Semi-empirical simulation of Zn/Cd binding site preference in the metal binding domains of mammalian metallothionein

The Association Between Renal Tubular Dysfunction and Zinc Level in a Chinese Population Environmentally Exposed to Cadmium

Studies in vivo and in vitro have shown a protective effect of zinc against renal dysfunction caused by cadmium exposure. However, limited human data is available. In this study, we evaluated the association between renal tubular dysfunction and body zinc burden in a Chinese population exposed to cadmium. A total of 331 subjects (170 women and 161 men) living in control and cadmium-polluted area were included. Blood cadmium (BCd), urinary cadmium (UCd), serum zinc (SZn), zinc in hair (HZn), Zn/Cd ratio, and urinary β₂Microglobulin (UBMG) were measured. The median UCd, BCd, SZn, and HZn were 2.8 and 13.6 μg/g cr, 1.3 and 12.2 μg/L, 1.31 and 1.12 mg/L, and 0.14 and 0.12 mg/g in subjects living in control and polluted areas. The UBMG level of subjects living in the polluted area was significantly higher than that of the control (0.27 vs 0.11 mg/g cr, p < 0.01). SZn, HZn, and Zn/Cd ratios were negatively correlated with UBMG (p < 0.05 or 0.01). Subjects with high SZn concentrations (≥ 1.62 mg/L) had reduced risks of elevated UBMG [(odds ratio (OR) = 0.26, 95% confidence interval (CI) 0.07-0.99)] after controlling for multiple covariates compared with those with lower zinc levels. A similar result was observed in subjects with high HZn (OR = 0.09, 95% CI 0.02-0.48). The ORs of the second, third, and fourth quartiles of Zn/Cd ratio were 0.40 (95% CI 0.19-0.84), 0.14 (95% CI 0.06-0.37), and 0.01 (95% CI 0.02-0.18) for renal dysfunction compared with those of the first quartile, respectively. For those subjects with high level of UCd, high level of SZn and HZn also had reduced risks of elevated UBMG. The results of the present study show that high zinc body burden is associated with a decrease risk of renal tubular dysfunction induced by cadmium. Zinc nutritional status should be considered in evaluating cadmium-induced renal damage.

Structural and computational insights into the versatility of cadmium binding to proteins

Cadmium is a highly toxic group XII metal, similar to zinc and mercury. Unlike zinc, which is one of the most common metal cofactors in biology, cadmium is highly toxic. Many Zn(2+)-binding proteins can bind Cd(2+)-ions without significantly affecting their structures. Here, the protein data bank is analysed with regard to protein-cadmium interactions, which shows that cadmium can bind to a variety of ion binding sites in proteins. Statistical analysis of Cd(2+)-side chain interactions is compared with a similar analysis of other ions. This analysis reveals that with regard to amino acid side-chain preference, Cd(2+) is more similar to Mn(2+) than to Zn(2+) or Hg(2+). Finally, the interaction energies of three native metal binding proteins are calculated where Cd(2+) binds instead of Zn(2+), Ca(2+) or Cu(2+). The interaction energies are decomposed into individual components whose contributions are discussed.

Metallothionein polymorphisms in pathological processes

Metallothioneins (MTs) are a class of metal-binding proteins characterized by a high cysteine content and low molecular weight. MTs play an important role in metal metabolism and protect cells against the toxic effects of radiation, alkylating agents and oxygen free radicals. The evidence that individual genetic characteristics of MTs play an important role in physiological and pathological processes associated with antioxidant defense and detoxification inspired targeted studies of genetic polymorphisms in a clinical context. In recent years, common MT polymorphisms were identified and associated with, particularly, western lifestyle diseases such as cancer, complications of atherosclerosis, and type 2 diabetes mellitus along with related complications. This review summarizes all evidence regarding MT polymorphisms of major human MTs (MT1, MT2, MT3 and MT4), their relation to pathological processes, and outlines specific applications of MTs as a set of genetic markers for certain pathologies.

Association of Zn, Cu, Cd and Pb with protein fractions and sub-cellular partitioning in the digestive gland of Octopus vulgaris living in habitats with different metal levels

Zinc, Cu, Cd and Pb concentrations were determined in protein fractions of digestive gland and in the whole digestive gland of Octopus vulgaris collected from two areas of the Portuguese coast. Approximately 95% of Zn, 99% of Cu, 85-96% of Cd and 77-86% of Pb were stored in the cytosol, suggesting the predominance of cytosolic proteins in the trapping these elements. Gel filtration chromatography evidenced the presence of two major groups of proteins, with high molecular weight (HMW, 144 000-130 000 Da) and low molecular weight (LMW, 11000-6000 Da). The following metal-protein associations were found: Zn was distributed between HMW and LMW; Cu and Cd in LMW proteins with a minor association with HMW; and Pb in HMW proteins. The strong positive correlations between Cd, Zn and Cu and LMW proteins point to the presence of metalloproteins with high affinity to these elements. A shift was registered between the maximum of the ratio 254:280 nm and metal concentrations in the chromatographic profiles. This shift may result from metallothioneins having a small participation in the metal binding or protein purification was insufficient and various LMW proteins may be interfering.

A system-based comparison of gene expression reveals alterations in oxidative stress, disruption of ubiquitin-proteasome system and altered cell cycle regulation after exposure to cadmium and methylmercury in mouse embryonic fibroblast

Environmental and occupational exposures to heavy metals such as methylmercury (MeHg) and cadmium (Cd) pose significant health risks to humans, including neurotoxicity. The underlying mechanisms of their toxicity, however, remain to be fully characterized. Our previous studies with Cd and MeHg have demonstrated that the perturbation of the ubiquitin-proteasome system (UPS) was associated with metal-induced cytotoxicity and apoptosis. We conducted a microarray-based gene expression analysis to compare metal-altered gene expression patterns with a classical proteasome inhibitor, MG132 (0.5 microM), to determine whether the disruption of the UPS is a critical mechanism of metal-induced toxicity. We treated mouse embryonic fibroblast cells at doses of MeHg (2.5 microM) and Cd (5.0 microM) for 24 h. The doses selected were based on the neutral red-based cell viability assay where initial statistically significant decreases in variability were detected. Following normalization of the array data, we employed multilevel analysis tools to explore the data, including group comparisons, cluster analysis, gene annotations analysis (gene ontology analysis), and pathway analysis using GenMAPP and Ingenuity Pathway Analysis (IPA). Using these integrated approaches, we identified significant gene expression changes across treatments within the UPS (Uchl1 and Ube2c), antioxidant and phase II enzymes (Gsta2, Gsta4, and Noq1), and genes involved in cell cycle regulation pathways (ccnb1, cdc2a, and cdc25c). Furthermore, pathway analysis revealed significant alterations in genes implicated in Parkinson's disease pathogenesis following metal exposure. This study suggests that these pathways play a critical role in the development of adverse effects associated with metal exposures.

Zinc, cadmium, and mercury 1,2-benzenedithiolates with intramolecular NH...S hydrogen bonds

Mononuclear Zn, Cd, and Hg 1,2-benzenedithiolates with intramolecular NH...S hydrogen bonds, [M(II){1,2-S2-3,6-(RCONH)2C6H2}2](2-) (R = CH 3, t-Bu; M = Zn, Cd, Hg), were synthesized and characterized by X-ray analysis and spectral measurements. The presence of intramolecular NH...S hydrogen bonds was established by the IR spectra. (199)Hg and (113)Cd nuclear magnetic resonance showed a stabilized four-thiolate coordinated structure and suggested the influence of the NH...S hydrogen bonds to ppi(Hg)-ppi(S) interactions. The NH stretching bands show that the NH...S hydrogen bonds in Cd and Hg complexes are stronger than those in the corresponding Zn complex. These results are supported by theoretical calculations. The experimental and theoretical results suggested that the NH...S hydrogen bond influences the efficient capture of toxic Cd and Hg ions by metallothioneins.

Characterization of the conformational changes in recombinant human metallothioneins using ESI-MS and molecular modeling

Electrospray ionization mass spectrometry (ESI-MS) data and molecular modeling calculations were used to gain mechanistic, conformational, and domain-specific information from the acid-induced demetallation reactions of human metallothionein. The recombinant proteins studied were the single α- and β-rhMT-1a domains and the βα- and αβ-rhMT-1a two-domain species, based on the human metallothionein 1a sequence. Complete molecular models (MM3/MD) for all the fully metallated and demetallated species using a modified force field are reported for the first time. Basic residues that contribute to the ESI-MS charge states are identified from the molecular models. Demetallation took place under equilibrium conditions within a narrow pH range. For the two-domain proteins, these results support a demetallation mechanism involving the initial complete demetallation of one domain followed by the other for both βα-rhMT and αβ-rhMT. Based on the stability of the separate domains, the β domain is predicted to demetallate first in the two-domain rhMTs. Both the α domain and the β domain were observed to bind an excess of one Cd2+ ion. The metallated rhMT structures were shown to have very stable conformations, but only when fully metallated. Two or more conformations were observed at low pH in the ESI-MS data, which are interpreted as arising from the presence of structure, as opposed to a random coil, in the apo-rhMT. This is the first report of the existence of a structure in the two-domain metal-free apo-MT proteins. Only at extremely low pH does the structure open fully to give the highest charge distribution, which is associated with a random coil. Pre-existing structural features in the apo-MT would explain why the metallation reactions occur so rapidly.Key words: recombinant human metallothionein-1 (rhMT1), electrospray ionization mass spectrometry (ESI-MS), circular dichroism (CD), molecular mechanics/molecular dynamics (MM3/MD).

Peptide folding, metal-binding mechanisms, and binding site structures in metallothioneins

This minireview specifically focuses on recent studies carried out on structural aspects of metal-free metallothionein (MT), the mechanism of metal binding for copper and arsenic, structural studies using x-ray absorption spectroscopy and molecular mechanics modeling, and speciation studies of a novel cadmium and arsenic binding algal MT. Molecular mechanics-molecular dynamics calculations of apo-MT show that significant secondary structural features are retained by the polypeptide backbone upon sequential removal of the metal ions, which is stabilized by a possible H-bonding network. In addition, the cysteinyl sulfurs were shown to rotate from within the domain core, where they are found in the metallated state, to the exterior surface of the domain, suggesting an explanation for the rapid metallation reactions that were measured. Mixing Cu6beta-MT with Cd4alpha-MT and Cu6alpha-MT with Cd3beta-MT resulted in redistribution of the metal ions to mixed metal species in each domain; however, the Cu+ ions preferentially coordinated to the beta domain in each case. Reaction of As3+ with the individual metal-free beta and alpha domains of MT resulted in three As3+ ions coordinating to each of the domains, respectively, in a proposed distorted trigonal pyramid structure. Kinetic analysis provides parameters that allow simulation of the binding of each of the As3+ ions. X-ray absorption spectroscopy provides detailed information about the coordination environment of the absorbing element. We have combined measurement of x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) data with extensive molecular dynamics calculations to determine accurate metal-thiolate structures. Simulation of the XANES data provides a powerful technique for probing the coordination structures of metals in metalloproteins. The metal binding properties of an algal MT, Fucus vesiculosus, has been investigated by UV absorption and circular dichroism spectroscopy and electrospray ionization-mass spectrometry. The 16 cysteine residues of this algal MT were found to coordinate six Cd2+ ions in two domains with stoichiometries of a novel Cd3S7 cluster and a beta-like Cd3S9 cluster.

Mn,Cd-metallothionein-2: A room temperature magnetic protein

Naturally occurring metallothionein (MT) is a metal binding protein, which binds to seven Zn2+ through 20 conserved cysteines and forms two metal binding clusters with a Zinc-Blende structure. We demonstrate that the MT, when substituting the Zn2+ ions by Mn2+ and Cd2+, exhibits magnetic hysteresis loop observable by SQUID from 10 to 330 K. The magnetic moment may have originated from the bridging effect of the sulfur atoms between the metal ions that leads to the alignment of the electron spins of the Mn2+ ions inside the clusters. The protein backbone may restrain the net spin moment of Mn2+ ions from thermal fluctuation. The modified magnetic-metallothionein is a novel approach to creating molecular magnets with operating temperatures up to 330 K.

Molecular dynamics study on the folding and metallation of the individual domains of metallothionein

De novo synthesis of metallothionein (MT) initially forms the metal-free protein, which must, in a posttranslational reaction, coordinate metal ions via the cysteine sulfur ligands to form the fully folded protein structure. In this article, we use molecular dynamics (MD) and molecular mechanics (MM) to investigate the metal-dependent folding steps of the individual domains of recombinant human metallothionein (MT). The divalent metals were removed sequentially from the metal-sulfur M4(Scys)11 and M3(Scys)9 clusters within the alpha- and beta- domains of MT, respectively, after protonation of the previously coordinating sulfurs. With each of the four (alpha) or three (beta) sites defined, an order of metal release could be determined on the basis of a comparison of the strain energies for each combination by selecting the lowest energy demetallated conformations. The effect of an additional noninteracting, 34-residue peptide sequence on the demetallation order was assessed when bound to either the N- or C-termini of the individual domain fragments to identify the differences in cluster stability between one- and two-domain proteins. The N-terminal-bound peptide had no effect on the order of metal removal; however, addition to the C-terminus significantly altered the sequence. The number of hydrogen bonds was calculated for each energy-minimized demetallated structure and was increased on metal removal, indicating a possible stabilization mechanism for the protein structure via a hydrogen-bonding network. On complete demetallation, the cysteinyl sulfurs were shown to move to the exterior surface of the peptide chain.

Functional differentiation in the mammalian metallothionein gene family: metal binding features of mouse MT4 and comparison with its paralog MT1

This paper reports on the characterization of the metal binding abilities of mammalian MT4 and their comparison with those of the well known MT1. Heterologous Escherichia coli expression in cultures supplemented with zinc, cadmium, or copper was achieved for MT4 and for its separate alphaMT4 and betaMT4 domains as well as for MT1 and its alphaMT1 domain in cadmium-enriched medium. The in vivo conformed metal complexes and the in vitro substituted zinc/cadmium and zinc/copper MT4 aggregates were characterized. Biosynthesis of MT4 and betaMT4 in Cd(II)-supplemented medium revealed that these peptides failed to form the same homometallic species as MT1, thus appearing less effective for cadmium coordination. Conversely, the entire MT4 and both of its domains showed better Cu(I) binding properties than MT1, affording Cu(10)-MT4, Cu(5)-alphaMT4 and Cu(7)-betaMT4, stoichiometries that make the domain dependence toward Cu(I) clear. Overall results allow consideration of MT4 as a novel copper-thionein, made up of two copper-thionein domains, the first of this class reported in mammals, and by extension in vertebrates. Furthermore, the in silico protein sequence analyses corroborated the copper-thionein nature of the MT4 peptides. As a consequence, there is the suggestion of a possible physiological role played by MT4 related with copper requirements in epithelial differentiating tissues, where MT4 is expressed.

Reversible folding of cysteine-rich metallothionein by an overcritical reaction path

A first-order-like state transition is considered to be involved in the restoration of the activities of a few proteins by correctly folding the protein [Phys. Rev. E 66 (2002) 021903]. In order to understand the general applicability of this mechanism, we studied a metallothionein (MT) protein with an unconventional structure, i.e., without any alpha-helix or beta-sheet. MT is a 61 amino-acid peptide. There are 6-7 Zn(2+) ions, which bind avidly to 20 conserved cysteines (Cys) of MT. These properties indicate that the structure of MT is quite different from those of the other proteins. Similar to our previous findings, the denatured MT can be folded without any aggregation via a designated stepwise quasi-static process (an over-critical reaction path). The particle size of folded MT intermediates, determined by dynamic light scattering, shrank right after the first folding stage. It is consistent with a collapse-model. In addition, results from both atomic absorption and circular dichroism (CD) indicate that the stable intermediates may fold to the native conformation but with only partial Zn(2+) binding, which in turn implies that those folding intermediates are in a molten globular state. These reversible unfolding and folding processes indicate that Cys-rich protein, MT, may also be folded by way of a first-order-like state transition mechanism. We suspect that this process may likely be involved in the reaction of the metal substitution process in metal containing enzymes.

Interactions between cadmium and zinc in the organism

It is well known that many toxic effects of cadmium (Cd) action result from interactions with essential elements, including zinc (Zn). These interactions can take place at different stages of absorption, distribution in the organism and excretion of both metals and at the stage of Zn biological functions. Exposure to Cd leads to disturbance in Zn in the organism on the one hand, while dietary Zn intake has an important effect on Cd absorption, accumulation and toxicity on the other. The Zn status of the body is important in relation to development of Cd toxicity. Numerous data show that increased Zn supply may reduce Cd absorption and accumulation and prevent or reduce the adverse actions of Cd, whereas Zn deficiency can intensify Cd accumulation and toxicity. In this review, the interactions between these two trace elements in humans and animals are discussed on the basis of the available literature and our own results, against the background of general population exposure to Cd and common nutritional deficiency of Zn.

Bioremediation of Heavy Metal Pollution Exploiting Constituents, Metabolites and Metabolic Pathways of Livings. A Review

Removal of heavy metals from the soil and water or their remediation from the waste streams "at source" has been a long-term challenge. During the recent era of environmental protection, the use of microorganisms for the recovery of metals from waste streams as well as employment of plants for landfill applications has generated growing attention. Many studies have demonstrated that both prokaryotes and eukaryotes have the ability to remove metals from contaminated water or waste streams. They sequester metals from soils and sediments or solubilize them to aid their extraction. The proposed microbial processes for bioremediation of toxic metals and radionuclides from waste streams employ living cells and non-living biomass or biopolymers as biosorbents. Microbial biotransformation of metals or metalloids results in an alteration of their oxidation state or in their alkylation and subsequent precipitation or volatilization. Specific metabolic pathways leading to precipitation of heavy metals as metal sulfides, phosphates or carbonates possess significance for possible biotechnology application. Moreover, the possibility of altering the properties of living species used in heavy metal remediation or constructing chimeric organisms possessing desirable features using genetic engineering is now under study in many laboratories. The encouraging evidence as to the usefulness of living organisms and their constituents as well as metabolic pathways for the remediation of metal contamination is reviewed here. A review with 243 references.

Replacement of terminal cysteine with histidine in the metallothionein alpha and beta domains maintains its binding capacity

To generate novel forms of metal-binding proteins, six mutant mouse metallothionein (MT) 1 fragments, in which a terminal cysteine residue was replaced by histidine, were expressed in Escherichia coli. The spectroscopic and analytical results showed that the alphaMT (C33H, C36H, C41H, C57H) and betaMT (C5H, C13H) mutant forms bound 4 and 3 Zn(II) atoms per molecule of protein to the nearest integer, even though in C41H and C5H, species of lower stoichiometry were also detected. In Cd(II) titrations, all the Zn(II) ions bound to the mutant proteins were displaced from the binding sites, giving rise to Cd-mutated MT forms with 4 and 3 Cd(II), respectively. However, although Cys-to-His substitutions maintained the binding capacity of the MT fragments, they caused structural changes with respect to the wild-type proteins. While C13H, C36H and C57H seem to contain Zn(II)-aggregates that are closely related to those of the wild-type proteins, only C41H and C57H gave rise to Cd(II)-aggregates similar to those of Cd4-alphaMT, where the His residue plays the role of the substituted Cys. Despite the structural implications of the Cys-to-His replacement, the dissociation constants showed no major decrease in the Cd-binding affinity in any of the mutants assayed compared with the wild-type.

Aspects of the mechanism of catalysis of glucose oxidase: a docking, molecular mechanics and quantum chemical study

The complex structure of glucose oxidase (GOX) with the substrate glucose was determined using a docking algorithm and subsequent molecular dynamics simulations. Semiempirical quantum chemical calculations were used to investigate the role of the enzyme and FAD co-enzyme in the catalytic oxidation of glucose. On the basis of a small active site model, substrate binding residues were determined and heats of formation were computed for the enzyme substrate complex and different potential products of the reductive half reaction. The influence of the protein environment on the active site model was estimated with a point charge model using a mixed QM/MM method. Solvent effects were estimated with a continuum model. Possible modes of action are presented in relation to experimental data and discussed with respect to related enzymes. The calculations indicate that the redox reaction of GOX differs from the corresponding reaction of free flavins as a consequence of the protein environment. One of the active site histidines is involved in substrate binding and stabilization of potential intermediates, whereas the second histidine is a proton acceptor. The former one, being conserved in a series of oxidoreductases, is also involved in the stabilization of a C4a-hydroperoxy dihydroflavin in the course of the oxidative half reaction.