Conversion in the peptides coating cadmium:sulfide crystallites in Candida glabrata

Biogenic Sulfur-Based Chalcogenide Nanocrystals: Methods of Fabrication, Mechanistic Aspects, and Bio-Applications

Bio-nanotechnology has emerged as an efficient and competitive methodology for the production of added-value nanomaterials (NMs). This review article gathers knowledge gleaned from the literature regarding the biosynthesis of sulfur-based chalcogenide nanoparticles (S-NPs), such as CdS, ZnS and PbS NPs, using various biological resources, namely bacteria, fungi including yeast, algae, plant extracts, single biomolecules, and viruses. In addition, this work sheds light onto the hypothetical mechanistic aspects, and discusses the impact of varying the experimental parameters, such as the employed bio-entity, time, pH, and biomass concentration, on the obtained S-NPs and, consequently, on their properties. Furthermore, various bio-applications of these NMs are described. Finally, key elements regarding the whole process are summed up and some hints are provided to overcome encountered bottlenecks towards the improved and scalable production of biogenic S-NPs.

Cadmium-induced formation of sulphide and cadmium sulphide particles in the aquatic hyphomycete Heliscus lugdunensis

Freshwater fungi which can survive under metal exposure receive increasing scientific attention. Enhanced synthesis of sulphide and glutathione but no phytochelatin synthesis in response to cadmium (up to 80 μM Cd(2+) in the medium) was measured in the aquatic hyphomycete Heliscus lugdunensis. Up to 25 μmol g(-1) dry mass the fungus formed sulphide in an exponentially Cd(2+)-concentration-dependent manner. Using light microscopy, precipitates were observed outside of the hyphae which could be determined as amorphous particles by X-ray diffraction (XRD). Energy dispersive X-ray spectroscopy (EDS) analysis indicated that these particles were mainly composed of Cd and S with an atomic ratio of 1:1, but some elements of the culture medium such as P and Cl were also present. Fungal cells exposed to Cd(2+) accumulated 12-28 μmol metal g(-1) dry mass over a period of 7-28 days. The results may indicate that sulphide could sequester excess Cd(2+) under oxygen deprived conditions and thereby reduce its toxicity via an additional avoidance mechanism of this fungus.

Characterization of lead induced metal-phytochelatin complexes in Chlamydomonas reinhardtii

Accumulation of Pb and induction of phytochelatin synthesis were observed in Chlamydomonas reinhardtii upon Pb(II) exposure. Our aim was to examine whether Pb(II) is bound by phytochelatins (PCs) in C. reinhardtii and to examine formed complexes for their stoichiometry and composition. Metal-phytochelatin (Me-PC) complexes induced by Pb were isolated by size-exclusion chromatography in 13 collected fractions, which were analyzed for their PC and metal content by high-performance liquid chromatography and inductively coupled plasma mass spectrometry. A recovery of more than 90% of Pb from standard Pb-PC₂ complexes within the total volume of the size-exclusion column indicated the adequacy of the method for Pb-PC(n) complex separation and characterization. Phytochelatins were detected mainly in a molecular weight ranging from 1,000 to 5,300 daltons (Da), indicating the formation of complexes with various stoichiometries. Approximately 72% of total PC₂ eluted in the range from 1,000 to 1,600 Da, and 80% of total PC₃ eluted in the molecular weight range from 1,600 to 2,300 Da. The distribution of Cu, Zn, and Pb showed that more than 70% of these metals were associated with the high-molecular-weight fractions. Copper, zinc, and lead were also observed in PC-containing fractions, suggesting the formation of various Me-PC complexes. The results of the present study indicate that the role of PCs in Pb detoxification is minor, because only 13% of total Pb was associated with PCs.

Nucleic acid and nucleotide-mediated synthesis of inorganic nanoparticles

Since the advent of practical methods for achieving DNA metallization, the use of nucleic acids as templates for the synthesis of inorganic nanoparticles (NPs) has become an active area of study. It is now widely recognized that nucleic acids have the ability to control the growth and morphology of inorganic NPs. These biopolymers are particularly appealing as templating agents as their ease of synthesis in conjunction with the possibility of screening nucleotide composition, sequence and length, provides the means to modulate the physico-chemical properties of the resulting NPs. Several synthetic procedures leading to NPs with interesting photophysical properties as well as studies aimed at rationalizing the mechanism of nucleic acid-templated NP synthesis are now being reported. This progress article will outline the current understanding of the nucleic acid-templated process and provides an up to date reference in this nascent field.

Molecular mechanisms of resistance to heavy metals in the protist Euglena gracilis

The biochemical mechanisms of resistance to several heavy metals, which are associated with their accumulation (binding by high-affinity chelating molecules such as thiol-compounds together with their compartmentalization into organelles), are analyzed for the photosynthetic, free-living protist Euglena gracilis. The complete understanding of these mechanisms may facilitate the rational design of strategies for bioremediation of heavy metal polluted water and soil systems.

Phytochelatin-cadmium-sulfide high-molecular-mass complexes of Euglena gracilis

High-molecular-mass PC complexes (PC-HMWCs) constituted by phytochelatins (PCs), cadmium and sulfide are synthesized by several organisms after exposure to cadmium. In this study, PC-HMWCs were isolated from photoheterotrophic Euglena gracilis and purified to homogeneity, resulting in compounds of molecular mass 50-380 kDa depending on the CdCl2 and sulfate concentrations in the culture medium. In contrast with plants and some yeasts, PC-HMWCs from E. gracilis mainly comprise (57-75%) monothiol molecules (Cys, gamma-glutamylcysteine, GSH) and, to a lesser extent (25-43%), PCs. A similar acid-soluble thiol compound composition was found in whole cell extracts. The -SH/Cd2+ and S2-/Cd2+ ratios found in purified PC-HMWCs were 1.5 and 1.8, respectively; the (-SH + S2-)/Cd2+ ratio was 3.2. PC-HMWCs of molecular mass 60 and 100 kDa were also localized inside Percoll-purified chloroplasts, in which cadmium and PCs were mainly compartmentalized. Cadmium and sulfur-rich clusters with similar sulfur/cadmium stoichiometries to those of the purified PC-HMWCs were detected in the chloroplast and throughout the cell by energy dispersive microanalysis and atomic resolution electron microscopy. The presence of PC-HMWCs in primitive photosynthetic eukaryotes such as the protist, E. gracilis, suggests that their function as the final cadmium-storage-inactivation process is widespread. Their particular intracellular localization suggests that chloroplasts may play a major role in the cadmium-resistance mechanism in organisms lacking a plant-like vacuole.

Simultaneous Cd2+, Zn2+, and Pb2+ uptake and accumulation by photosynthetic Euglena gracilis

The ability of Euglena gracilis to simultaneously remove and accumulate Zn2+, Cd2+, and Pb2+ from culture up- to media was evaluated. E. gracilis was able to remove up to 80% of the Cd2+ present in the medium when cultured with 20 or 50 microM CdCl2. Higher external Cd2+ concentrations increased Cd2+ accumulation per cell but decreased cell growth, thus decreasing the capacity of the cell culture to remove Cd2+. E. gracilis removed 70% to 80% of the Zn2+ present in the medium when cultured with 5 to 50 microM ZnSO4. Zn2+ did not affect Cd2+ removal capacity. E. gracilis was much less efficient in removing Pb2+ (<15%) when cultured with 100 or 200 microM Pb(NO3)2. Moreover, Pb2+ decreased the efficiency to remove Cd2+, but it did not affect Zn2+ removal. Cd2+ induced a generalized increase in the cellular thiol compounds, including phytochelatins, and Pb2+ had an additive effect only at 200 microM. Zn2+ did not stimulate phytochelatin synthesis. Cd2+ and Pb2+ colocated in the same cytosolic high-molecular-weight fraction. Because Pb2+ is a weak phytochelatin inducer, competition between Pb2+ and Cd2+ for transportation across the plasma membrane and binding to phytochelatins and other thiol compounds is proposed to explain the detrimental effects of Pb2+ on the Cd2+ removal capacity of E. gracilis.

Species-specific Cd-stress response in the white rot basidiomycetes Abortiporus biennis and Cerrena unicolor

The effect of cadmium (Cd) on fungal growth, Cd bioaccumulation and biosorption, and on the formation of potential heavy metal response indicators such as thiols, oxalate, and laccase was investigated in the white rot fungi Cerrena unicolor andAbortiporus biennis. Only the highest Cd concentration employed (200 microM) inhibited growth of C. unicolor, whereas already lower Cd concentrations caused decreasing mycelia dry weights in A. biennis. Cd biosorption onto the mycelial surface was the predominant Cd sequestration mechanism in C. unicolor. Surface-bound and bioaccumulated Cd concentrations were essentially in the same range in A. biennis, leading to considerably higher intracellular Cd concentrations in A. biennis than in C. unicolor. Oxalate and laccase were produced by both of the fungal strains and their extracellular levels were elevated upon Cd exposure. Oxalate concentrations and laccase titres were considerably higher in C. unicolor than in A. biennis. Both fungi responded to increasing Cd concentrations by increasing intracellular amounts of thiol compounds (cysteine, gamma-glutamylcysteine, glutathione in both its reduced and oxidized form) but Cd application increased the amounts of thiols to a higher extend in A. biennis. Taken together, these species-specific responses towards Cd suggest that C. unicolor possesses a more efficient system than A. biennis to keep intracellular Cd concentrations low.

Sulfur assimilation and glutathione metabolism under cadmium stress in yeast, protists and plants

Glutathione (gamma-glu-cys-gly; GSH) is usually present at high concentrations in most living cells, being the major reservoir of non-protein reduced sulfur. Because of its unique redox and nucleophilic properties, GSH serves in bio-reductive reactions as an important line of defense against reactive oxygen species, xenobiotics and heavy metals. GSH is synthesized from its constituent amino acids by two ATP-dependent reactions catalyzed by gamma-glutamylcysteine synthetase and glutathione synthetase. In yeast, these enzymes are found in the cytosol, whereas in plants they are located in the cytosol and chloroplast. In protists, their location is not well established. In turn, the sulfur assimilation pathway, which leads to cysteine biosynthesis, involves high and low affinity sulfate transporters, and the enzymes ATP sulfurylase, APS kinase, PAPS reductase or APS reductase, sulfite reductase, serine acetyl transferase, O-acetylserine/O-acetylhomoserine sulfhydrylase and, in some organisms, also cystathionine beta-synthase and cystathionine gamma-lyase. The biochemical and genetic regulation of these pathways is affected by oxidative stress, sulfur deficiency and heavy metal exposure. Cells cope with heavy metal stress using different mechanisms, such as complexation and compartmentation. One of these mechanisms in some yeast, plants and protists is the enhanced synthesis of the heavy metal-chelating molecules GSH and phytochelatins, which are formed from GSH by phytochelatin synthase (PCS) in a heavy metal-dependent reaction; Cd(2+) is the most potent activator of PCS. In this work, we review the biochemical and genetic mechanisms involved in the regulation of sulfate assimilation-reduction and GSH metabolism when yeast, plants and protists are challenged by Cd(2+).

Cadmium induces a novel metallothionein and phytochelatin 2 in an aquatic fungus

Cadmium stress response was measured at the thiol peptide level in an aquatic hyphomycete (Heliscus lugdunensis). In liquid culture, 0.1 mM cadmium increased the glutathione (GSH) content and induced the synthesis of additional thiol peptides. HPLC, electrospray ionization mass spectrometry, and Edman degradation confirmed that a novel small metallothionein as well as phytochelatin (PC2) were synthesized. The metallothionein has a high homology to family 8 metallothioneins (http://www.expasy.ch/cgi-bin/lists?metallo.txt). The bonding of at least two cadmium ions to the metallothionein was demonstrated by mass spectrometry (MALDI MS). This is the first time that simultaneous induction of metallothionein and phytochelatin accompanied by an increase in GSH level has been shown in a fungus under cadmium stress, indicating a potential function of these complexing agents for in vivo heavy metal detoxification. The method presented here should be applicable as biomarker tool.

Structure and function of metal chelators produced by plants: the case for organic acids, amino acids, phytin, and metallothioneins

Plants produce a range of ligands for cadmium (Cd), copper (Cu), nickel (Ni), and zinc (Zn). Cd- and Zn-citrate complexes are prevalent in leaves, even though malate is more abundant. In the xylem sap moving from roots to leaves, citrate and histidine are the principal ligands for Cu, Ni, and Zn. Phosphorus-rich globular bodies in young roots are probably Zn-phytate. Metallothioneins (MTs) are cysteine (Cys)-rich ligands. Plants produce class II MTs (MT-IIs) which differ from the archetypal mammalian MT-I in the location and number of Cys. The Ec protein from wheat embryos has Cys in three domains, binds Zn, and disappears with seedling development. The first 59 amino acids have been sequenced for the protein. Fifty-eight genes for MT-IIs, from a range of plants and tissues, predict proteins with Cys in two domains. Most of the predicted proteins have not been isolated, and their metal binding is poorly documented. Three protein bands, corresponding to six MT genes, have been isolated from Arabidopsis, and the amino acids sequenced for nine fragments. The MT-IIIs are atypical, nontranslationally synthesized polypeptides with variously repeating gamma-glutamylcysteine units. Of the five families known, those with carboxy-terminal glycine are the most widespread among plants, algae, and certain yeasts. A heterogeneous grouping of these molecules form Cd-binding complexes with tetrahedral coordination and a Cd-sulfur interatomic distance of 2.52 A. One complex is cytosolic, the dominant one is vacuolar. Together, they can bind a large proportion of cellular Cd; other ligands may also function. Little is known about the counterpart situation for Cu and Zn.

A role for HEM2 in cadmium tolerance

A Candida glabrata cadmium-sensitive mutant partially defective in glutathione production and exhibiting a complete absence of phytochelatins was used to clone a gene required for Cd tolerance. Transformation of the Cd-sensitive mutant with a genomic library from the wild-type C. glabrata led to the cloning of a gene that restored Cd tolerance and formation of Cd-glutathione and Cd-phytochelatin complexes. The cloned gene showed high levels of nucleic acid and protein sequence homology to the HEM2 genes, encoding porphobilinogen synthases, from several sources. It was shown that the C, glabrata Cd-sensitive mutant indeed exhibited a significant reduction in porphobilinogen synthase levels. The cloned C. glabrata gene complemented a hem2 mutant of Saccharomyces cerevisiae and restored porphobilinogen synthase activity in the mutant. The Cd sensitive mutant predictably showed decreased levels of sulfite reductase that requires siroheme, a metabolite produced in the heme biosynthetic pathway. The addition of cysteine, but not methionine, increased glutathione levels and Cd tolerance of both the wild-type and the mutant strain. However, addition of hemin chloride and methionine together restored Cd tolerance indicating that heme was required for transsulfuration of homocysteine to cysteine.

Characteristics of glutathione-capped ZnS nanocrystallites

The titration of increasing equivalents of inorganic sulfide into preformed Zn-glutathione led to the appearance of UV/VIS spectral features attributable to ZnS nanocrystallites. Glutathione-ZnS complexes upon irradiation caused reduction of methylviologen confirming their semiconductor properties. Size-fractionation of glutathione-ZnS samples on a gel filtration column showed the formation of a range of complexes whose spectral properties were correlated with the sulfide content. The stoichiometry of Zn:glutathione increased from 1:2 to a maximum of about 7:1 as the sulfide/Zn ratios increase from 0 to approximately 1.0 in Zn-glutathione complex indicating up to 14-fold increase in the Zn-binding capacity of glutathione upon sulfide incorporation. Spectral characteristics of GSH-capped ZnS nanocrystallites were significantly influenced by pH and by the stoichiometry of Zn, sulfide and glutathione in the complex. Samples containing least glutathione and highest sulfide showed maximal luminescence at pH 6, whereas those with higher glutathione and lower sulfide content showed maximal luminescence at pH 11.

Identification of SLF1 as a new copper homeostasis gene involved in copper sulfide mineralization in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, at least 12 genes are important for cells to propagate in medium containing elevated concentrations of copper salts (J. Welch, S. Fogel, C. Buchman, and M. Karin, EMBO J. 8:255-260, 1989). Complementation studies were carried out on a copper-sensitive mutation (cup14) from this group. A new yeast gene, designated SLF1, was identified as a multicopy suppressor of the cup14 mutation. Slf1 is important for the physiological process of copper sulfide (CuS) mineralization on the surface of cells cultured in medium containing copper salts. CuS mineralization causes the cells to turn brown. Disruption of SLF1, which is located close to the telomere region of chromosome IV, leads to limited copper sensitivity, and the resulting cells lack the normal brownish coloration when grown in CuSO4-containing medium. Overproduction of Slf1 in wild-type cells confers superresistance to CuSO4 and enhances the coloration of cells cultured in the presence of CuSO4. Upon addition of KCN to Cu-grown cells, the brownish coloration was bleached instantly, and copper ions were solubilized. These data are consistent with Slf1-dependent accumulation of CuS complexes on the cell surface. Disruption of SFL1 also results in loss of the ability of yeast cells to deplete Cu but not Cd ions from the growth medium, whereas overexpression enhances Ca depletion ability and the resulting deposition of CuS particles. It is proposed that Slfl participates in a copper homeostasis pathway, distinct from the Cup1 detoxification system, that leads to sulfide generation and CuS biomineralization on the cell surface. This process may coordinate with the Cup1 pathway at different copper concentrations to prevent copper-induced toxicity.

Glutathione-mediated transfer of Cu(I) into phytochelatins

Room temperature luminescence attributable to Cu(I)-thiolate clusters has been used to probe the transfer of Cu(I) from Cu(I)-glutathione complex to rabbit liver thionein-II and plant metal-binding peptides phytochelatins (gamma-Glu-Cys)2Gly, (gamma-Glu-Cys)3Gly and (gamma-Glu-Cys)4Gly. Reconstitutions were also performed using CuC1. The Cu(I)-binding stoichiometry of metallothionein or phytochelatins was generally independent of the Cu(I) donor. However, the luminescence of the reconstituted metallothionein or phytochelatins was higher when Cu(I)-GSH was the donor. This higher luminescence is presumably due to the stabilizing effect of GSH on Cu(I)-thiolate clusters. As expected, 12 Cu(I) ions were bound per molecule of metallothionein. The Cu(I) binding to phytochelatins depended on their chain length; the binding stoichiometries being 1.25, 2.0 and 2.5 for (gamma-Glu-Cys)2Gly, (gamma-Glu-Cys)3Gly and (gamma-Glu-Cys)4Gly respectively at neutral pH. A reduced stoichiometry for the longer phytochelatins was observed at alkaline pH. No GSH was found to associate with phytochelatins by a gel-filtration assay. The Cu(I) binding to (gamma-Glu-Cys)2Gly and (gamma-Glu-Cys)3Gly occurred in a biphasic manner in the sense that the relative luminescence increased approximately linearly with the amount of Cu(I) up to a certain molar ratio whereafter luminescence increased dramatically upon the binding of additional Cu(I). The luminescence intensity declined once the metal-binding sites were saturated. In analogy with the studies on metallothioneins, biphasic luminescence suggests the formation of two types of Cu(I) clusters in phytochelatins.