 cancer cells was monitored using the fluorophore 2',7'-dichlorodihydrofluorescin diacetate, and results indicate a concentration-dependent propensity toward ROS generation by 1 and 2 that mirrors their antitumoral behavior. DNA interaction studies, using fluorescence and viscosity measurements, were conducted in tandem with the DNA-targeting drugs actinomycin D and pentamidine, using calf thymus DNA, poly[d(A-T)(2)], and poly[d(G-C)(2)], with intercalation of 1 and 2 at the minor groove appearing to be the likely interaction mode. DNA cleavage reactions using superhelical plasmid DNA, in the presence of exogenous reductant, l-ascorbic acid, revealed excellent agreement between double-stranded DNA scission capability and antitumoral IC(50) concentration. The presence of double-strand DNA breaks (DSBs) was confirmed within SK-OV-3 cancer cells using immunodetection of γ-H2AX foci by confocal microscopy and flow cytometry, with complex 1 quantitatively producing superior numbers of DSBs compared with complex 2. Superoxide dismutase and catalase mimetic activity assays were conducted, and these activities are related to the ability of both complexes to cleave DNA through free radical generation.

The CopRS two-component system is responsible for resistance to copper in the cyanobacterium Synechocystis sp. PCC 6803

Photosynthetic organisms need copper for cytochrome oxidase and for plastocyanin in the fundamental processes of respiration and photosynthesis. However, excess of free copper is detrimental inside the cells and therefore organisms have developed homeostatic mechanisms to tightly regulate its acquisition, sequestration, and efflux. Herein we show that the CopRS two-component system (also known as Hik31-Rre34) is essential for copper resistance in Synechocystis sp. PCC 6803. It regulates expression of a putative heavy-metal efflux-resistance nodulation and division type copper efflux system (encoded by copBAC) as well as its own expression (in the copMRS operon) in response to the presence of copper in the media. Mutants in this two-component system or the efflux system render cells more sensitive to the presence of copper in the media and accumulate more intracellular copper than the wild type. Furthermore, CopS periplasmic domain is able to bind copper, suggesting that CopS could be able to detect copper directly. Both operons (copMRS and copBAC) are also induced by the photosynthetic inhibitor 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone but this induction requires the presence of copper in the media. The reduced response of two mutant strains to copper, one lacking plastocyanin and a second one impaired in copper transport to the thylakoid, due to the absence of the P(I)-type ATPases PacS and CtaA, suggests that CopS can detect intracellular copper. In addition, a tagged version of CopS with a triple HA epitope localizes to both the plasma and the thylakoid membranes, suggesting that CopS could be involved in copper detection in both the periplasm and the thylakoid lumen.

Periplasmic proteins encoded by VCA0261-0260 and VC2216 genes together with copA and cueR products are required for copper tolerance but not for virulence in Vibrio cholerae

The bacterial pathogen Vibrio cholerae requires colonizination of the human small intestine to cause cholera. The anaerobic and slightly acidic conditions predominating there enhance toxicity of low copper concentrations and create a selective environment for bacteria with evolved detoxifying mechanisms. We reported previously that the VCA0260, VCA0261 and VC2216 gene products were synthesized only in V. cholerae grown in microaerobiosis or anaerobiosis. Here we show that ORFs VCA0261 and VCA0260 are actually combined into a single gene encoding a 18.7 kDa protein. Bioinformatic analyses linked this protein and the VC2216 gene product to copper tolerance. Following the approach of predict-mutate and test, we describe for the first time, to our knowledge, the copper tolerance systems operating in V. cholerae. Copper susceptibility analyses of mutants in VCA0261-0260, VC2216 or in the putative copper-tolerance-related VC2215 (copA ATPase) and VC0974 (cueR), under aerobic and anaerobic growth, revealed that CopA represents the main tolerance system under both conditions. The VC2216-encoded periplasmic protein contributes to resistance only under anaerobiosis in a CopA-functional background. The locus tag VCA0261-0260 encodes a copper-inducible, CueR-dependent, periplasmic protein, which mediates tolerance in aerobiosis, but under anaerobiosis its role is only evident in CopA knock-out mutants. None of the genes involved in copper homeostasis were required for V. cholerae virulence or colonization in the mouse model. We conclude that copper tolerance in V. cholerae, which lacks orthologues of the periplasmic copper tolerance proteins CueO, CusCFBA and CueP, involves CopA and CueR proteins along with the periplasmic Cot (VCA0261-0260) and CopG (VC2216) V. cholerae homologues.

Flexibility of the metal-binding region in apo-cupredoxins

Protein-mediated electron transfer is an essential event in many biochemical processes. Efficient electron transfer requires the reorganization energy of the redox event to be minimized, which is ensured by the presence of rigid donor and acceptor sites. Electron transfer copper sites are present in the ubiquitous cupredoxin fold, able to bind one or two copper ions. The low reorganization energy in these metal centers has been accounted for by assuming that the protein scaffold creates an entatic/rack-induced state, which gives rise to a rigid environment by means of a preformed metal chelating site. However, this notion is incompatible with the need for an exposed metal-binding site and protein-protein interactions enabling metallochaperone-mediated assembly of the copper site. Here we report an NMR study that reveals a high degree of structural heterogeneity in the metal-binding region of the nonmetallated Cu(A)-binding cupredoxin domain, arising from microsecond to second dynamics that are quenched upon metal binding. We also report similar dynamic features in apo-azurin, a paradigmatic blue copper protein, suggesting a general behavior. These findings reveal that the entatic/rack-induced state, governing the features of the metal center in the copper-loaded protein, does not require a preformed metal-binding site. Instead, metal binding is a major contributor to the rigidity of electron transfer copper centers. These results reconcile the seemingly contradictory requirements of a rigid, occluded center for electron transfer, and an accessible, dynamic site required for in vivo copper uptake.

Dynamic multibody protein interactions suggest versatile pathways for copper trafficking

As part of intracellular copper trafficking pathways, the human copper chaperone Hah1 delivers Cu(+) to the Wilson's Disease Protein (WDP) via weak and dynamic protein-protein interactions. WDP contains six homologous metal binding domains (MBDs) connected by flexible linkers, and these MBDs all can receive Cu(+) from Hah1. The functional roles of the MBD multiplicity in Cu(+) trafficking are not well understood. Building on our previous study of the dynamic interactions between Hah1 and the isolated fourth MBD of WDP, here we study how Hah1 interacts with MBD34, a double-domain WDP construct, using single-molecule fluorescence resonance energy transfer (smFRET) combined with vesicle trapping. By alternating the positions of the smFRET donor and acceptor, we systematically probed Hah1-MBD3, Hah1-MBD4, and MBD3-MBD4 interaction dynamics within the multidomain system. We found that the two interconverting interaction geometries were conserved in both intermolecular Hah1-MBD and intramolecular MBD-MBD interactions. The Hah1-MBD interactions within MBD34 are stabilized by an order of magnitude relative to the isolated single-MBDs, and thermodynamic and kinetic evidence suggest that Hah1 can interact with both MBDs simultaneously. The enhanced interaction stability of Hah1 with the multi-MBD system, the dynamic intramolecular MBD-MBD interactions, and the ability of Hah1 to interact with multiple MBDs simultaneously suggest an efficient and versatile mechanism for the Hah1-to-WDP pathway to transport Cu(+).

Molecular and biochemical characterization of a unique mutation in CCS, the human copper chaperone to superoxide dismutase

Copper (Cu) is a trace metal that readily gains and donates electrons, a property that renders it desirable as an enzyme cofactor but dangerous as a source of free radicals. To regulate cellular Cu metabolism, an elaborate system of chaperones and transporters has evolved, although no human Cu chaperone mutations have been described to date. We describe a child from a consanguineous family who inherited homozygous mutations in the SLC33A1, encoding an acetyl CoA transporter, and in CCS, encoding the Cu chaperone for superoxide dismutase. The CCS mutation, p.Arg163Trp, predicts substitution of a highly conserved arginine residue at position 163, with tryptophan in domain II of CCS, which interacts directly with superoxide dismutase 1 (SOD1). Biochemical analyses of the patient's fibroblasts, mammalian cell transfections, immunoprecipitation assays, and Lys7Δ (CCS homolog) yeast complementation support the pathogenicity of the mutation. Expression of CCS was reduced and binding of CCS to SOD1 impaired. As a result, this mutation causes reduced SOD1 activity and may impair other mechanisms important for normal Cu homeostasis. CCS-Arg163Trp represents the primary example of a human mutation in a gene coding for a Cu chaperone.

Variations in methanobactin structure influences copper utilization by methane-oxidizing bacteria

Methane-oxidizing bacteria are nature's primary biological mechanism for suppressing atmospheric levels of the second-most important greenhouse gas via methane monooxygenases (MMOs). The copper-containing particulate enzyme is the most widespread and efficient MMO. Under low-copper conditions methane-oxidizing bacteria secrete the small copper-binding peptide methanobactin (mbtin) to acquire copper, but how variations in the structures of mbtins influence copper metabolism and species selection are unknown. Methanobactins have been isolated from Methylocystis strains M and hirsuta CSC1, organisms that can switch to using an iron-containing soluble MMO when copper is limiting, and the nonswitchover Methylocystis rosea. These mbtins are shorter, and have different amino acid compositions, than the characterized mbtin from Methylosinus trichosporium OB3b. A coordinating pyrazinedione ring in the Methylocystis mbtins has little influence on the Cu(I) site structure. The Methylocystis mbtins have a sulfate group that helps stabilize the Cu(I) forms, resulting in affinities of approximately 10(21) M(-1). The Cu(II) affinities vary over three orders of magnitude with reduction potentials covering approximately 250 mV, which may dictate the mechanism of intracellular copper release. Copper uptake and the switchover from using the iron-containing soluble MMO to the copper-containing particulate enzyme is faster when mediated by the native mbtin, suggesting that the amino acid sequence is important for the interaction of mbtins with receptors. The differences in structures and properties of mbtins, and their influence on copper utilization by methane-oxidizing bacteria, have important implications for the ecology and global function of these environmentally vital organisms.

In vitro thermodynamic dissection of human copper transfer from chaperone to target protein

Transient protein-protein and protein-ligand interactions are fundamental components of biological activity. To understand biological activity, not only the structures of the involved proteins are important but also the energetics of the individual steps of a reaction. Here we use in vitro biophysical methods to deduce thermodynamic parameters of copper (Cu) transfer from the human copper chaperone Atox1 to the fourth metal-binding domain of the Wilson disease protein (WD4). Atox1 and WD4 have the same fold (ferredoxin-like fold) and Cu-binding site (two surface exposed cysteine residues) and thus it is not clear what drives metal transfer from one protein to the other. Cu transfer is a two-step reaction involving a metal-dependent ternary complex in which the metal is coordinated by cysteines from both proteins (i.e., Atox1-Cu-WD4). We employ size exclusion chromatography to estimate individual equilibrium constants for the two steps. This information together with calorimetric titration data are used to reveal enthalpic and entropic contributions of each step in the transfer process. Upon combining the equilibrium constants for both steps, a metal exchange factor (from Atox1 to WD4) of 10 is calculated, governed by a negative net enthalpy change of ∼10 kJ/mol. Thus, small variations in interaction energies, not always obvious upon comparing protein structures alone, may fuel vectorial metal transfer.

Rhodanese functions as sulfur supplier for key enzymes in sulfur energy metabolism

How microorganisms obtain energy is a challenging topic, and there have been numerous studies on the mechanisms involved. Here, we focus on the energy substrate traffic in the hyperthermophilic bacterium Aquifex aeolicus. This bacterium can use insoluble sulfur as an energy substrate and has an intricate sulfur energy metabolism involving several sulfur-reducing and -oxidizing supercomplexes and enzymes. We demonstrate that the cytoplasmic rhodanese SbdP participates in this sulfur energy metabolism. Rhodaneses are a widespread family of proteins known to transfer sulfur atoms. We show that SbdP has also some unusual characteristics compared with other rhodaneses; it can load a long sulfur chain, and it can interact with more than one partner. Its partners (sulfur reductase and sulfur oxygenase reductase) are key enzymes of the sulfur energy metabolism of A. aeolicus and share the capacity to use long sulfur chains as substrate. We demonstrate a positive effect of SbdP, once loaded with sulfur chains, on sulfur reductase activity, most likely by optimizing substrate uptake. Taken together, these results lead us to propose a physiological role for SbdP as a carrier and sulfur chain donor to these key enzymes, therefore enabling channeling of sulfur substrate in the cell as well as greater efficiency of the sulfur energy metabolism of A. aeolicus.

Response to copper stress in Streptomyces lividans extends beyond genes under direct control of a copper-sensitive operon repressor protein (CsoR)

A copper-sensitive operon repressor protein (CsoR) has been identified in Streptomyces lividans (CsoR(Sl)) and found to regulate copper homeostasis with attomolar affinity for Cu(I). Solution studies reveal apo- and Cu(I)-CsoR(Sl) to be a tetramer assembly, and a 1.7-Å resolution crystal structure of apo-CsoR(Sl) reveals that a significant conformational change is necessary to enable Cu(I) binding. In silico prediction of the CsoR regulon was confirmed in vitro (EMSA) and in vivo (RNA-seq), which highlighted that next to the csoR gene itself, the regulon consists of two Cu(I) efflux systems involving a CopZ-like copper metallochaperone protein and a CopA P(1)-type ATPase. Although deletion of csoR has only minor effects on S. lividans development when grown under high copper concentrations, mutations of the Cu(I) ligands decrease tolerance to copper as a result of the Cu(I)-CsoR mutants failing to disengage from the DNA targets, thus inhibiting the derepression of the regulon. RNA-seq experiments carried out on samples incubated with exogenous copper and a ΔcsoR strain showed that the set of genes responding to copper stress is much wider than anticipated and largely extends beyond genes targeted by CsoR. This suggests more control levels are operating and directing other regulons in copper homeostasis beside the CsoR regulon.

Metal transport across biomembranes: emerging models for a distinct chemistry

Transition metals are essential components of important biomolecules, and their homeostasis is central to many life processes. Transmembrane transporters are key elements controlling the distribution of metals in various compartments. However, due to their chemical properties, transition elements require transporters with different structural-functional characteristics from those of alkali and alkali earth ions. Emerging structural information and functional studies have revealed distinctive features of metal transport. Among these are the relevance of multifaceted events involving metal transfer among participating proteins, the importance of coordination geometry at transmembrane transport sites, and the presence of the largely irreversible steps associated with vectorial transport. Here, we discuss how these characteristics shape novel transition metal ion transport models.

Importance of electrostatic polarizability in calculating cysteine acidity constants and copper(I) binding energy of Bacillus subtilis CopZ

CopZ is a copper chaperone from Bacillus subtilis. It is an important part of Cu(I) trafficking. We have calculated pK(a) values for the CXXC motif of this protein, which is responsible for the Cu(I) binding, and the Cu(I) binding constants. Polarizable and fixed-charges formalisms were used, and solvation parameters for the both models have been refitted. We had to partially redevelop parameters for the protonated and deprotonated cysteine residues. We have discovered that the polarizable force field (PFF) is qualitatively superior and allows a uniformly better level of energetic results. The PFF pK(a) values for cysteine are within about 0.8-2.8 pH units of the experimental data, while the fixed-charges OPLS formalism yields errors of up to tens of units. The PFF magnitude of the copper binding energy is about 10 kcal/mol or 50% higher than the experimental value, while the using the refitted OPLS parameters leads to an overall positive binding energy, thus predicting no thermodynamically stable complex. At the same time, the agreement of the polarizable S···Cu(I) distances with the experimental results is within 0.08 Å range, and the nonpolarizable calculations lead to an error of about 0.4 Å. Moreover, the accuracy of the PFF has been achieved without any explicit fitting to either pK(a) or CopZ···Cu(I) binding energies. We believe that this makes our polarizable technique a choice method in reproducing protein-copper binding and further supports the notion that explicit treatment of electrostatic polarization is crucial in many biologically relevant studies, especially ion binding and transport.

Chemistry and biology of the copper chelator methanobactin

Methanotrophic bacteria, organisms that oxidize methane, produce a small copper chelating molecule called methanobactin (Mb). Mb binds Cu(I) with high affinity and is hypothesized to mediate copper acquisition from the environment. Recent advances in Mb characterization include revision of the chemical structure of Mb from Methylosinus trichosporium OB3b and further investigation of its biophysical properties. In addition, Mb production by several other methanotroph strains has been investigated, and preliminary characterization suggests diversity in chemical composition. Initial clues into Mb biosynthesis have been obtained by identification of a putative precursor gene in the M. trichosporium OB3b genome. Finally, direct uptake of intact Mb into the cytoplasm of M. trichosporium OB3b cells has been demonstrated, and studies of the transport mechanism have been initiated. Taken together, these advances represent significant progress and set the stage for exciting new research directions.

Cellular transport and homeostasis of essential and nonessential metals

Metals can have a number of detrimental or beneficial effects in the cell, but first they must get in. Organisms have evolved transport mechanisms to get metals that are required, or essential into the cell. Nonessential metals often enter the cell through use of the machinery provided for essential metals. Much work has been done to advance our understanding of how these metals are transported across plasma and organelle membranes. This review provides an overview of essential and nonessential metal transport and homeostatic processes.

Visualization of Cu²⁺ uptake and release in plant cells by fluorescence lifetime imaging microscopy

A principal objective in life sciences is the visualization of biochemical processes. Fluorescence-based techniques are widely used to demonstrate transport of relevant substances across cellular membranes. In this paper we report a novel noninvasive, real-time fluorescence lifetime imaging microscopy method for visualizing uptake and release of divalent copper ions (Cu(2+) ) in vivo. For this purpose, we employed a green fluorescent protein (GFP) form able to change its fluorescence lifetime upon Cu(2+) binding. We demonstrate that this technique is selective for Cu(2+) . We show the reversible decrease of the fluorescence lifetime of GFP from 2.2 to 1.6 ns in Escherichia coli and from 1.8 to 1.3 ns in root cells of Arabidopsis after the addition of Cu(2+) . Cu(2+) uptake of epidermal tobacco cells leads to a drop of the GFP lifetime from 2.5 to 2.2 ns. In summary, the spatially resolved visualization of Cu(2+) distribution in vivo is demonstrated in prokaryote and eukaryote cells.

Transcriptome sequencing identifies SPL7-regulated copper acquisition genes FRO4/FRO5 and the copper dependence of iron homeostasis in Arabidopsis

The transition metal copper (Cu) is essential for all living organisms but is toxic when present in excess. To identify Cu deficiency responses comprehensively, we conducted genome-wide sequencing-based transcript profiling of Arabidopsis thaliana wild-type plants and of a mutant defective in the gene encoding SQUAMOSA PROMOTER BINDING PROTEIN-LIKE7 (SPL7), which acts as a transcriptional regulator of Cu deficiency responses. In response to Cu deficiency, FERRIC REDUCTASE OXIDASE5 (FRO5) and FRO4 transcript levels increased strongly, in an SPL7-dependent manner. Biochemical assays and confocal imaging of a Cu-specific fluorophore showed that high-affinity root Cu uptake requires prior FRO5/FRO4-dependent Cu(II)-specific reduction to Cu(I) and SPL7 function. Plant iron (Fe) deficiency markers were activated in Cu-deficient media, in which reduced growth of the spl7 mutant was partially rescued by Fe supplementation. Cultivation in Cu-deficient media caused a defect in root-to-shoot Fe translocation, which was exacerbated in spl7 and associated with a lack of ferroxidase activity. This is consistent with a possible role for a multicopper oxidase in Arabidopsis Fe homeostasis, as previously described in yeast, humans, and green algae. These insights into root Cu uptake and the interaction between Cu and Fe homeostasis will advance plant nutrition, crop breeding, and biogeochemical research.

Near-infrared fluorescent sensor for in vivo copper imaging in a murine Wilson disease model

Copper is an essential metal nutrient that is tightly regulated in the body because loss of its homeostasis is connected to severe diseases such as Menkes and Wilson diseases, Alzheimer's disease, prion disorders, and amyotrophic lateral sclerosis. The complex relationships between copper status and various stages of health and disease remain challenging to elucidate, in part due to a lack of methods for monitoring dynamic changes in copper pools in whole living organisms. Here we present the synthesis, spectroscopy, and in vivo imaging applications of Coppersensor 790, a first-generation fluorescent sensor for visualizing labile copper pools in living animals. Coppersensor 790 combines a near-infrared emitting cyanine dye with a sulfur-rich receptor to provide a selective and sensitive turn-on response to copper. This probe is capable of monitoring fluctuations in exchangeable copper stores in living cells and mice under basal conditions, as well as in situations of copper overload or deficiency. Moreover, we demonstrate the utility of this unique chemical tool to detect aberrant increases in labile copper levels in a murine model of Wilson disease, a genetic disorder that is characterized by accumulation of excess copper. The ability to monitor real-time copper fluxes in living animals offers potentially rich opportunities to examine copper physiology in health and disease.

Role of metal in folding and stability of copper proteins in vitro

Metal coordination is required for function of many proteins. For biosynthesis of proteins coordinating a metal, the question arises if the metal binds before, during or after folding of the polypeptide. Moreover, when the metal is bound to the protein, how does its coordination affect biophysical properties such as stability and dynamics? Understanding how metals are utilized by proteins in cells on a molecular level requires accurate descriptions of the thermodynamic and kinetic parameters involved in protein-metal complexes. Copper is one of the essential transition metals found in the active sites of many key proteins. To avoid toxicity of free copper ions, living systems have developed elaborate copper-transport systems that involve dedicated proteins that facilitate efficient and specific delivery of copper to target proteins. This review describes in vitro and in silico biophysical work assessing the role of copper in folding and stability of copper-binding proteins. Examples of proteins discussed are: a blue-copper protein (Pseudomonas aeruginosa azurin), members of copper-transport systems (bacterial CopZ, human Atox1 and ATP7B domains) and multi-copper ferroxidases (yeast Fet3p and human ceruloplasmin). The consequences of interactions between copper proteins and platinum-complexes are also discussed. This article is part of a Special Issue entitled: Cell Biology of Metals.

Transcriptional and posttranscriptional events control copper-responsive expression of a Rhodobacter capsulatus multicopper oxidase

The copper-regulated Rhodobacter capsulatus cutO (multicopper oxidase) gene confers copper tolerance and is carried in the tricistronic orf635-cutO-cutR operon. Transcription of cutO strictly depends on the promoter upstream of orf635, as demonstrated by lacZ reporter fusions to nested promoter fragments. Remarkably, orf635 expression was not affected by copper availability, whereas cutO and cutR were expressed only in the presence of copper. Differential regulation was abolished by site-directed mutations within the orf635-cutO intergenic region, suggesting that this region encodes a copper-responsive mRNA element. Bioinformatic predictions and RNA structure probing experiments revealed an intergenic stem-loop structure as the candidate mRNA element. This is the first posttranscriptional copper response mechanism reported in bacteria.

Elevated labile Cu is associated with oxidative pathology in Alzheimer disease

Oxidative stress is implicated in Alzheimer disease (AD) pathogenesis, for which evidence indicates that radical species are generated by the redox-active biometal Cu. The contribution of labile Cu to the oxidative stress observed in AD has not been evaluated. The Cu content of postmortem cortical tissue from nondemented elderly controls and AD cases was measured using inductively coupled plasma mass spectroscopy, and the proportion of labile Cu was assessed using the Cu-phenanthroline assay. Further, the capacity of the tissue to stabilize Cu(2+) was evaluated using immobilized metal-affinity chromatography, and the level of tissue oxidative damage was determined by the presence of thiobarbituric acid-reactive compounds. We identified elevated levels of exchangeable Cu(2+), which were correlated with tissue oxidative damage; additionally, we noted an increased capacity of AD cortical tissue samples to bind Cu(2+). This deranged Cu homeostasis reflects the homeostatic breakdown of Cu observed in AD and supports biometal metabolism as a therapeutic target.

Toward a molecular understanding of metal transport by P(1B)-type ATPases

The P(1B) family of P-type ATPases couples the transport of cytoplasmic transition metals across biological membranes to the hydrolysis of ATP. These ubiquitous transporters function in maintaining cytoplasmic metal quotas and in the assembly of metalloproteins, and have been classified into subfamilies (P(1B-1)-P(1B-5)) on the basis of their transported substrates (Cu(+), Zn(2+), Cu(2+), and Co(2+)) and signature sequences in their transmembrane segments. In addition, each subgroup presents a characteristic membrane topology and specific regulatory cytoplasmic metal-binding domains. In recent years, significant major aspects of their transport mechanism have been described, including the stoichiometry of transport and the delivery of substrates to transport sites by metallochaperones. Toward understanding their structure, the metal coordination by transport sites has been characterized for Cu(+) and Zn(2+)-ATPases. In addition, atomic resolution structures have been determined, providing key insight into the elements that enable transition metal transport. Because the Cu(+)-transporting ATPases are found in humans and are linked to disease, this subfamily has been the focus of intense study. As a result, significant progress has been made toward understanding Cu(+)-ATPase function on the molecular level, using both the human proteins and the bacterial homologs, most notably the CopA proteins from Archaeoglobus fulgidus, Bacillus subtilis, and Thermotoga maritima. This chapter thus focuses on the mechanistic and structural information obtained by studying these latter Cu(+)-ATPases, with some consideration of how these aspects might differ for the other subfamilies of P(1B)-ATPases.

Iron transport machinery of human cells: players and their interactions

Organisms, like cells, maintain tight control of iron. In humans as well as other mammals, control is achieved through the regulation of iron uptake into the body rather than through the excretion of iron. The mechanisms by which humans and mice regulate both iron uptake and the distribution of iron within the body and cells are reviewed. Special emphasis is given to the iron transporters involved in this process.

Metal export by CusCFBA, the periplasmic Cu(I)/Ag(I) transport system of Escherichia coli

High levels of metal ions have the potential to cause cellular toxicity through a variety of mechanisms; therefore, cells have developed numerous systems that regulate their intracellular concentrations. The Cus resistance system aids in protection of Escherichia coli from high levels of Cu(I) and Ag(I) by actively transporting these metal ions to the extracellular environment. The Cus system forms a continuous complex, CusCBA, that spans the inner membrane, periplasm, and outer membrane of Gram-negative bacteria, together with a novel fourth component, the periplasmic metallochaperone, CusF. The metal-binding sites of CusA, CusB, and CusF are exquisitely tuned for Cu(I) and Ag(I), and thus effectively discriminate these ions for transport from other metals that may be required in the cell. Furthermore, direct transfer of metal from protein to protein within the Cus system during the transport process is likely to reduce the potential toxicity posed by the free metal ions. Here we review the wealth of structural, biochemical, and genetic information on the Cus system, which demonstrates the many intriguing aspects of function for metal-transporting efflux systems.

Elemental economy: microbial strategies for optimizing growth in the face of nutrient limitation

Microorganisms play a dominant role in the biogeochemical cycling of nutrients. They are rightly praised for their facility for fixing both carbon and nitrogen into organic matter, and microbial driven processes have tangibly altered the chemical composition of the biosphere and its surrounding atmosphere. Despite their prodigious capacity for molecular transformations, microorganisms are powerless in the face of the immutability of the elements. Limitations for specific elements, either fleeting or persisting over eons, have left an indelible trace on microbial genomes, physiology, and their very atomic composition. We here review the impact of elemental limitation on microbes, with a focus on selected genetic model systems and representative microbes from the ocean ecosystem. Evolutionary adaptations that enhance growth in the face of persistent or recurrent elemental limitations are evident from genome and proteome analyses. These range from the extreme (such as dispensing with a requirement for a hard to obtain element) to the extremely subtle (changes in protein amino acid sequences that slightly, but significantly, reduce cellular carbon, nitrogen, or sulfur demand). One near-universal adaptation is the development of sophisticated acclimation programs by which cells adjust their chemical composition in response to a changing environment. When specific elements become limiting, acclimation typically begins with an increased commitment to acquisition and a concomitant mobilization of stored resources. If elemental limitation persists, the cell implements austerity measures including elemental sparing and elemental recycling. Insights into these fundamental cellular properties have emerged from studies at many different levels, including ecology, biological oceanography, biogeochemistry, molecular genetics, genomics, and microbial physiology. Here, we present a synthesis of these diverse studies and attempt to discern some overarching themes.

Cyanobacterial metallochaperone inhibits deleterious side reactions of copper

Copper metallochaperones supply copper to cupro-proteins through copper-mediated protein-protein-interactions and it has been hypothesized that metallochaperones thereby inhibit copper from causing damage en route. Evidence is presented in support of this latter role for cyanobacterial metallochaperone, Atx1. In cyanobacteria Atx1 contributes towards the supply of copper to plastocyanin inside thylakoids but it is shown here that in copper-replete medium, copper can reach plastocyanin without Atx1. Unlike metallochaperone-independent copper-supply to superoxide dismutase in eukaryotes, glutathione is not essential for Atx1-independent supply to plastocyanin: Double mutants missing atx1 and gshB (encoding glutathione synthetase) accumulate the same number of atoms of copper per cell in the plastocyanin pool as wild type. Critically, Δatx1ΔgshB are hypersensitive to elevated copper relative to wild type cells and also relative to ΔgshB single mutants with evidence that hypersensitivity arises due to the mislocation of copper to sites for other metals including iron and zinc. The zinc site on the amino-terminal domain (ZiaA(N)) of the P(1)-type zinc-transporting ATPase is especially similar to the copper site of the Atx1 target PacS(N), and ZiaA(N) will bind Cu(I) more tightly than zinc. An NMR model of a substituted-ZiaA(N)-Cu(I)-Atx1 heterodimer has been generated making it possible to visualize a juxtaposition of residues surrounding the ZiaA(N) zinc site, including Asp(18), which normally repulse Atx1. Equivalent repulsion between bacterial copper metallochaperones and the amino-terminal regions of P(1)-type ATPases for metals other than Cu(I) is conserved, again consistent with a role for copper metallochaperones to withhold copper from binding sites for other metals.

Mitochondrial effects of plant-made compounds

SIGNIFICANCE

Plants produce many small molecules with biomedical potential. Their absorption from foods, metabolism, their effects on physiological and pathological processes, and the mechanisms of action are intensely investigated. Many are known to affect multiple cellular functions. Mitochondria are coming to be recognized as a major target for these compounds, especially redox-active ones, but the mechanisms involved still need clarification. At the same time, frontline research is uncovering the importance of processes involving these organelles for the cell and for an array of physiological and pathological processes. We review the major functions and possible dysfunctions of mitochondria, identify signaling pathways through which plant-derived molecules have an impact, and show how this may be relevant for major pathologies.

RECENT ADVANCES

Antioxidant, protective effects may arise as a reaction to a low-level pro-oxidant activity, largely taking place at mitochondria. Some plant-derived molecules can activate AMP-dependent kinase, with a consequent upregulation of mitochondrial biogenesis and a potential favorable impact on aging, pathologies like diabetes and neurodegeneration, and on ischemic damage.

CRITICAL ISSUES

The extrapolation of in vitro results and the verification of paradigms in vivo is a key issue for current research on both plant-derived compounds and mitochondria. The low bioavailability of many of these molecules poses a problem for both the study of their activities and their utilization.

FUTURE DIRECTIONS

The further clarification of the role of mitochondria in the activities of plant dietary compounds and their metabolites, mitochondrial targeting, the development of analogs and pro-drugs are all topics for promising research.

Characterization of the peroxide sensitivity of COX-deficient yeast strains reveals unexpected relationships between COX assembly proteins

A number of distinct cuproproteins of the mitochondrial inner membrane are required for the assembly of cytochrome oxidase (COX), thought to function in a "bucket brigade" fashion to provide copper to the Cu(A) and Cu(B) sites. In yeast, the loss of two these proteins, Sco1p and Cox11p, leads to respiratory deficiency and a specific inability to survive exposure to hydrogen peroxide (H(2)O(2)). Using a quantitative assay, we have identified subtle differences in the peroxide-sensitive phenotypes between sco1 and cox11 mutant strains. Interestingly, the peroxide sensitivity of the sco1 null strain can be suppressed by overexpressing either SCO2 or COX11, although overexpression of neither SCO1 nor SCO2 can rescue the cox11 null strain. We also find that overexpression of either CTT1, encoding the cytosolic catalase T, or CTA1, encoding the mitochondrial matrix catalase, suppresses the peroxide sensitivity in both the sco1 and the cox11 null mutants. Direct measurement of peroxide metabolism shows that sco1 and cox11 null strains fail to degrade a significant amount of exogenously provided H(2)O(2). Taken together, our data demonstrate that although Cox11p and Sco1p play distinct roles in COX assembly, they seem to play overlapping or related roles in peroxide metabolism that require further investigation.

Molecular basis of the cooperative binding of Cu(I) and Cu(II) to the CopK protein from Cupriavidus metallidurans CH34

The bacterium Cupriavidus metallidurans CH34 is resistant to high environmental concentrations of many metal ions. Upon copper challenge, it upregulates the periplasmic protein CopK (8.3 kDa). The function of CopK in the copper resistance response is ill-defined, but CopK demonstrates an intriguing cooperativity: occupation of a high-affinity Cu(I) binding site generates a high-affinity Cu(II) binding site, and the high-affinity Cu(II) binding enhances Cu(I) binding. Native CopK and targeted variants were examined by chromatographic, spectroscopic, and X-ray crystallographic probes. Structures of two distinct forms of Cu(I)Cu(II)-CopK were defined, and structural changes associated with occupation of the Cu(II) site were demonstrated. In solution, monomeric Cu(I)Cu(II)-CopK features the previously elucidated Cu(I) site in Cu(I)-CopK, formed from four S(δ) atoms of Met28, -38, -44, and -54 (site 4S). Binding of Cu(I) to apo-CopK induces a conformational change that releases the C-terminal β-strand from the β-sandwich structure. In turn, this allows His70 and N-terminal residues to form a large loop that includes the Cu(II) binding site. In crystals, a polymeric form of Cu(I)Cu(II)-CopK displays a Cu(I) site defined by the S(δ) atoms of Met26, -38, and -54 (site 3S) and an exogenous ligand (modeled as H(2)O) and a Cu(II) site that bridges dimeric CopK molecules. The 3S Cu(I) binding mode observed in crystals was demonstrated in solution in protein variant M44L where site 4S is disabled. The intriguing copper binding chemistry of CopK provides molecular insight into Cu(I) transfer processes. The adaptable nature of the Cu(I) coordination sphere in methionine-rich clusters allows copper to be relayed between clusters during transport across membranes in molecular pumps such as CusA and Ctr1.

Mfc1 is a novel forespore membrane copper transporter in meiotic and sporulating cells

To gain insight in the molecular basis of copper homeostasis during meiosis, we have used DNA microarrays to analyze meiotic gene expression in the model yeast Schizosaccharomyces pombe. Profiling data identified a novel meiosis-specific gene, termed mfc1(+), that encodes a putative major facilitator superfamily-type transporter. Although Mfc1 does not exhibit any significant sequence homology with the copper permease Ctr4, it contains four putative copper-binding motifs that are typically found in members of the copper transporter family of copper transporters. Similarly to the ctr4(+) gene, the transcription of mfc1(+) was induced by low concentrations of copper. However, its temporal expression profile during meiosis was distinct to ctr4(+). Whereas Ctr4 was observed at the plasma membrane shortly after induction of meiosis, Mfc1 appeared later in precursor vesicles and, subsequently, at the forespore membrane of ascospores. Using the fluorescent copper-binding tracker Coppersensor-1 (CS1), labile cellular copper was primarily detected in the forespores in an mfc1(+)/mfc1(+) strain, whereas an mfc1Δ/mfc1Δ mutant exhibited an intracellular dispersed punctate distribution of labile copper ions. In addition, the copper amine oxidase Cao1, which localized primarily in the forespores of asci, was fully active in mfc1(+)/mfc1(+) cells, but its activity was drastically reduced in an mfc1Δ/mfc1Δ strain. Furthermore, our data showed that meiotic cells that express the mfc1(+) gene have a distinct developmental advantage over mfc1Δ/mfc1Δ mutant cells when copper is limiting. Taken together, the data reveal that Mfc1 serves to transport copper for accurate and timely meiotic differentiation under copper-limiting conditions.

Differential affinity of BsSCO for Cu(II) and Cu(I) suggests a redox role in copper transfer to the Cu(A) center of cytochrome c oxidase

SCO (synthesis of cytochrome c oxidase) proteins are involved in the assembly of the respiratory chain enzyme cytochrome c oxidase acting to assist in the assembly of the Cu(A) center contained within subunit II of the oxidase complex. The Cu(A) center receives electrons from the reductive substrate ferrocytochrome c, and passes them on to the cytochrome a center. Cytochrome a feeds electrons to the oxygen reaction site composed of cytochrome a(3) and Cu(B). Cu(A) consists of two copper ions positioned within bonding distance and ligated by two histidine side chains, one methionine, a backbone carbonyl and two bridging cysteine residues. The complex structure and redox capacity of Cu(A) present a potential assembly challenge. SCO proteins are members of the thioredoxin family which led to the early suggestion of a disulfide exchange function for SCO in Cu(A) assembly, whereas the copper binding capacity of the Bacillus subtilis version of SCO (i.e., BsSCO) suggests a direct role for SCO proteins in copper transfer. We have characterized redox and copper exchange properties of apo- and metalated-BsSCO. The release of copper (II) from its complex with BsSCO is best achieved by reducing it to Cu(I). We propose a mechanism involving both disulfide and copper exchange between BsSCO and the apo-Cu(A) site. This article is part of a Special Issue entitled: Biogenesis/Assembly of Respiratory Enzyme Complexes.

Superoxide dismutases: role in redox signaling, vascular function, and diseases

Excessive reactive oxygen species Revised abstract, especially superoxide anion (O₂•-), play important roles in the pathogenesis of many cardiovascular diseases, including hypertension and atherosclerosis. Superoxide dismutases (SODs) are the major antioxidant defense systems against (O₂•-), which consist of three isoforms of SOD in mammals: the cytoplasmic Cu/ZnSOD (SOD1), the mitochondrial MnSOD (SOD2), and the extracellular Cu/ZnSOD (SOD3), all of which require catalytic metal (Cu or Mn) for their activation. Recent evidence suggests that in each subcellular location, SODs catalyze the conversion of (O₂•-), H2O2, which may participate in cell signaling. In addition, SODs play a critical role in inhibiting oxidative inactivation of nitric oxide, thereby preventing peroxynitrite formation and endothelial and mitochondrial dysfunction. The importance of each SOD isoform is further illustrated by studies from the use of genetically altered mice and viral-mediated gene transfer. Given the essential role of SODs in cardiovascular disease, the concept of antioxidant therapies, that is, reinforcement of endogenous antioxidant defenses to more effectively protect against oxidative stress, is of substantial interest. However, the clinical evidence remains controversial. In this review, we will update the role of each SOD in vascular biologies, physiologies, and pathophysiologies such as atherosclerosis, hypertension, and angiogenesis. Because of the importance of metal cofactors in the activity of SODs, we will also discuss how each SOD obtains catalytic metal in the active sites. Finally, we will discuss the development of future SOD-dependent therapeutic strategies.

Designed to dissolve: suppression of colloidal aggregation of Cu(I)-selective fluorescent probes in aqueous buffer and in-gel detection of a metallochaperone

Due to the lipophilicity of the metal-ion receptor, previously reported Cu(I)-selective fluorescent probes form colloidal aggregates, as revealed by dynamic light scattering. To address this problem, we have developed a hydrophilic triarylpyrazoline-based fluorescent probe, CTAP-2, that dissolves directly in water and shows a rapid, reversible, and highly selective 65-fold fluorescence turn-on response to Cu(I) in aqueous solution. CTAP-2 proved to be sufficiently sensitive for direct in-gel detection of Cu(I) bound to the metallochaperone Atox1, demonstrating the potential for cation-selective fluorescent probes to serve as tools in metalloproteomics for identifying proteins with readily accessible metal-binding sites.

Thermodynamics of copper and zinc distribution in the cyanobacterium Synechocystis PCC 6803

Copper is supplied to plastocyanin for photosynthesis and cytochrome c oxidase for respiration in the thylakoids of Synechocystis PCC 6803 by the membrane-bound P-type ATPases CtaA and PacS, and the metallochaperone Atx1. We have determined the Cu(I) affinities of all of the soluble proteins and domains in this pathway. The Cu(I) affinities of the trafficking proteins range from 5 × 10(16) to 5 × 10(17) M(-1) at pH 7.0, consistent with values for homologues. Unusually, Atx1 binds Cu(I) significantly tighter than the metal-binding domains (MBDs) of CtaA and PacS (CtaA(N) and PacS(N)), and equilibrium copper exchange constants of approximately 0.2 are obtained for transfer to the MBDs. Dimerization of Atx1 increases the affinity for Cu(I), but the loop 5 His61 residue has little influence. The MBD of the zinc exporter ZiaA (ZiaA(N)) exhibits an almost identical Cu(I) affinity, and Cu(I) exchange with Atx1, as CtaA(N) and PacS(N), and the relative stabilities of the complexes must enable the metallochaperone to distinguish between the MBDs. The binding of potentially competing zinc to the trafficking proteins has been studied. ZiaA(N) has the highest Zn(II) affinity and thermodynamics could be important for zinc removal from the cell. Plastocyanin has a Cu(I) affinity of 2.6 × 10(17) M(-1), 15-fold tighter than that of the Cu(A) site of cytochrome c oxidase, highlighting the need for specific mechanisms to ensure copper delivery to both of these targets. The narrow range of Cu(I) affinities for the cytoplasmic copper proteins in Synechocystis will facilitate relocation when copper is limiting.

Determination of copper(II) ion concentration by lifetime measurements of green fluorescent protein

The understanding of cellular processes and functions and the elucidation of their physiological mechanisms is an important aim in the life sciences. One important aspect is the uptake and the release of essential substances as well as their interactions with the cellular environment. As green fluorescent protein (GFP) can be genetically encoded in cells it can be used as an internal sensor giving a deeper insight into biochemical pathways. Here we report that the presence of copper(II) ions leads to a decrease of the fluorescence lifetime (τ(fl)) of GFP and provide evidence for Förster resonance energy transfer (FRET) as the responsible quenching mechanism. We identify the His(6)-tag as the responsible binding site for Cu(2+) with a dissociation constant K(d) = 9 ± 2 μM and a Förster radius R(0) = 2.1 ± 0.1 nm. The extent of the lifetime quenching depends on [Cu(2+)] which is comprehended by a mathematical titration model. We envision that Cu(2+) can be quantified noninvasively and in real-time by measuring τ(fl) of GFP.

Chloroplastic and mitochondrial metal homeostasis

Transition metal deficiency has a strong impact on the growth and survival of an organism. Indeed, transition metals, such as iron, copper, manganese and zinc, constitute essential cofactors for many key cellular functions. Both photosynthesis and respiration rely on metal cofactor-mediated electron transport chains. Chloroplasts and mitochondria are, therefore, organelles with high metal ion demand and represent essential components of the metal homeostasis network in photosynthetic cells. In this review, we describe the metal requirements of chloroplasts and mitochondria, the acclimation of their functions to metal deficiency and recent advances in our understanding of their contributions to cellular metal homeostasis, the control of the cellular redox status and the synthesis of metal cofactors.

A targetable fluorescent sensor reveals that copper-deficient SCO1 and SCO2 patient cells prioritize mitochondrial copper homeostasis

We present the design, synthesis, spectroscopy, and biological applications of Mitochondrial Coppersensor-1 (Mito-CS1), a new type of targetable fluorescent sensor for imaging exchangeable mitochondrial copper pools in living cells. Mito-CS1 is a bifunctional reporter that combines a Cu(+)-responsive fluorescent platform with a mitochondrial-targeting triphenylphosphonium moiety for localizing the probe to this organelle. Molecular imaging with Mito-CS1 establishes that this new chemical tool can detect changes in labile mitochondrial Cu(+) in a model HEK 293T cell line as well as in human fibroblasts. Moreover, we utilized Mito-CS1 in a combined imaging and biochemical study in fibroblasts derived from patients with mutations in the two synthesis of cytochrome c oxidase 1 and 2 proteins (SCO1 and SCO2), each of which is required for assembly and metalation of functionally active cytochrome c oxidase (COX). Interestingly, we observe that although defects in these mitochondrial metallochaperones lead to a global copper deficiency at the whole cell level, total copper and exchangeable mitochondrial Cu(+) pools in SCO1 and SCO2 patient fibroblasts are largely unaltered relative to wild-type controls. Our findings reveal that the cell maintains copper homeostasis in mitochondria even in situations of copper deficiency and mitochondrial metallochaperone malfunction, illustrating the importance of regulating copper stores in this energy-producing organelle.

The combined actions of the copper-responsive repressor CsoR and copper-metallochaperone CopZ modulate CopA-mediated copper efflux in the intracellular pathogen Listeria monocytogenes

We have characterized the csoR-copA-copZ copper resistance operon of the important human intracellular pathogen Listeria monocytogenes. Transcription of the operon is specifically induced by copper, and mutants lacking the P₁-type ATPase CopA have reduced copper tolerance and over-accumulate copper relative to wild type. The copper-responsive repressor CsoR autoregulates transcription by binding to a single 32 bp site spanning the -10 and -35 elements of the promoter. Copper co-ordination by CsoR derepresses transcription of the operon and alters CsoR:DNA complex assembly as determined by DNase I footprinting and electrophoretic mobility shift assays, with some DNA-binding capacity being retained in the presence of 2 mole equivalents of copper. Analysis of the CsoR copper sensory site demonstrated that substitution of Cys⁴² with Ala generated a CsoR variant that was unresponsive to copper. Importantly, in the absence of CopZ, copper responsiveness of csoR-copA-copZ expression is substantially increased, implying that CopZ reduces the access of CsoR to copper. Furthermore, CopZ is shown to confer copper resistance in mutants lacking copper-inducible csoR-copA-copZ expression, thus providing protection from the deleterious effects of copper within the cytoplasm.

ATP7A-related copper transport diseases-emerging concepts and future trends

This Review summarizes recent advances in understanding copper-transporting ATPase 1 (ATP7A), and examines the neurological phenotypes associated with dysfunction of this protein. Involvement of ATP7A in axonal outgrowth, synapse integrity and neuronal activation underscores the fundamental importance of copper metabolism to neurological function. Defects in ATP7A cause Menkes disease, an infantile-onset, lethal condition. Neonatal diagnosis and early treatment with copper injections enhance survival in patients with this disease, and can normalize clinical outcomes if mutant ATP7A molecules retain small amounts of residual activity. Gene replacement rescues a mouse model of Menkes disease, suggesting a potential therapeutic approach for patients with complete loss-of-function ATP7A mutations. Remarkably, a newly discovered ATP7A disorder-isolated distal motor neuropathy-has none of the characteristic clinical or biochemical abnormalities of Menkes disease or its milder allelic variant occipital horn syndrome (OHS), instead resembling Charcot-Marie-Tooth disease type 2. These findings indicate that ATP7A has a crucial but previously unappreciated role in motor neuron maintenance, and that the mechanism underlying ATP7A-related distal motor neuropathy is distinct from Menkes disease and OHS pathophysiology. Collectively, these insights refine our knowledge of the neurology of ATP7A-related copper transport diseases and pave the way for further progress in understanding ATP7A function.

Transition metal homeostasis: from yeast to human disease

Transition metal ions are essential nutrients to all forms of life. Iron, copper, zinc, manganese, cobalt and nickel all have unique chemical and physical properties that make them attractive molecules for use in biological systems. Many of these same properties that allow these metals to provide essential biochemical activities and structural motifs to a multitude of proteins including enzymes and other cellular constituents also lead to a potential for cytotoxicity. Organisms have been required to evolve a number of systems for the efficient uptake, intracellular transport, protein loading and storage of metal ions to ensure that the needs of the cells can be met while minimizing the associated toxic effects. Disruptions in the cellular systems for handling transition metals are observed as a number of diseases ranging from hemochromatosis and anemias to neurodegenerative disorders including Alzheimer's and Parkinson's disease. The yeast Saccharomyces cerevisiae has proved useful as a model organism for the investigation of these processes and many of the genes and biological systems that function in yeast metal homeostasis are conserved throughout eukaryotes to humans. This review focuses on the biological roles of iron, copper, zinc, manganese, nickel and cobalt, the homeostatic mechanisms that function in S. cerevisiae and the human diseases in which these metals have been implicated.