Coordination of Zn2+ (and Cd2+) by prokaryotic metallothionein. Involvement of his-imidazole

Heavy metals assessment in water, soil, vegetables and their associated health risks via consumption of vegetables, District Kasur, Pakistan

The consumption of contaminated vegetables has a great impact on human health. Due to this fact, we conduct the study to estimate the heavy metals in groundwater, soil, and vegetables by using the atomic adsorption spectroscopy (AAS) and find out the health risk using THQ and TCR caused by using these vegetables. The mean concentrations of As (0.015–0.40 mg/L), Cd (0.02–0.029 mg/L), Co (0.31–0.38 mg/L), Cr (1.02–1.09 mg/L), Cu (2.14–2.17 mg/L), and Hg (0.01–0.04 mg/L) are high in groundwater from threshold values given by WHO. The mean concentrations of As (22.17–23.14 mg/kg), Cd (4.21–4.54 mg/kg), Cu (21.24–24.36 mg/kg), and Pb (32.12–33.48 mg/kg) are high in soil samples from threshold values given by WHO. The mean concentrations of As, Cd, Pb, Cr, Fe, Hg, and Mn values exceeded the recommended values with concentration ranges: 1.75–4.56, 0.41–0.67, 2.12–3.12, 1.44–4.56, 87.12–135.25, 2.09–2.64, and 33.41–129.32 mg/Kg, respectively. The vegetable sample’s average concentration of heavy metals was in decreasing order cabbage ˃ brinjal ˃ okra ˃ tomato. The EDI values for As, Co, and Hg calculated for both adults and children is high. The target hazard quotients (THQ) for As, Co, and Hg are greater than the threshold value by consuming vegetables, which indicated the health risk for both adults and children. Similarly, HI due to tomato, cabbage, okra, and brinjal’s consumption is ˃ 1, with HI values 8.1975, 15.3077, 8.7312, and 10.2306, respectively. This advised the possible health effect in this area by using these vegetables. Target Cancer risk (TCR) exposed the adverse cancer risk persuaded by As, Cr, and Hg as their values exceeded the normal range by USEPA by consumption of these vegetables. This study concluded that vegetables imply the total health risk on local people, and regular monitoring of heavy metals is strongly suggested in this region.

Diversity, structure and regulation of microbial metallothionein: metal resistance and possible applications in sequestration of toxic metals

Metallothioneins (MTs) are a group of cysteine-rich, universal, low molecular weight proteins distributed widely in almost all major taxonomic groups ranging from tiny microbes to highly organized vertebrates. The primary function of this protein is storage, transportation and binding of metals, which enable microorganisms to detoxify heavy metals. In the microbial world, these peptides were first identified in a cyanobacterium Synechococcus as the SmtA protein which exhibits high affinity towards rising level of zinc and cadmium to preserve metal homeostasis in a cell. In yeast, MTs aid in reserving copper and confer protection against copper toxicity by chelating excess copper ions in a cell. Two MTs, CUP1 and Crs5, originating from Saccharomyces cerevisiae predominantly bind to copper though are capable of binding with zinc and cadmium ions. MT superfamily 7 is found in ciliated protozoa which show high affinity towards copper and cadmium. Several tools and techniques, such as western blot, capillary electrophoresis, inductively coupled plasma, atomic emission spectroscopy and high performance liquid chromatography, have been extensively utilized for the detection and quantification of microbial MTs which are utilized for the efficient remediation and sequestration of heavy metals from a contaminated environment.

Selenium cytotoxicity in Tetrahymena thermophila: New clues about its biological effects and cellular resistance mechanisms

Selenium is an essential micronutrient but at high concentrations can produce severe cytotoxicity and genomic damage. We have evaluated the cytotoxicity, ultrastructural and mitochondrial alterations of the two main selenium inorganic species; selenite and selenate, in the eukaryotic microorganism Tetrahymena thermophila. In this ciliate, selenite is more toxic than selenate. Their LC₅₀ values were calculated as 27.65 μM for Se(IV) and 56.88 mM for Se(VI). Significant levels of peroxides/hydroperoxides are induced under low-moderate selenite or selenate concentrations. Se(VI) exposures induce an immediate mitochondrial membrane depolarization. Selenium treated cells show an intense vacuolization and some of them present numerous discrete and small electrondense particles, probably selenium deposits. Mitochondrial fusion, an intense swelling in peripheral mitochondria and mitophagy are detected in selenium treated cells, especially in those exposed to Se (IV). qRT-PCR analysis of diverse genes, encoding relevant antioxidant enzymes or other proteins, like metallothioneins, involved in an environmental general stress response, have shown that they may be crucial against Se(IV) and/or Se (VI) cytotoxicity.

Responses exhibited by various microbial groups relevant to uranium exposure

There is a strong interest in knowing how various microbial systems respond to the presence of uranium (U), largely in the context of bioremediation. There is no known biological role for uranium so far. Uranium is naturally present in rocks and minerals. The insoluble nature of the U(IV) minerals keeps uranium firmly bound in the earth's crust minimizing its bioavailability. However, anthropogenic nuclear reaction processes over the last few decades have resulted in introduction of uranium into the environment in soluble and toxic forms. Microbes adsorb, accumulate, reduce, oxidize, possibly respire, mineralize and precipitate uranium. This review focuses on the microbial responses to uranium exposure which allows the alteration of the forms and concentrations of uranium within the cell and in the local environment. Detailed information on the three major bioprocesses namely, biosorption, bioprecipitation and bioreduction exhibited by the microbes belonging to various groups and subgroups of bacteria, fungi and algae is provided in this review elucidating their intrinsic and engineered abilities for uranium removal. The survey also highlights the instances of the field trials undertaken for in situ uranium bioremediation. Advances in genomics and proteomics approaches providing the information on the regulatory and physiologically important determinants in the microbes in response to uranium challenge have been catalogued here. Recent developments in metagenomics and metaproteomics indicating the ecologically relevant traits required for the adaptation and survival of environmental microbes residing in uranium contaminated sites are also included. A comprehensive understanding of the microbial responses to uranium can facilitate the development of in situ U bioremediation strategies.

Extreme metal adapted, knockout and knockdown strains reveal a coordinated gene expression among different Tetrahymena thermophila metallothionein isoforms

Metallothioneins (MT) constitute a superfamily of small cytosolic proteins that are able to bind metal cations through numerous cysteine (Cys) residues. Like other organisms the ciliate Tetrahymena thermophila presents several MT isoforms, which have been classified into two subfamilies (Cd- and Cu-metallothioneins). The main aim of this study was to examine the specific functions and transcriptional regulation of the five MT isoforms present in T. thermophila, by using several strains of this ciliate. After a laboratory evolution experiment over more than two years, three different T. thermophila strains adapted to extreme metal stress (Cd²⁺, Cu²⁺ or Pb²⁺) were obtained. In addition, three knockout and/or knockdown strains for different metallothionein (MT) genes were generated. These strains were then analyzed for expression of the individual MT isoforms. Our results provide a strong basis for assigning differential roles to the set of MT isoforms. MTT1 appears to have a key role in adaptation to Cd. In contrast, MTT2/4 are crucial for Cu-adaptation and MTT5 appears to be important for Pb-adaptation and might be considered as an “alarm” MT gene for responding to metal stress. Moreover, results indicate that likely a coordinated transcriptional regulation exists between the MT genes, particularly among MTT1, MTT5 and MTT2/4. MTT5 appears to be an essential gene, a first such report in any organism of an essential MT gene.

The Tetrahymena metallothionein gene family: twenty-one new cDNAs, molecular characterization, phylogenetic study and comparative analysis of the gene expression under different abiotic stressors

Background

Ciliate metallothioneins (MTs) are included in family 7 of the MT superfamily. This family has been divided into two main subfamilies: 7a or CdMTs and 7b or CuMTs. All ciliate MTs reported have been isolated from different Tetrahymena species and present unique features with regard to standard MTs. Likewise, an expression analysis has been carried out on some of MT genes under metal stress, corroborating their classification into two subfamilies.

Results

We isolated 21 new cDNAs from different Tetrahymena species to obtain a wider view of the biodiversity of these conserved genes. Structural analysis (cysteine patterns) and an updated phylogenetic study both corroborated the previous classification into two subfamilies. A new CuMT from a Tetrahymena-related species Ichthyophthirius multifiliis was also included in this general analysis. We detected a certain tendency towards the presentation of a CdMT tri-modular structure in Borealis group species with respect to Australis group. We report for the first time a semi-complete paralog duplication of a CdMT gene originating a new CdMT gene isoform in T. malaccensis. An asymmetry of the codon usage for glutamine residues was detected between Cd- and CuMTs, and the phylogenetic implications are discussed. A comparative gene expression analysis of several MT genes by qRT-PCR revealed differential behavior among them under different abiotic stressors in the same Tetrahymena species.

Conclusions

The Tetrahymena metallothionein family represents a quite conserved protein structure group with unique features with respect to standard MTs. Both Cd- and CuMT subfamilies present very defined and differentiated characteristics at several levels: cysteine patterns, modular structure, glutamine codon usage and gene expression under metal stress, among others. Gene duplication through evolution seems to be the major genetic mechanism for creating new MT gene isoforms and increasing their functional diversity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2658-6) contains supplementary material, which is available to authorized users.

Improvement of Cd(2+) uptake ability of SmtA protein by Lys/Cys mutation; experimental and theoretical studies

The improved Cd(2+) surface affinity characteristics of a mutated cyanobacterial metallothionein SmtA (K45C) were investigated via experimental and theoretical methods. Molecular dynamics simulations were carried out using a model of Cd(2+) and other ions enclosed in a fully hydrated simulation box with the wild-type or mutated SmtA protein. The theoretical results suggested that mutated SmtA was more powerful in absorption of Cd(2+) than the wild-type protein. Then, the mutated smtA gene (from Synechococcus PCC 7942) was synthesized by simplified gene synthesis method and expressed on isopropyl-beta-d-thiogalactopyranoside induction. The protein expression was investigated by SDS-PAGE and verified by Western blotting. Finally, cadmium uptake ratio of mutant protein toward wild type was analyzed by atomic absorption. This study is the first example of cytoplasmic expression of a mutant protein. Experimental results also verified that the mutation intensifies uptake of Cd(2+) ions.

Cadmium

Cadmium is not an essential element for life. It is geologically marginal but anthropogenic activities have contributed significantly to its dispersion in the environment and to cadmium exposure of living species. The natural speciation of the divalent cation Cd2+ is dominated by its high propensity to bind to sulfur ligands, but Cd2+ may also occupy sites providing imidazole and carboxylate ligands. It binds to cell walls by passive adsorption (bio-sorption) and it may interact with surface receptors. Cellular uptake can occur by ion mimicry through a variety of transporters of essential divalent cations, but not always. Once inside cells, Cd2+ preferentially binds to thiol-rich molecules. It can accumulate in intracellular vesicles. It may also be transported over long distances within multicellular organisms and be trapped in locations devoid of efficient excretion systems. These locations include the renal cortex of animals and the leaves of hyper-accumulating plants. No specific regulatory mechanism monitors Cd2+ cellular concentrations. Thiol recruitment by cadmium is a major interference mechanism with many signalling pathways that rely on thiolate-disulfide equilibria and other redox-related processes. Cadmium thus compromises the antioxidant intracellular response that relies heavily on molecules with reactive thiolates. These biochemical features dominate cadmium toxicity, which is complex because of the diversity of the biological targets and the consequent pleiotropic effects. This chapter compares the cadmium-handling systems known throughout phylogeny and highlights the basic principles underlying the impact of cadmium in biology.

Cloning and Expression of a Seed-Specific Metallothionein-Like Protein from Sesame

A cDNA clone, SiMT encoding an Ec type of metallothionein (MT)-like protein, was isolated from maturing seeds of sesame (Sesamum indicum L.), and its deduced protein sequence shared 47-65% similarity to other known Ec type of MT-like proteins with three highly conserved cysteine-rich segments. The transcript of SiMT was exclusively accumulated in maturing seeds from two weeks after flowering to the end of seed maturation. The results of a southern blot analysis suggested that one SiMT and one SiMT-like gene were present in the sesame genome. Recombinant SiMT fused with glutathione-S-transferase (GST) was over-expressed in Escherichia coli, and purified to homogeneity by affinity chromatography. Recombinant SiMT released from GST was harvested after factor Xa cleavage. Migration of the recombinant SiMT during SDS-PAGE was accelerated when its binding metal ions were depleted by EDTA. The metal-binding capability of recombinant SiMT was measured by inductively-coupled plasma atomic emission spectrometry. Our results show that the recombinant SiMT could trap zinc or copper ions, but not manganese ions, with a stoichiometric ratio (metal ion/SiMT) of approximately 2.

Insights into the interactions of cyanobacteria with uranium

Due to various activities associated with nuclear industry, uranium is migrated to aquatic environments like groundwater, ponds or oceans. Uranium forms stable carbonate complexes in the oxic waters of pH 7-10 which results in a high degree of uranium mobility. Microorganisms employ various mechanisms which significantly influence the mobility and the speciation of uranium in aquatic environments. Uranyl bioremediation studies, this far, have generally focussed on low pH conditions and related to adsorption of positively charged UO2 (2+) onto negatively charged microbial surfaces. Sequestration of anionic uranium species, i.e. [UO2(CO3) 2 (2-) ] and [UO2(CO3) 3 (4-) ] onto microbial surfaces has received only scant attention. Marine cyanobacteria are effective metal adsorbents and represent an important sink for metals in aquatic environment. This article addresses the cyanobacterial interactions with toxic metals in general while stressing on uranium. It focusses on the possible mechanisms employed by cyanobacteria to sequester uranium from aqueous solutions above circumneutral pH where negatively charged uranyl carbonate complexes dominate aqueous uranium speciation. The mechanisms demonstrated by cyanobacteria are important components of biogeochemical cycle of uranium and are useful for the development of appropriate strategies, either to recover or remediate uranium from the aquatic environments.

Lessons on the critical interplay between zinc binding and protein structure and dynamics

Zinc is one of the most important micronutrients for virtually all living organisms, and hence, it is important to understand the molecular mechanisms for its homeostasis. Besides proteins involved in transmembrane transport, both extra- and intracellular zinc-binding proteins play important roles in the respective metabolic networks. Important examples for extracellular zinc transporters are mammalian serum albumins, and for intracellular zinc handling, certain metallothioneins are of relevance. The availability of protein structures including relevant metal binding sites is a fundamental prerequisite to decipher the mechanisms that govern zinc binding dynamics in these proteins, but their determination can prove to be surprisingly challenging. Due to the spectroscopic silence of Zn(2+), combinations of biophysical techniques including electrospray ionisation mass spectrometry (ESI-MS) and multinuclear NMR, isothermal titration calorimetry (ITC) and extended X-ray absorption fine structure (EXAFS) spectroscopy, coupled with site-directed mutagenesis and molecular modelling have proven to be valuable approaches to understand not only the zinc-binding properties of metallothioneins and albumins, but also the influence of other physiologically relevant competing agents. These studies have demonstrated why the bacterial metallothionein SmtA contains a site inert towards exchange with Cd(2+), why the plant metallothionein EC from wheat is partially unfolded in the presence of Cd(2+), and how fatty acids impact on the zinc-binding ability of mammalian serum albumins.

Cd2+ complex of a triazole-based calix[4]arene conjugate as a selective fluorescent chemosensor for Cys

An N,N-Dimethylamine ethylimino-appended triazole-linked calix[4]arene conjugate, L, has been synthesized and characterized, and its Cd(2+) complex has been isolated and characterized. The structure of [CdL] was established by computational calculation using B3LYP/LANL2DZ. Time-dependent density functional theory calculations were performed to demonstrate the electronic properties of [CdL]. This highly fluorescing [CdL] has been used to recognize Cys selectively among the 20 naturally occurring amino acids. [CdL] exhibits a minimum detection limit of 58 ppb for Cys, with reusability and reversibility being imparted to the system during sensing. Thus, the sensing of Cys was well demonstrated using various techniques, viz., fluorescence, absorption, visual color change, electrospray ionization MS, (1)H NMR, and live cell imaging experiments.

Cadmium chloride-induced oxidative stress and DNA damage in the human Jurkat T cell line is not linked to intracellular trace elements depletion

Cadmium (Cd) is a widespread environmental contaminant. Cd affects the cellular homeostasis and generates damage via complex mechanisms involving interactions with other metals, induction of oxidative stress and apoptotic or necrotic cell death, depending on the cell type and the concentration. The goal of the present study was to investigate the effect of exposure to CdCl(2) on the intracellular trace elements levels, the antioxidant enzyme activities and on DNA damage in the Jurkat T cell line. Cells were exposed to 5, 25 and 50 μM of CdCl(2) for 24 h. Cd significantly reduced the viability of Jurkat T cells and induced a dose-dependent increase in DNA damage with statistically significant differences relative to controls (p<0.001); the superoxide dismutase and glutathione peroxidase activities were significantly decreased. Lipid peroxidation and protein carbonyl levels were significantly increased while glutathione and the total intracellular sulfhydryl groups were decreased showing clearly that an oxidative stress was generated by Cd. Surprisingly the treatment with Cd induced a significant increase in the intracellular levels of all the trace elements measured. The results indicate that cellular pro-oxidative stress induced by Cd is most likely mediated by disruption of redox homeostasis associated to a mishandling of redox-active transition metals and causes lipid and protein oxidation and oxidative DNA damage in Jurkat T cells.

Metallothioneins: unparalleled diversity in structures and functions for metal ion homeostasis and more

Metallothioneins have been the subject of intense study for five decades, and have greatly inspired the development of bio-analytical methodologies including multi-dimensional and multi-nuclear NMR.With further advancements in molecular biology, protein science, and instrumental techniques, recent years have seen a renaissance of research into metallothioneins. The current report focuses on in vitro studies of so-called class II metallothioneins from a variety of phyla, highlighting the diversity of metallothioneins in terms of structure, biological functions, and molecular functions such as metal ion specificity, thermodynamic stabilities, and kinetic reactivity. We are still far from being able to predict any of these properties, and further efforts will be required to generate the knowledge that will enable a better understanding of what governs the biological and chemical properties of these unusual and intriguing small proteins.

Zinc-handling in cyanobacteria: an update

Zinc is a constituent of all six classes of enzymes, plays important roles in gene regulation, and is thought to be essential for most organisms. Despite initial discoveries of cyanobacterial metallothioneins, zinc efflux pumps and uptake systems, and zinc sensors, our knowledge of the zinc requirements, uptake, and detoxification mechanisms of cyanobacteria is still limited. Although cyanobacteria occupy extremely diverse habitats, most available data pertains to freshwater species, and almost no studies of zinc-handling mechanisms have been conducted in marine species. The current report highlights what is known about zinc homeostasis in cyanobacteria, and presents an analysis of the 40 sequenced cyanobacterial genomes.

Applications of heteronuclear NMR spectroscopy in biological and medicinal inorganic chemistry

There is a wide range of potential applications of inorganic compounds, and metal coordination complexes in particular, in medicine but progress is hampered by a lack of methods to study their speciation. The biological activity of metal complexes is determined by the metal itself, its oxidation state, the types and number of coordinated ligands and their strength of binding, the geometry of the complex, redox potential and ligand exchange rates. For organic drugs a variety of readily observed spin I = 1/2 nuclei can be used (1H, 13C, 15N, 19F, 31P), but only a few metals fall into this category. Most are quadrupolar nuclei giving rise to broad lines with low detection sensitivity (for biological systems). However we show that, in some cases, heteronuclear NMR studies can provide new insights into the biological and medicinal chemistry of a range of elements and these data will stimulate further advances in this area.

Distinct characteristics of Ag+ and Cd2+ binding to CopZ from Bacillus subtilis

The chaperone CopZ together with the P-type ATPase transporter CopA constitute a copper-detoxification system in Bacillus subtilis that is commonly found in bacteria and higher cells. Previous studies of the regulation of the copZA operon showed that expression is significantly upregulated in response to elevated concentrations of environmental silver and cadmium, as well as copper. Here, we have used spectroscopic and bioanalytical methods to investigate in detail the capacity of CopZ to bind these metal ions (as Ag(+) and Cd(2+)). We demonstrate that Ag(+) binding mimics closely that of Cu(+): Ag(+)-mediated dimerisation of the protein occurs, and distinct Ag(+)-bound species are formed at higher Ag(+) loadings. Cd(2+) also binds to CopZ, but exhibits significantly different behaviour. Cd(2+)-mediated dimerisation is only observed at low loadings, such that at 0.5 and one Cd(2+) per CopZ the protein is present mainly in a monomeric form; and multinuclear higher-order forms of Cd(2+)-CopZ are not observed. Competition binding studies reveal that Ag(+) binds with an affinity very similar to that of Cu(+), while Cd(2+) binding is significantly weaker. These data provide support for the proposal that CopZ may be involved in the detoxification of silver and cadmium, in addition to copper.

Metallothioneins with unusual residues: histidines as modulators of zinc affinity and reactivity

For many years, paradigms regarding metallothioneins comprised the exclusive metal coordination by thiolates from cysteine residues and the absence of aromatic residues. As more sequence and in vitro data on metallothioneins, in particular from non-vertebrate organisms, has become available, both the occurrence of and metal coordination by histidine residues in metallothioneins is emerging as a more frequent feature than expected. We discuss the general implications of histidines versus cysteines in zinc binding sites, and review some recent results from literature and our own lab. We conclude that histidines can stabilise metallothionein clusters by reducing the overall charge, offering the ability to help with structural organisation by supplying H-bond donor and acceptor properties, reducing the likelihood for disulfide bond formation, whilst maintaining a high affinity towards metal ions, in particular the borderline zinc ion.

Histidine ligands in bacterial metallothionein enhance cluster stability

The cyanobacterial metallothionein (MT) SmtA is the prototype for bacterial MTs and protects against elevated levels of zinc. In contrast to mammalian MTs, bacterial MTs coordinate to metal ions not only via cysteine sulfurs, but unusually for MTs, also via histidine nitrogens. To investigate whether histidine coordination in these metal-sulfur clusters provides advantages over S-coordination only, we mutated the two metal-binding histidine residues in the cyanobacterial MT SmtA from Synechococcus PCC7942 to cysteines. We show that the mutant proteins are still capable of binding up to four zinc ions as is the wild-type protein. However, the mutations perturb protein folding and metal-binding dynamics. Interestingly, several homologues of SmtA also show variations in these two residues. We conclude that histidine residues in Synechococcus PCC7942 SmtA have a stabilising effect due to electrostatic interactions that impact on protein folding and metal cluster charge, and are involved in fine-tuning the reactivity of the bound metal ions.

Single and double metallothionein knockout in the nematode C. elegans reveals cadmium dependent and independent toxic effects on life history traits

The genome of the nematode Caenorhabditis elegans contains two metallothionein genes, both involved in metal homeostasis and/or detoxification. Single metallothionein knockout mutants have been created and now, for the first time, a double mutant has been isolated. Life history studies in the presence or absence of cadmium showed that all metallothionein mutants are viable. Although cadmium did not influence longevity, a dose dependent reduction in total brood size and volumetric growth was observed in wild type animals, which was magnified in single knockouts and further exacerbated in the double knockout. However, the metallothionein deletion caused two effects that are independent of cadmium exposure, namely all knockout strains displayed a reduced total brood size and the deletion of both metallothionein loci caused a significant reduction in volumetric growth. In summary, metallothionein is undoubtedly an important player in cadmium detoxification, but evidently also an important factor in cadmium independent pathways.

The Saccharomyces cerevisiae Crs5 Metallothionein metal-binding abilities and its role in the response to zinc overload

Crs5 is a Saccharomyces cerevisiae Metallothionein (MT), non-homologous to the paradigmatic Cu-thionein Cup1. Although considered a secondary copper-resistance agent, we show here that it determines survival under zinc overload in a CUP1-null background. Its overexpression prevents the deleterious effects exhibited by CUP1-CRS5-null cells when exposed to combined Zn/Cu, as it does the mouse MT1 Zn-thionein, but not Cup1. The detailed characterization of Crs5 in vivo and in vitro Zn(II)-, Cd(II)- and Cu(I)-binding abilities fully supports its resemblance to mammalian MTs. Hence, Crs5 exhibits a good divalent metal-binding ability, yielding homometallic, highly chiral and stable Zn and Cd complexes when expressed in media enriched with these metal ions. In Cu-supplemented cultures, heterometallic Zn,Cu complexes are recovered, unless aeration is kept to a minimum. These features define a Crs5 dual metal-binding behaviour that is significantly closer to Zn-thioneins than to Cu-thioneins. Protein sequence similarities fully support these findings. Overall, a Crs5 function in global metal cell homeostasis, based on its Zn-binding features, is glimpsed. The comparative evaluation of Crs5 in the framework of MT functional differentiation and evolution allows its consideration as a representative of the primeval eukaryotic forms that progressively evolved to give rise to the Zn-thionein lineage.

Construction of an additional metal-binding site in human metallothionein-2

We have constructed a new metal-binding site in the human metallothionein-2 (hMT-2), using the protein as a scaffold to investigate the structure and function of metal-binding. Potential metal-binding sites were designed within hMT-2 on the basis of structures generated by homology modeling. Amino acid residues D11, C13, C26 and S28 in the beta-domain of hMT-2 (hMT-2beta) were found, by computer search, to form a potential tetrahedral Cys4 metal-binding site. Six mutant proteins were constructed with the following amino acid substitutions: D11C, S28C and D11C/S28C in hMT-2 and the same mutations in hMT-2beta, respectively. These single-mutant and double-mutant proteins bound one gram atom of cadmium or zinc ions per gram molecule of protein more than the corresponding wild-type proteins. The circular dichroism spectra suggested that the structures of the single-mutant proteins that bound Cd or Zn were similar to that of the D11C/S28C double-mutant proteins. To evaluate the metal-binding affinity of the mutant proteins, we performed pH titrations of wild-type and mutant proteins. The stability with changes in pH of all the mutant proteins was higher than that of the wild-type proteins, and that of the double-mutant D11C/S28C protein was highest. Consequently, it appears that we were able to create novel proteins that bound metal ions at high density and with high affinity.

Differential responses of a mine tailings Pseudomonas isolate to cadmium and lead exposures

We examined cadmium and lead resistance in Pseudomonas sp. S8A, an isolate obtained from mine tailings-contaminated soil. Resistant to soluble metal concentrations up to 200 mg l(-1) cadmium and 300 mg l(-1) lead, S8A produced both exopolymer and biosurfactant. Upon growth, this pseudomonad diverged into two morphologically distinct colony subtypes; small and round or large and flat. In the presence of lead and in the no metal control the large morphotype appeared only in late stationary phase. With cadmium the large morphotype appeared immediately following exposure. Results show that the large morphotype produced greater amounts of surfactant than the small morphotype, suggesting a unique subpopulation response to cadmium toxicity. Results also indicate that an unidentified 28 kDa protein was expressed following exposure to >10 mg l(-1) cadmium. This study demonstrates new links between surfactant production, differential subpopulation response and metal exposure.

How to hide zinc in a small protein

Small cysteine-rich proteins (metallothioneins) and related domains of some large proteins (e.g., lysine methyltransferases) bind tri- and tetranuclear zinc clusters with topologies resembling fragments of Zn(II) sulfide minerals. These clusters are ubiquitous in animals, plants, and bacteria. Bacterial metallothioneins can also contain histidines as cluster ligands and embed Zn(II) with a "treble-clef"-like finger fold. This unusual embedded Zn(II) is "hidden" and surprisingly inert toward Zn or Cd exchange. Clearly, proteins can exert fine control over both the thermodynamics and kinetics of zinc binding in thiolate clusters. Genome sequences suggest that related zinc-finger sites are common in a variety of bacteria.

Arsenic binding to Fucus vesiculosus metallothionein

The seaweed Fucus vesiculosus is a member of the brown algae family. Kille and co-workers [Biochem. J. 338 (1999) 553] reported that this species contains the gene for metallothionein. Metallothionein is a metalloprotein having low molecular weight, and high cysteine content, which binds a range of metals. F. vesiculosus bioaccumulates arsenic from the aquatic environment [Mar. Chem. 18 (1986) 321]. In this paper we describe arsenic binding to F. vesiculosus metallothionein, characterized by electrospray ionization mass spectrometry. Five arsenic-MT species were detected with increasing As to protein ratios. These results provide important information about the metal-chelation behaviour of this novel algal metallothionein which is a putative model for arsenic binding to F. vesiculosus in vivo.

Escherichia coli thioredoxin inhibition by cadmium: two mutually exclusive binding sites involving Cys32 and Asp26

Observations of thioredoxin inhibition by cadmium and of a positive role for thioredoxin in protection from Cd(2+) led us to investigate the thioredoxin-cadmium interaction properties. We used calorimetric and spectroscopic methods at different pH values to explore the relative contribution of putative binding residues (Cys32, Cys35, Trp28, Trp31 and Asp26) within or near the active site. At pH 8 or 7.5 two binding sites were identified by isothermal titration calorimetry with affinity constants of 10 x 10(6) m(-1) and 1 x 10(6) m(-1). For both sites, a proton was released upon Cd(2+) binding. One mole of Cd(2+) per mole of reduced thioredoxin was measured by mass spectrometry at these pH values, demonstrating that the two binding sites were partially occupied and mutually exclusive. Cd(2+) binding at either site totally inhibited the thiol-disulfide transferase activity of Trx. The absence of Cd(2+) interaction detected for oxidized or alkylated Trx and the inhibition of the enzymatic activity of thioredoxin by Cd(2+) supported the role of Cys32 at the first site. The fluorescence profile of Cd(2+)-bound thioredoxin differed, however, from that of oxidized thioredoxin, indicating that Cd(2+) was not coordinated with Cys32 and Cys35. From FTIR spectroscopy, we inferred that the second site might involve Asp26, a buried residue that deprotonates at a rather high and unusual pK(a) for a carboxylate (7.5/9.2). The pK(a) of the two residues Cys32 and Asp26 have been shown to be interdependent [Chivers, T. P. (1997) Biochemistry36, 14985-14991]. A mechanism is proposed in which Cd(2+) binding at the solvent-accessible thiolate group of Cys32 induces a decrease of the pK(a) of Asp26 and its deprotonation. Conversely, interaction between the carboxylate group of Asp26 and Cd(2+) at a second binding site induces Cys32 deprotonation and thioredoxin inhibition, so that Cd(2+) inhibits thioredoxin activity not only by binding at the Cys32 but also by interacting with Asp26.

Zn(II) metabolism in prokaryotes

It is difficult to over-state the importance of Zn(II) in biology. It is a ubiquitous essential metal ion and plays a role in catalysis, protein structure and perhaps as a signal molecule, in organisms from all three kingdoms. Of necessity, organisms have evolved to optimise the intracellular availability of Zn(II) despite the extracellular milieu. To this end, prokaryotes contain a range of Zn(II) import, Zn(II) export and/or binding proteins, some of which utilise either ATP or the chemiosmotic potential to drive the movement of Zn(II) across the cytosolic membrane, together with proteins that facilitate the diffusion of this ion across either the outer or inner membranes of prokaryotes. This review seeks to give an overview of the systems currently classified as altering Zn(II) availability in prokaryotes.

Zn, Cu and Co in cyanobacteria: selective control of metal availability

Homeostatic systems for essential and non-essential metals create the cellular environments in which the correct metals are acquired by metalloproteins while the incorrect ones are somehow avoided. Cyanobacteria have metal requirements often absent from other bacteria; copper in thylakoidal plastocyanin, zinc in carboxysomal carbonic anhydrase, cobalt in cobalamin but magnesium in chlorophyll, molybdenum in heterocystous nitrogenase, manganese in thylakoidal water-splitting oxygen-evolving complex. This article reviews: an intracellular trafficking pathway for inward copper supply, the sequestration of surplus zinc by metallothionein (also present in other bacteria) and the detection and export of excess cobalt. We consider the influence of homeostatic proteins on selective metal availability.

Identification of a Metallothionein in Synechococcus by Capillary Electrophoresis Hyphenated with Inductively Coupled Plasma Mass Spectrometry

A home-made system hyphenating capillary electrophoresis with an inductively coupled plasma mass spectrometer (CE-ICP-MS) for cadmium speciation of protein-binding and free cadmium ions in solution is presented. The CE-ICP-MS interface consisted of an acrylic block with an internal volume ca. 20 microL in which a platinum electrode, a capillary column, and a connection to an ICP nebulizer were inserted. A make-up electrolyte solution containing 50 mmol L(-1) Tris-HCl buffer solution (pH 9.0) was continuously flowed through the interface to the ICP nebulizer. The separation of free Cd ions, Cd-cysteine, and Cd bounded to metallothionein (MT) isoforms from rabbit liver was carried out by capillary electrophoresis, and the analytes were detected by ICP-MS. The feasibility to isolate metallothionein compounds extracted from the cyanobacterium Synechococcus PCC7942 was demonstrated. The Cd binding proteins were induced in Synechococcus PCC7942 and further analyzed by CE ICP-MS.

Multiple bacteria encode metallothioneins and SmtA-like zinc fingers

Zinc is essential but toxic in excess. Bacterial metallothionein, SmtA from Synechococcus PCC 7942, sequesters and detoxifies four zinc ions per molecule and contains a zinc finger structurally similar to eukaryotic GATA. The dearth of other reported bacterial metallothioneins has been surprising. Here we describe related bacterial metallothioneins (BmtA) from Anabaena PCC 7120, Pseudomonas aeruginosa and Pseudomonas putida that bind multiple zinc ions with high stability towards protons. Thiol modification demonstrates that cysteine coordinates zinc in all of these proteins. Additionally, (111)Cd-NMR, and (111)Cd-edited (1)H-NMR, identified histidine ligands in Anabaena PCC 7120 BmtA, analogous to SmtA. A related Escherichia coli protein bound only a single zinc ion, via four cysteine residues, with low stability towards protons; (111)Cd-NMR and (111)Cd-edited (1)H-NMR confirmed exclusive cysteine-coordination, and these cysteine residues reacted rapidly with 5,5'-dithiobis-(2-nitrobenzoic acid). (1)H-NMR of proteins from P. aeruginosa, Anabaena PCC 7120 and E. coli generated fingerprints diagnostic for the GATA-like zinc finger fold of SmtA. These studies reveal first the existence of multiple bacterial metallothioneins, and second proteins with SmtA-like lone zinc fingers, devoid of a cluster,and designated GatA. We have identified 12 smtA-like genes in sequence databases including four of the gatA type.

Surplus zinc is handled by Zym1 metallothionein and Zhf endoplasmic reticulum transporter in Schizosaccharomyces pombe

Homeostatic mechanisms prevent the accumulation of free zinc in the cytoplasm, raising questions regarding where surplus zinc is stored and how it is delivered to and from these stores. A genetic screen for zinc hypersensitivity in Schizosaccharomyces pombe identified a missense mutation truncating Zhf, an endoplasmic reticulum transporter. These cells were approximately 5-fold more zinc-sensitive than other independent mutants. The targeted disruption of zhf prevented growth on low zinc medium and caused hypersensitivity to elevated zinc/cobalt but resistance to cadmium. The exposure to elevated zinc but not copper also promotes the accumulation of transcripts encoding a metallothionein designated Zym1. The Sty1 pathway is required for maximal zym1 expression but is not obligatory for zinc perception. The targeted disruption of zym1 impaired cadmium tolerance but only slightly impaired zinc tolerance, whereas zym1 overexpression substantially rescued zinc hypersensitivity of zhf(-) cells. Four equivalents of zinc were displaced from Zym1 by up to 12 equivalents of p-(hydroxymercuri)phenylsulphonate. Zym1 thiols react rapidly with 5,5'-dithiobis-(2-nitrobenzoic acid) compared with bacterial zinc metallothionein (6.8 and 0.2 x 10(-4) s(-1), respectively). Zym1 is unlike known fungal metallothioneins that are induced by and sequester copper but not zinc. Less zinc but normal cadmium was accumulated by zym1Delta, consistent with zinc sequestration by Zym1 in vivo.

Structural characterization of distinct alpha3N and alpha5 metal sites in the cyanobacterial zinc sensor SmtB

SmtB is required for Synechococcus to effect a response to toxic concentrations of Zn(II) and other heavy metals. Direct binding of inducing metal ions to SmtB transcriptionally derepresses the expression of SmtA, a prokaryotic class II metallothionein. Homodimeric SmtB binds one Zn(II) or Co(II) per monomer in a cysteine thiolate-containing site in a tetrahedral coordination geometry [VanZile, M. L., et al. (2000) Biochemistry 39, 11818-11829]. In this report, characterization of a set of cysteine substitution mutants of SmtB reveals that SmtB homodimer binds Zn(II) or Co(II) in one of two mutually exclusive metal binding sites, termed alpha3N and alpha5, with very high equilibrium affinities. Both sites are characterized by similar affinities for Co(II) (K(Co) approximately equal to 2-5 x 10(9) M(-1)), while the Zn(II) affinities are at least 20-fold different (K(Zn)(alpha)(3N) > or = 10(13) M(-1); K(Zn)(alpha)(5) approximately equal to 5 x 10(11) M(-1)). Co(II) bound exclusively at the alpha5 sites is capable of rapid equilibration between the alpha3N and alpha5 sites upon reduction of the mixed disulfides in S-methylated SmtB. These results suggest that the alpha3N or alpha5 metal sites might play distinct roles in this Zn(II)-sensing protein, systematically investigated in the following paper [VanZile, M. L., Chen, X., and Giedroc, D. P. (2002) Biochemistry 41, 9776-9786]. Since both the alpha3N and alpha5 sites are present in many members of the SmtB/ArsR family of metal sensor proteins, the presence of these two metal binding sites may explain some of the functional diversity in metal responses across this family of proteins.

Microbial metallothioneins

Bacterial metallothioneins bind, sequester and buffer excess intracellular zinc. At present, the vast majority of the available experimental data relate to cyanobacterial metallothionein, SmtA, from Synechococcus PCC 7942. SmtA is required for normal resistance to zinc and smtA-mediated zinc resistance has been used as a selectable marker. The imidazole groups of histidine residues, in addition to the thiol groups of cysteine residues, co-ordinate zinc in bacterial metallothioneins. The structure of bacterial metallothionein must facilitate some discrimination between 'adventitious' and 'adventageous' zinc-binding sites such that under excess zinc conditions metal is predominantly scavenged from the former. It remains unclear whether or not bacterial metallothionein also acts as a zinc store that supplies zinc-requiring proteins or if under some conditions it deactivates a subset of proteins via zinc removal. Expression of smtA is induced in response to elevated concentrations of zinc via the action of SmtB. SmtB has some sequence similarity to the arsenic responsive repressor ArsR and genes encoding related proteins are present in many bacterial genomes. Metal perception by SmtB differs from ArsR. The latter contains a characteristic Cys-Val-Cys motif associated with a DNA-binding helix-turn-helix (the ArsR motif), while the former contains metal-binding motifs associated with a carboxyl-terminal alpha-helix that forms the interface between SmtB dimers (the SmtB motif). Some SmtB-ArsR family proteins, including the zinc sensor ZiaR from the cyanobacterium Synechocystis PCC 6803, have the metal-sensory motifs of both SmtB and ArsR. The mechanisms of action, and the features that allow discrimination between different metal ions by these sensors, are discussed.

A metallothionein containing a zinc finger within a four-metal cluster protects a bacterium from zinc toxicity

Zinc is essential for many cellular processes, including DNA synthesis, transcription, and translation, but excess can be toxic. A zinc-induced gene, smtA, is required for normal zinc-tolerance in the cyanobacterium Synechococcus PCC 7942. Here we report that the protein SmtA contains a cleft lined with Cys-sulfur and His-imidazole ligands that binds four zinc ions in a Zn(4)Cys(9)His(2) cluster. The thiolate sulfurs of five Cys ligands provide bridges between the two ZnCys(4) and two ZnCys(3)His sites, giving two fused six-membered rings with distorted boat conformations. The inorganic core strongly resembles the Zn(4)Cys(11) cluster of mammalian metallothionein, despite different amino acid sequences, a different linear order of the ligands, and presence of histidine ligands. Also, SmtA contains elements of secondary structure not found in metallothioneins. One of the two Cys(4)-coordinated zinc ions in SmtA readily exchanges with exogenous metal ((111)Cd), whereas the other is inert. The thiolate sulfur ligands bound to zinc in this site are buried within the protein. Regions of beta-strand and alpha-helix surround the inert site to form a zinc finger resembling the zinc fingers in GATA and LIM-domain proteins. Eukaryotic zinc fingers interact specifically with other proteins or DNA and an analogous interaction can therefore be anticipated for prokaryotic zinc fingers. SmtA now provides structural proof for the existence of zinc fingers in prokaryotes, and sequences related to the zinc finger motif can be identified in several bacterial genomes.

Recombinant PhpA protein, a unique histidine motif-containing protein from Streptococcus pneumoniae, protects mice against intranasal pneumococcal challenge

The multivalent pneumococcal conjugate vaccine is effective against both systemic disease and otitis media caused by serotypes contained in the vaccine. However, serotypes not covered by the current conjugate vaccine may still cause pneumococcal disease. To address these serotypes and the remaining otitis media due to Streptococcus pneumoniae, we have been evaluating antigenically conserved proteins from S. pneumoniae as vaccine candidates. A previous report identified a 20-kDa protein with putative human complement C3-proteolytic activity. By utilizing the publicly released pneumococcal genomic sequences, we found the gene encoding the 20-kDa protein to be part of a putative open reading frame of approximately 2,400 bp. We recombinantly expressed a 79-kDa fragment (rPhpA-79) that contains a repeated HxxHxH motif and evaluated it for vaccine potential. The antibodies elicited by the purified rPhpA-79 protein were cross-reactive to proteins from multiple strains of S. pneumoniae and were against surface-exposed epitopes. Immunization with rPhpA-79 protein adjuvanted with monophosphoryl lipid A (for subcutaneous immunization) or a mutant cholera toxin, CT-E29H (for intranasal immunization), protected CBA/N mice against death and bacteremia, as well as reduced nasopharyngeal colonization, following intranasal challenge with a heterologous pneumococcal strain. In contrast, immunization with the 20-kDa portion of the PhpA protein did not protect mice. These results suggest that rPhpA-79 is a potential candidate for use as a vaccine against pneumococcal systemic disease and otitis media.

The zinc metalloregulatory protein Synechococcus PCC7942 SmtB binds a single zinc ion per monomer with high affinity in a tetrahedral coordination geometry

The Synechococcus PCC7942 SmtB is a zinc-responsive transcriptional repressor and a member of the ArsR superfamily of prokaryotic metalloregulatory transcription factors. The mechanism of negative regulation by Zn(II) and other metals as well as the coordination chemistry (stoichiometry, affinity, and specificity) of SmtB is poorly understood. In contrast to previous results [Kar, S. R., Adams, A. C., Lebowitz, J., Taylor, K. B., and Hall, L. M. (1997) Biochemistry 36, 15343-15348], we find that fully reduced SmtB binds 1 mol equiv of Zn(II) with a very high affinity, K(Zn) in excess of 10(11) M(-1) (pH 7.4, 0.15 M KCl, 22 degrees C). Optical spectroscopic experiments reveal that SmtB binds 1 mol equiv of Co(II) in a tetrahedral or distorted tetrahedral environment with one or two cysteine thiolate ligands in the first coordination shell. Zn(II) and Co(II) EXAFS studies are consistent with the optical spectroscopic data, and further suggest the presence of a mixture of carboxylate and imidazole-containing ligands. K(Co) was determined to be 1.7 (+/-0.1) x 10(9) M(-1) in a chelator (EGTA) competition assay; 1 equiv of Zn(II) results in complete displacement of the bound Co(II). SmtB also binds 1 mol equiv of Ni(II), which, when formed at low Ni(II):SmtB molar ratios, adopts a non-native, six-coordinate complex characterized by at least two histidine and no thiolate ligands. The hierarchy of metal binding affinities is Zn(II) >> Co(II) >> Ni(II).

Bacterial metal-resistance proteins and their use in biosensors for the detection of bioavailable heavy metals

We have expressed and purified metal-resistance and metal regulatory proteins from the bacterial determinants of resistance to heavy metals and utilised these in the development of biosensors for heavy metals. Both the metallothionein from the cyanobacterium Synechococcus PCC 7942 and the MerR regulatory protein from transposon Tn501 allow the detection of non-specific metal binding down to 10(-15) M concentrations of Hg(II), Cu(II), Zn(II) and Cd(II) in pure solution. Differential effects of the metals can be detected at both low and high concentrations, and the shape of the capacitance curves may reflect biologically relevant responses of the proteins to metals. Further work is required to establish the relationship between the detected binding of metal and the biological response of the protein, or to provide biosensors of use in the natural environment.

Interactions of bismuth complexes with metallothionein(II)

Bismuth complexes are widely used as anti-ulcer drugs and can significantly reduce the side effects of platinum anti-cancer drugs. Bismuth is known to induce the synthesis of metallothionein (MT) in the kidney, but there are few chemical studies on the interactions of bismuth complexes with metallothionein. Here we show that Bi(3+) binds strongly to metallothionein with a stoichiometry bismuth:MT = 7:1 (Bi(7)MT) and can readily displace Zn(2+) and Cd(2+). Bismuth is still bound to the protein even in strongly acidic solutions (pH 1). Reactions of bismuth citrate with MT are faster than those of [Bi(EDTA)](-), and both exhibit biphasic kinetics. (1)H NMR data show that Zn(2+) is displaced faster than Cd(2+), and that both Zn(2+) and Cd(2+) in the beta-domain (three metal cluster) of MT are displaced by Bi(3+) much faster than from the alpha-domain (four metal cluster). The extended x-ray absorption fine structure spectrum of Bi(7)MT is very similar to that for the glutathione and N-acetyl-L-cysteine complexes [Bi(GS)(3)] and [Bi(NAC)(3)] with an inner coordination sphere of three sulfur atoms and average Bi-S distances of 2.55 A. Some sites appear to contain additional short Bi-O bonds of 2.2 A and longer Bi-S bonds of 3.1 A. The Bi(3+) sites in Bi(7)MT are therefore highly distorted in comparison with those of Zn(2+) and Cd(2+).

Synthesis and Characterization of Pseudotetrahedral N(2)O and N(2)S Zinc(II) Complexes of Two Heteroscorpionate Ligands: Models for the Binding Sites of Several Zinc Metalloproteins

Nine new pseudotetrahedral Zn(II) complexes of the heteroscorpionate ligands (3-tert-butyl-2-hydroxy(or thio)-5-methylphenyl)bis(3,5-dimethylpyrazolyl)methane, L1OH or L2SH, have been prepared and characterized (in most cases crystallographically). Complexes isolated include [(L1O)ZnCl], [(L1O)ZnI], [(L1OH)ZnI(2)], [(L1O)ZnCH(3)], [(L1O)ZnOAc], [(L1O)ZnSPh], [(L1O)ZnSBz], [(L2S)ZnCH(3)], [(L2S)ZnSPh], and [(L2S)(2)Zn]. In conjunction with the widely studied tris(pyrazolyl)borates this new series of heteroscorpionates provides a set of isolobal and isoelectronic ligands differing in donor set, i.e. N(3), N(2)O, and N(2)S. Preliminary reactivity studies with HX species, MeI, or trimethyl phosphate suggest differences between the three sets of ligands.