Structure and metal loading of a soluble periplasm cuproprotein

Contributions of polysaccharides to arsenate resistance in Chlamydomonas reinhardtii

Polysaccharides supply energy for various metabolic processes in cells. However, their roles in the arsenate (As(V)) resistance in microalgae remain largely unknown. Here, we explored the synthesis and transformation of polysaccharides in Chlamydomonas reinhardtii upon various levels of As(V) stress, using a number of physiological indexes along with transmission electron microscopic (TEM) and proteomic analyses. When exposed to low concentration of As(V) (0-20 μg/L), C. reinhardtii accumulated starch and produced more extracellular polysaccharides. At 50 μg/L As(V) treatment, starch accumulation gradually shifted to polysaccharides decomposition in the algal cells. Under higher As(V) concentration (500 μg/L), significantly more proteins in fatty acid metabolic pathway were differentially expressed, indicating that cells redirected carbon flux and transformed lipids into polysaccharides. The findings of this study demonstrate that polysaccharides may be critically involved in the As(V) resistance of C. reinhardtii.

Synthetic biology approaches to copper remediation: bioleaching, accumulation and recycling

One of the current aims of synthetic biology is the development of novel microorganisms that can mine economically important elements from the environment or remediate toxic waste compounds. Copper, in particular, is a high-priority target for bioremediation owing to its extensive use in the food, metal and electronic industries and its resulting common presence as an environmental pollutant. Even though microbe-aided copper biomining is a mature technology, its application to waste treatment and remediation of contaminated sites still requires further research and development. Crucially, any engineered copper-remediating chassis must survive in copper-rich environments and adapt to copper toxicity; they also require bespoke adaptations to specifically extract copper and safely accumulate it as a human-recoverable deposit to enable biorecycling. Here, we review current strategies in copper bioremediation, biomining and biorecycling, as well as strategies that extant bacteria use to enhance copper tolerance, accumulation and mineralization in the native environment. By describing the existing toolbox of copper homeostasis proteins from naturally occurring bacteria, we show how these modular systems can be exploited through synthetic biology to enhance the properties of engineered microbes for biotechnological copper recovery applications.

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

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

Silver in biology and medicine: opportunities for metallomics researchers

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

A cytosolic copper storage protein provides a second level of copper tolerance in Streptomyces lividans

Streptomyces lividans has a distinct dependence on the bioavailability of copper for its morphological development. A cytosolic copper resistance system is operative in S. lividans that serves to preclude deleterious copper levels. This system comprises of several CopZ-like copper chaperones and P₁-type ATPases, predominantly under the transcriptional control of a metalloregulator from the copper sensitive operon repressor (CsoR) family. In the present study, we discover a new layer of cytosolic copper resistance in S. lividans that involves a protein belonging to the newly discovered family of copper storage proteins, which we have named Ccsp (cytosolic copper storage protein). From an evolutionary perspective, we find Ccsp homologues to be widespread in Bacteria and extend through into Archaea and Eukaryota. Under copper stress Ccsp is upregulated and consists of a homotetramer assembly capable of binding up to 80 cuprous ions (20 per protomer). X-ray crystallography reveals 18 cysteines, 3 histidines and 1 aspartate are involved in cuprous ion coordination. Loading of cuprous ions to Ccsp is a cooperative process with a Hill coefficient of 1.9 and a CopZ-like copper chaperone can transfer copper to Ccsp. A Δccsp mutant strain indicates that Ccsp is not required under initial copper stress in S. lividans, but as the CsoR/CopZ/ATPase efflux system becomes saturated, Ccsp facilitates a second level of copper tolerance.

Handling of nutrient copper in the bacterial envelope

The insertion of copper into bacterial cuproenzymesin vivodoes not always require a copper-binding metallochaperone – why?

Bacterial copper storage proteins

Copper is essential for most organisms as a cofactor for key enzymes involved in fundamental processes such as respiration and photosynthesis. However, copper also has toxic effects in cells, which is why eukaryotes and prokaryotes have evolved mechanisms for safe copper handling. A new family of bacterial proteins uses a Cys-rich four-helix bundle to safely store large quantities of Cu(I). The work leading to the discovery of these proteins, their properties and physiological functions, and how their presence potentially impacts the current views of bacterial copper handling and use are discussed in this review.

Copper and Antibiotics: Discovery, Modes of Action, and Opportunities for Medicinal Applications

Copper is a ubiquitous element in the environment as well as living organisms, with its redox capabilities and complexation potential making it indispensable for many cellular functions. However, these same properties can be highly detrimental to prokaryotes and eukaryotes when not properly controlled, damaging many biomolecules including DNA, lipids, and proteins. To restrict free copper concentrations, all bacteria have developed mechanisms of resistance, sequestering and effluxing labile copper to minimize its deleterious effects. This weakness is actively exploited by phagocytes, which utilize a copper burst to destroy pathogens. Though administration of free copper is an unreasonable therapeutic antimicrobial itself, due to insufficient selectivity between host and pathogen, small-molecule ligands may provide an opportunity for therapeutic mimicry of the immune system. By modulating cellular entry, complex stability, resistance evasion, and target selectivity, ligand/metal coordination complexes can synergistically result in high levels of antibacterial activity. Several established therapeutic drugs, such as disulfiram and pyrithione, display remarkable copper-dependent inhibitory activity. These findings have led to development of new drug discovery techniques, using copper ions as the focal point. High-throughput screens for copper-dependent inhibitors against Mycobacterium tuberculosis and Staphylococcus aureus uncovered several new compounds, including a new class of inhibitors, the NNSNs. In this review, we highlight the microbial biology of copper, its antibacterial activities, and mechanisms to discover new inhibitors that synergize with copper.

c-Type Cytochrome Assembly Is a Key Target of Copper Toxicity within the Bacterial Periplasm

In the absence of a tight control of copper entrance into cells, bacteria have evolved different systems to control copper concentration within the cytoplasm and the periplasm. Central to these systems, the Cu⁺ ATPase CopA plays a major role in copper tolerance and translocates copper from the cytoplasm to the periplasm. The fate of copper in the periplasm varies among species. Copper can be sequestered, oxidized, or released outside the cells. Here we describe the identification of CopI, a periplasmic protein present in many proteobacteria, and show its requirement for copper tolerance in Rubrivivax gelatinosus. The ΔcopI mutant is more susceptible to copper than the Cu⁺ ATPase copA mutant. CopI is induced by copper, localized in the periplasm and could bind copper. Interestingly, copper affects cytochrome c membrane complexes (cbb₃ oxidase and photosystem) in both ΔcopI and copA-null mutants, but the causes are different. In the copA mutant, heme and chlorophyll synthesis are affected, whereas in ΔcopI mutant, the decrease is a consequence of impaired cytochrome c assembly. This impact on c-type cytochromes would contribute also to the copper toxicity in the periplasm of the wild-type cells when they are exposed to high copper concentrations.

Copper is an essential cation required as a cofactor in enzymes involved in vital processes such as respiration, photosynthesis, free radical scavenging, and pathogenesis. However, copper is highly toxic and has been implicated in disorders in all organisms, including humans, because it can catalyze the production of toxic reactive oxygen species and targets various biosynthesis pathways. Identifying copper targets, provides insights into copper toxicity and homeostatic mechanisms for copper tolerance. In this work, we describe for the first time a direct effect of excess copper on cytochrome c assembly. We show that excess copper specifically affects periplasmic and membrane cytochromes c, thus suggesting that the copper toxicity targets c-type cytochrome biogenesis.

Metals in cyanobacteria: analysis of the copper, nickel, cobalt and arsenic homeostasis mechanisms

Traces of metal are required for fundamental biochemical processes, such as photosynthesis and respiration. Cyanobacteria metal homeostasis acquires an important role because the photosynthetic machinery imposes a high demand for metals, making them a limiting factor for cyanobacteria, especially in the open oceans. On the other hand, in the last two centuries, the metal concentrations in marine environments and lake sediments have increased as a result of several industrial activities. In all cases, cells have to tightly regulate uptake to maintain their intracellular concentrations below toxic levels. Mechanisms to obtain metal under limiting conditions and to protect cells from an excess of metals are present in cyanobacteria. Understanding metal homeostasis in cyanobacteria and the proteins involved will help to evaluate the use of these microorganisms in metal bioremediation. Furthermore, it will also help to understand how metal availability impacts primary production in the oceans. In this review, we will focus on copper, nickel, cobalt and arsenic (a toxic metalloid) metabolism, which has been mainly analyzed in model cyanobacterium Synechocystis sp. PCC 6803.

Metal preferences and metallation

The metal binding preferences of most metalloproteins do not match their metal requirements. Thus, metallation of an estimated 30% of metalloenzymes is aided by metal delivery systems, with ∼ 25% acquiring preassembled metal cofactors. The remaining ∼ 70% are presumed to compete for metals from buffered metal pools. Metallation is further aided by maintaining the relative concentrations of these pools as an inverse function of the stabilities of the respective metal complexes. For example, magnesium enzymes always prefer to bind zinc, and these metals dominate the metalloenzymes without metal delivery systems. Therefore, the buffered concentration of zinc is held at least a million-fold below magnesium inside most cells.

Structural and mechanistic insights into an extracytoplasmic copper trafficking pathway in Streptomyces lividans

In Streptomyces lividans an extracytoplasmic copper-binding Sco protein plays a role in two unlinked processes: (i) initiating a morphological development switch and (ii) facilitating the co-factoring of the CuA domain of CcO (cytochrome c oxidase). How Sco obtains copper once secreted to the extracytoplasmic environment is unknown. In the present paper we report on a protein possessing an HX₆MX₂₁HXM motif that binds a single cuprous ion with subfemtomolar affinity. High-resolution X-ray structures of this extracytoplasmic copper chaperone-like protein (ECuC) in the apo- and Cu(I)-bound states reveal that the latter possesses a surface-accessible cuprous-ion-binding site located in a dish-shaped region of β-sheet structure. A cuprous ion is transferred under a favourable thermodynamic gradient from ECuC to Sco with no back transfer occurring. The ionization properties of the cysteine residues in the Cys⁸⁶xxxCys⁹⁰ copper-binding motif of Sco, together with their positional locations identified from an X-ray structure of Sco, suggests a role for Cys⁸⁶ in initiating an inter-complex ligand-exchange reaction with Cu(I)-ECuC. Generation of the genetic knockouts, Δsco, Δecuc and Δsco/ecuc, and subsequent in vivo assays lend support to the existence of a branched extracytoplasmic copper-trafficking pathway in S. lividans. One branch requires both Sco and to a certain extent ECuC to cofactor the CuA domain, whereas the other uses only Sco to deliver copper to a cuproenzyme to initiate morphological development.

Copper-transporting P-type ATPases use a unique ion-release pathway

Heavy metals in cells are typically regulated by PIB-type ATPases. The first structure of the class, a Cu(+)-ATPase from Legionella pneumophila (LpCopA), outlined a copper transport pathway across the membrane, which was inferred to be occluded. Here we show by molecular dynamics simulations that extracellular water solvated the transmembrane (TM) domain, results indicative of a Cu(+)-release pathway. Furthermore, a new LpCopA crystal structure determined at 2.8-Å resolution, trapped in the preceding E2P state, delineated the same passage, and site-directed-mutagenesis activity assays support a functional role for the conduit. The structural similarities between the TM domains of the two conformations suggest that Cu(+)-ATPases couple dephosphorylation and ion extrusion differently than do the well-characterized PII-type ATPases. The ion pathway explains why certain Menkes' and Wilson's disease mutations impair protein function and points to a site for inhibitors targeting pathogens.

Morphological development and cytochrome c oxidase activity in Streptomyces lividans are dependent on the action of a copper bound Sco protein

Copper has an important role in the life cycle of many streptomycetes, stimulating the developmental switch between vegetative mycelium and aerial hyphae concomitant with the production of antibiotics. In streptomycetes, a gene encoding for a putative Sco-like protein has been identified and is part of an operon that contains two other genes predicted to handle cellular copper. We report on the Sco-like protein from Streptomyces lividans (Sco^Sl) and present a series of experiments that firmly establish a role for Sco^Sl as a copper metallochaperone as opposed to a role as a thiol-disulphide reductase that has been assigned to other bacterial Sco proteins. Under low copper concentrations, a Δsco mutant in S. lividans displays two phenotypes; the development switch between vegetative mycelium and aerial hyphae stalls and cytochrome c oxidase (CcO) activity is significantly decreased. At elevated copper levels, the development and CcO activity in the Δsco mutant are restored to wild-type levels and are thus independent of Sco^Sl. A CcO knockout reveals that morphological development is independent of CcO activity leading us to suggest that Sco^Sl has at least two targets in S. lividans. We establish that one Sco^Sl target is the dinuclear Cu_A domain of CcO and it is the cupric form of Sco^Sl that is functionally active. The mechanism of cupric ion capture by Sco^Sl has been investigated, and an important role for a conserved His residue is identified.

A fresh view of the cell biology of copper in enterobacteria

Copper ions are essential but also very toxic. Copper resistance in bacteria is based on export of the toxic ion, oxidation from Cu(I) to Cu(II), and sequestration by copper-binding metal chaperones, which deliver copper ions to efflux systems or metal-binding sites of copper-requiring proteins. In their publication in this issue, Osman et al. (2013) demonstrate how tightly copper resistance, homeostasis and delivery pathways are interwoven in Salmonella enterica sv. Typhimurium. Copper is transported from the cytoplasm by the two P-type ATPases CopA and GolT to the periplasm and transferred to SodCII by CueP, a periplasmic copper chaperone. When copper levels are higher, SodCII is also able to bind copper without the help of CueP. This scheme raises the question as to why copper ions present in the growth medium have to make the detour through the cytoplasm. The data presented in the publication by Osman et al. (2013) change our view of the cell biology of copper in enterobacteria.

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.

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.

Metal sensing in Salmonella: implications for pathogenesis

Both the essentiality and toxicity of transition metals are exploited as part of mammalian immune defenses against bacterial infection. Salmonella serovars continue to cause serious medical and veterinary problems worldwide and detecting deficiency and excess of different metal ions (such as copper, iron, zinc, manganese, nickel, and cobalt) is fundamental to their virulence. This involves multiple DNA-binding metal-responsive transcription factors that discriminate between elements and trigger expression of genes that mediate appropriate responses to metal fluxes. This review focuses on the metal stresses encountered by Salmonella during infection and the roles of the different metal-sensing regulatory proteins and their target genes in adapting to these changing metal levels. Current knowledge regarding the mechanisms of metal-regulated gene expression and the structural features of sensory metal binding sites are described. In addition, the principles governing the ability of the different sensors to detect specific metals within a cell to control cytosolic metal levels are also discussed. These proteins represent potential targets for the development of new therapeutic approaches.