Reexamination of lead(II) coordination preferences in sulfur-rich sites: implications for a critical mechanism of lead poisoning

Lead(II)-Based Coordination Polymer Exhibiting Reversible Color Switching and Selective CO2 Photoreduction Properties

Herein, we report a new photofunctional Pb-S-based coordination polymer (CP) with the formula [Pb(ATAT)(OAc)]n (ATAT = 3-amino-5-mercapto-1,2,4-triazole, OAc = acetate, CP1). Apart from its photoactive one-dimensional (1D) (-Pb-S-)n chain, CP1 is also composed of another 1D (-Pb-O-)n chain that originates from the coordination with acetate. The coordinated acetate can be exchanged with water (H2O) or dimethyl sulfoxide (DMSO), leading to the formation of a CP1-H2O or CP1-DMSO structure that exhibits a distinct change in optical properties, including a white-to-yellow color change. The structural transformation of CP1 to CP1-H2O and CP1-DMSO, and its subsequent recovery to the original CP1 structure could be controlled by the presence or absence of acetic acid vapor; the transformation was completely reversible. CP1 absorbed light with wavelengths shorter than 390 nm, with an estimated bandgap of 3.18 eV. Density functional theory calculations indicated that the valence band of CP1 is mainly formed by N and S orbitals originating from the ATAT unit, whereas the conduction band is composed of the Pb orbitals. Even without any modification, such as the incorporation of a molecular catalyst, CP1 reduced CO2 into formate under UV light with >99% selectivity.

AsIII Selectively Induces a Disorder-to-Order Transition in the Metalloid Binding Region of the AfArsR Protein

Arsenic is highly toxic and a significant threat to human health, but certain bacteria have developed defense mechanisms initiated by AsIII binding to AsIII-sensing proteins of the ArsR family. The transcriptional regulator AfArsR responds to AsIII and SbIII by coordinating the metalloids with three cysteines, located in a short sequence of the same monomer chain. Here, we characterize the binding of AsIII and HgII to a model peptide encompassing this fragment of the protein via solution equilibrium and spectroscopic/spectrometric techniques (pH potentiometry, UV, CD, NMR, PAC, EXAFS, and ESI-MS) combined with DFT calculations and MD simulations. Coordination of AsIII changes the peptide structure from a random-coil to a well-defined structure of the complex. A trigonal pyramidal AsS3 binding site is formed with almost exactly the same structure as observed in the crystal structure of the native protein, implying that the peptide possesses all of the features required to mimic the AsIII recognition and response selectivity of AfArsR. Contrary to this, binding of HgII to the peptide does not lead to a well-defined structure of the peptide, and the atoms near the metal binding site are displaced and reoriented in the HgII model. Our model study suggests that structural organization of the metal site by the inducer ion is a key element in the mechanism of the metalloid-selective recognition of this protein.

Development of Meta-Chemical Surface by Dip-Pen Nanolithography for Precise Electrochemical Lead Sensing

Dip-pen nanolithography (DPN) is a powerful and unique technique for precisely depositing tiny nano-spherical cap shapes (nanoclusters) onto a desired surface. In this study, a meta-chemical surface (MCS; a pattern with advanced features) is developed by DPN and applied to electrochemical lead sensing, yielding a calibration curve in the ppb range. An ink mixture of PMMA and NTPH (which binds to Pb (II), as supported by DFT calculations) is patterned over a Pt surface. The average height of the nanoclusters is ≈13 nm with a high surface area-to-volume ratio, which depends on the ink composition and the MCS surface. This ratio affected the sensitivity of the MCS as a detecting tool. The results indicate that the sensor's features can be controlled by the ability to control the size of the nanoclusters, attributed to the unique properties of the DPN production method. These results are significant for the water-source purification industry.

Constructing sulfur and oxygen super-coordinated main-group electrocatalysts for selective and cumulative H2O2 production

Direct electrosynthesis of hydrogen peroxide (H2O2) via the two-electron oxygen reduction reaction presents a burgeoning alternative to the conventional energy-intensive anthraquinone process for on-site applications. Nevertheless, its adoption is currently hindered by inferior H2O2 selectivity and diminished H2O2 yield induced by consecutive H2O2 reduction or Fenton reactions. Herein, guided by theoretical calculations, we endeavor to overcome this challenge by activating a main-group Pb single-atom catalyst via a local micro-environment engineering strategy employing a sulfur and oxygen super-coordinated structure. The main-group catalyst, synthesized using a carbon dot-assisted pyrolysis technique, displays an industrial current density reaching 400 mA cm-2 and elevated accumulated H2O2 concentrations (1358 mM) with remarkable Faradaic efficiencies. Both experimental results and theoretical simulations elucidate that S and O super-coordination directs a fraction of electrons from the main-group Pb sites to the coordinated oxygen atoms, consequently optimizing the *OOH binding energy and augmenting the 2e- oxygen reduction activity. This work unveils novel avenues for mitigating the production-depletion challenge in H2O2 electrosynthesis through the rational design of main-group catalysts.

Pb(II) coordination to the nonclassical zinc finger tristetraprolin: retained function with an altered fold

Tristetraprolin (TTP) is a nonclassical CCCH zinc finger (ZF) that plays a crucial role in regulating inflammation. TTP regulates cytokine mRNAs by specific binding of its two conserved ZF domains (CysX₈CysX₅CysX₃His) to adenylate-uridylate-rich sequences (AREs) at the 3'-untranslated region, leading to degradation of the RNA. Dysregulation of TTP in animal models has demonstrated several cytokine-related syndromes, including chronic inflammation and autoimmune disorders. Exposure to Pb(II), a prevalent environmental toxin, is known to contribute to similar pathologies, in part by disruption of and/or competition with cysteine-rich metalloproteins. TTP's role during stress as a ubiquitous translational regulator of cell signaling (and dysfunction), which may underpin various phenotypes of Pb(II) toxicity, highlights the importance of understanding the interaction between TTP and Pb(II). The impact of Pb(II) binding on TTP's fold and RNA-binding function was analyzed via UV-Vis spectroscopy, circular dichroism, X-ray absorption spectroscopy, nuclear magnetic resonance spectroscopy, and fluorescence anisotropy. A construct containing the two ZF domains of TTP (TTP-2D) bound to Pb(II) with nanomolar affinity and exhibited a different geometry and fold in comparison to Zn₂-TTP-2D. Despite the altered secondary structure, Pb(II)-substituted TTP-2D bound a canonical ARE sequence more selectively than Zn₂-TTP-2D. Taken together, these data suggest that Pb(II) may interfere with proper TTP regulation and hinder the cell's ability to respond to inflammation.

Geophagia: Benefits and potential toxicity to human-A review

Geophagy is the habit of consuming clay soil such as chalk or kaolin. Though it is globally practiced, the safety of those involved is yet to be fully established. It is thought to be highly prevalent in pregnant women because of its antinausea or therapeutic effects. This practice is also thought to be provoked by some nutritional needs, but in modern society its etiology is obscure. The mineralogical and chemical compositions of clay may vary from one region to another and even in all form of rocks clay constitutes. Published articles in geophagy indicate lack of adequate investigations into the toxicity of geophagy, though it is globally practiced and more prevalent in Africa (as a continent) or in Africans migrants. Some studies have helped to identify some minerals that are toxic to human if ingested. In most cases, the potential toxicity emphasized by these studies is based on the detection of the presence of these nuisance elements in the geophagic materials. Scientifically, a lot has been done in the light of detection of toxic matter, but more investigations on metabolic studies are still necessary. The variability of clay content with respect to source motivated this review on geophagy and its potential toxicity to human. This review is aimed at bringing out findings that would enable a better understanding of the toxicity potential of geophagy across context and taxa.

On the Quantum Chemical Nature of Lead(II) “Lone Pair”

We study the quantum chemical nature of the Lead(II) valence basins, sometimes called the lead “lone pair”. Using various chemical interpretation tools, such as molecular orbital analysis, natural bond orbitals (NBO), natural population analysis (NPA) and electron localization function (ELF) topological analysis, we study a variety of Lead(II) complexes. A careful analysis of the results shows that the optimal structures of the lead complexes are only governed by the 6s and 6p subshells, whereas no involvement of the 5d orbitals is found. Similarly, we do not find any significant contribution of the 6d. Therefore, the Pb(II) complexation with its ligand can be explained through the interaction of the 6s² electrons and the accepting 6p orbitals. We detail the potential structural and dynamical consequences of such electronic structure organization of the Pb (II) valence domain.

Ultra-deep removal of Pb by functionality tuned UiO-66 framework: A combined experimental, theoretical and HSAB approach

A specific functionality in the adsorbent materials plays a significant role for the selective capture of heavy metals based on Pearson's Hard-Soft-Acid-Base (HSAB) concept. Herein, we introduced single and double amino- and thiol-functionalities into the UiO-66 framework, which acted as hard and soft base sites for heavy metal adsorption, respectively. The synthesized adsorbents (labelled as NH₂-UiO-66, (NH₂)₂-UiO-66, SH-UiO-66 and (SH)₂-UiO-66) were applied for the selective removal of lead (Pb) ions from contaminated water. The removal efficiency of Pb was about 64, 85, 75 and 99% (pH = 6, T = 30 °C, sample dosage = 10 mg, Pb concentration = 100 mg L^-1), respectively, based on available number of interacting sites in the respective adsorbent. To elaborate HSAB concept, the interacting sites of these functional groups towards Pb were explored by identifying their possible types of interactions in terms of soft acid-base affinity, coordinate and covalent bonding, chelation, π-π interactions and synergetic effect of bonding. Density functional theory (DFT) simulation was used to confirm these interactions and to help the better understanding of adsorption mechanism. Model fitting and characterization of Pb-sorbed adsorbents were also performed to reveal kinetics, order of adsorptive reaction, thermodynamics and adsorption mechanism. Moreover, the optimization of adsorptive removal was performed by controlled parameters including time, initial concentration, pH and temperature. The reusability and selectivity of these adsorbents along with recovery of Pb(II) were also assessed. This study presents the conceptual framework for the design of functional adsorbents in the removal of heavy metals using the HSAB principle as an intended guideline.

Critical aspects of the physiological interactions between lead and magnesium

Despite technological progress, exposure to lead is an ongoing problem. There are many mechanisms governing the toxic effects of lead on the human body. One such mechanism involves the interaction of this xenobiotic with bivalent metal ions, including magnesium. Literature data suggest that the competition between these elements for binding sites at the molecular and cellular levels, as well as at the systemic level, may represent an important aspect of lead toxicity in the human body. This is especially clear in the context of oxidative stress, immune response, and gene expression modifications. This review aims to summarize current knowledge regarding these issues.

Open Reading Frame 1 Protein of the Human Long Interspersed Nuclear Element 1 Retrotransposon Binds Multiple Equivalents of Lead

The human long interspersed nuclear element 1 (LINE1) has been implicated in numerous diseases and has been suggested to play a significant role in genetic evolution. Open reading frame 1 protein (ORF1p) is one of the two proteins encoded in this self-replicating mobile genetic element, both of which are essential for retrotransposition. The structure of the three-stranded coiled-coil domain of ORF1p was recently solved and showed the presence of tris-cysteine layers in the interior of the coiled-coil that could function as metal binding sites. Here, we demonstrate that ORF1p binds Pb(II). We designed a model peptide, GRCSL16CL23C, to mimic two of the ORF1p Cys3 layers and crystallized the peptide both as the apo-form and in the presence of Pb(II). Structural comparison of the ORF1p with apo-(GRCSL16CL23C)3 shows very similar Cys3 layers, preorganized for Pb(II) binding. We propose that exposure to heavy metals, such as lead, could influence directly the structural parameters of ORF1p and thus impact the overall LINE1 retrotransposition frequency, directly relating heavy metal exposure to genetic modification.

Rapid Speciation of Lead in Human Blood and Urine Samples Based on MWCNTs@DMP by Dispersive Ionic Liquid-Suspension-Micro-Solid Phase Extraction

An efficient sorbent based on 2,3-dimercapto-1-propanol immobilized on multi-wall carbon nanotubes (MWCNTs@DMP) was developed for separation/speciation of organic and inorganic lead (alkyl-Pb, Pb²⁺) in human blood, urine, and water samples by dispersive ionic liquid-suspension-micro-solid phase extraction (DIL-S-μ-SPE). By procedure, the MWCNTs@DMP as solid phase, acetone, and ionic liquid (IL, [HMIM][PF₆]) were mixed and injected to 10 mL of the liquid phase at pH = 6.5. After shaking, the Pb(II) was extracted in MWCNTs@DMP and settled down in a conical tube with IL by centrifuging (Pb²⁺→: SH-SiO₂@CNTs). The lead (Pb²⁺) was back-extracted from sorbent/IL in acidic pH and measured by atom trap atomic absorption spectrometry (AT-AAS). In addition, the organic lead (R-Pb, alkyl lead) converted to Pb(II) and total lead (T-Pb) was determined in the same conditions by UV radiation in 95 °C. Under the optimal conditions, the linear range (9.5-480 μg L^-1), LOD (3.2 μg L^-1), and enrichment factor (10.4) were obtained (RSD < 5%). The adsorption capacity of the MWCNTs@DMP and MWCNTs was achieved as 191.6 mg g^-1 and 25.8 mg g^-1, respectively. The method was validated by standard reference materials (SRM 1643d, SRM 955, and SRM 2668), ET-AAS, and ICP-MS analysis in real samples. Graphical abstract.

Elucidating the Quenching Mechanism in Carbon Dot-Metal Interactions-Designing Sensitive and Selective Optical Probes

Overexposure to metals has significant adverse effects on human and animal health coupled with nefarious consequences to the environment. Sensitive tools to measure low contaminant levels exist, but often come at a high cost and require tedious procedures. Thus, there exists a need for the development of affordable metal sensors that can offer high sensitivity and selectivity while being accessible on a global scale. Here, carbon dots, prepared in a one-pot synthesis using glutathione and formamide, have been developed as dual fluorescent metal sensing probes. Following extensive characterization of their physico-chemical properties, it is demonstrated that dual fluorescence can be exploited to build a robust ratiometric sensor with low-ppb detection sensitivity in water. This investigation shows that these optical probes are selective for Pb2+ and Hg2+ ions. Using steady-state and dynamic optical characterization techniques, coupled with hard and soft acid-base theory, the underlying reason for this selective behavior was identified. These findings shed light on the nature of metal-carbon dot interactions, which can be used to tailor their properties to target specific metal ions. Finally, these findings can be applicable to other fluorescent nanoparticle systems that are targeted for development as metal sensors.

Molecular dynamics simulation of the Pb(II) coordination in biological media via cationic dummy atom models

The coordination of Pb(II) in aqueous solutions containing thiols is a pivotal topic to the understanding of the pollutant potential of this cation. Based on its hard/soft borderline nature, Pb(II) forms stable hydrated ions as well as stable complexes with the thiol groups of proteins. In this paper, the modeling of Pb(II) coordination via classical molecular dynamics simulations was investigated to assess the possible use of non-bonded potentials for the description of the metal–ligand interaction. In particular, this study aimed at testing the capability of cationic dummy atom schemes—in which part of the mass and charge of the Pb(II) is fractioned in three or four sites anchored to the metal center—in reproducing the correct coordination geometry and, also, in describing the hard/soft borderline character of this cation. Preliminary DFT calculations were used to design two topological schemes, PB3 and PB4, that were subsequently implemented in the Amber force field and employed in molecular dynamics simulation of either pure water or thiol/thiolate-containing aqueous solutions. The PB3 scheme was then tested to model the binding of Pb(II) to the lead-sensing protein pbrR. The potential use of CDA topological schemes in the modeling of Pb(II) coordination was here critically discussed.

Review of recent progress on DNA-based biosensors for Pb2+ detection

Lead (Pb) is a highly toxic heavy metal of great environmental and health concerns, and interestingly Pb²⁺ has played important roles in nucleic acids chemistry. Since 2000, using DNA for selective detection of Pb²⁺ has become a rapidly growing topic in the analytical community. Pb²⁺ can serve as the most active cofactor for RNA-cleaving DNAzymes including the GR5, 17E and 8-17 DNAzymes. Recently, Pb²⁺ was found to promote a porphyrin metalation DNAzyme named T30695. In addition, Pb²⁺ can tightly bind to various G-quadruplex sequences inducing their unique folding and binding to other molecules such as dyes and hemin. The peroxidase-like activity of G-quadruplex/hemin complexes was also used for Pb²⁺ sensing. In this article, these Pb²⁺ recognition mechanisms are reviewed from fundamental chemistry to the design of fluorescent, colorimetric, and electrochemical biosensors. In addition, various signal amplification mechanisms such as rolling circle amplification, hairpin hybridization chain reaction and nuclease-assisted methods are coupled to these sensing methods to drive up sensitivity. We mainly cover recent examples published since 2015. In the end, some practical aspects of these sensors and future research opportunities are discussed.

Applying CRISPR-Cas12a as a Signal Amplifier to Construct Biosensors for Non-DNA Targets in Ultralow Concentrations

Efficient signal amplification is essential to construct ultrasensitive biosensors for biologically relevant species with abundant concomitant interferences. Here, we apply LbaCas12a as a signal amplifier to develop a versatile CRISPR-Cas12a platform to detect a wide range of analytes in ultralow concentrations. The platform relies on the indiscriminate single-stranded DNase activity of LbaCas12a, which recognizes single-stranded DNA intermediates generated by non-DNA targets down to femtomolar concentrations and subsequently enhances the fluorescence signal output. With the help of functional nucleotides (DNAzyme and aptamer), ultrasensitive bioassays for Pb²⁺ and Acinetobacter baumannii have been designed with a limit of detection down to ∼0.053 nM and ∼3 CFU/mL, respectively. It also allows simultaneous detection of four microRNAs (miRNAs) at a picomolar concentration without significant interferences by other counterparts, suggesting the potential of multiplexed miRNA expression profiles analysis in high throughput. Given the versatility and generality of the CRISPR-Cas12a platform, we expect the current work to advance the application of CRISPR-Cas-based platforms in bioanalysis and provide new insights into ultrasensitive biosensor design.

Infrared and 2-Dimensional Correlation Spectroscopy Study of the Effect of CH3NH3PbI3 and CH3NH3SnI3 Photovoltaic Perovskites on Eukaryotic Cells

We studied the effect of the exposure of human A549 and SH-SY5Y cell lines to aqueous solutions of organic/inorganic halide perovskites CH₃NH₃PbI₃ (MAPbI₃) and CH₃NH₃SnI₃ (MASnI₃) at the molecular level by using Fourier transform infrared microspectroscopy. We monitored the infrared spectra of some cells over a few days following exposure to the metals and observed the spectroscopic changes dominated by the appearance of a strong band at 1627 cm⁻¹. We used Infrared (IR) mapping to show that this change was associated with the cell itself or the cellular membrane. It is unclear whether the appearance of the 1627 cm⁻¹ band and heavy metal exposure are related by a direct causal relationship. The spectroscopic response of exposure to MAPbI₃ and MASnI₃ was similar, indicating that it may arise from a general cellular response to stressful environmental conditions. We used 2D correlation spectroscopy (2DCOS) analysis to interpret spectroscopic changes. In a novel application of the method, we demonstrated the viability of 2DCOS for band assignment in spatially resolved spectra. We assigned the 1627 cm⁻¹ band to the accumulation of an abundant amide or amine containing compound, while ruling out other hypotheses. We propose a few tentative assignments to specific biomolecules or classes of biomolecules, although additional biochemical characterization will be necessary to confirm such assignments.

Asymmetric Synthesis of Heterocyclic γ-Amino-Acid and Diamine Derivatives by Three-Component Radical Cascade Reactions

An enantioselective three-component radical reaction of quinolines or pyridines with enamides and α-bromo carbonyl compounds by dual photoredox and chiral Brønsted acid catalysis is presented. A range of valuable chiral γ-amino-acid derivatives are accessible in high chemo-, regio-, and enantioselectivity from simple, readily available starting materials under mild reaction conditions. Using the same strategy, the asymmetric synthesis of 1,2-diamine derivatives is also reported.

Coordination promiscuity guarantees metal substrate selection in transmembrane primary-active Zn2+ pumps

Metal selectivity in P1B-type ATPase pumps appears to be determined by amino acid motifs on their transmembrane helices. We reveal the principles governing substrate promiscuity towards first-, second- and third-row transition metals in a transmembrane Zn2+/Cd2+/Hg2+/Pb2+ P-type ATPase (ZntA), by dissecting its coordination chemistry. Atomic resolution characterization in detergent micelles and lipid bilayers reveals a "plastic" transmembrane metal-binding site that selects substrates by unique and diverse, yet defined, coordination geometries and ligand-metal distances.

Fluorescence Characteristics of Aqueous Synthesized Tin Oxide Quantum Dots for the Detection of Heavy Metal Ions in Contaminated Water

Tin oxide quantum dots were synthesized in aqueous solution via a simple hydrolysis and oxidation process. The morphology observation showed that the quantum dots had an average grain size of 2.23 nm. The rutile phase SnO₂ was confirmed by the structural and compositional characterization. The fluorescence spectroscopy of quantum dots was used to detect the heavy metal ions of Cd²⁺, Fe³⁺, Ni²⁺ and Pb²⁺, which caused the quenching effect of photoluminescence. The quantum dots showed the response of 2.48 to 100 ppm Ni²⁺. The prepared SnO₂ quantum dots exhibited prospective in the detection of heavy metal ions in contaminated water, including deionized water, deionized water with Fe³⁺, reclaimed water and sea water. The limit of detection was as low as 0.01 ppm for Ni²⁺ detection. The first principle calculation based on the density function theory demonstrated the dependence of fluorescence response on the adsorption energy of heavy metal ions as well as ion radius. The mechanism of fluorescence response was discussed based on the interaction between Sn vacancies and Ni²⁺ ions. A linear correlation of fluorescence emission intensity against Ni²⁺ concentration was obtained in the logarithmic coordinates. The density of active Sn vacancies was the crucial factor that determined fluorescence response of SnO₂ QDs to heavy metal ions.

Ni(II), Hg(II), and Pb(II) Coordination in the Prokaryotic Zinc-Finger Ros87

Zinc ion binding is a principal event in the achievement of the correct fold in classical zinc finger domains since the motif is largely unfolded in the absence of metal. In the case of a prokaryotic zinc finger, the larger βββαα domain contributes to the folding mechanism with a larger hydrophobic core. For these reasons, following the great amount of attention devoted to unveiling the effect of xenobiotic metal ion replacement in zinc fingers and in zinc-containing proteins in general, the prokaryotic zinc finger domain appears to be an interesting model for studying metal ion interaction with metalloproteins. Here, we explore the binding of Ni(II), Hg(II), and Pb(II) to Ros87, the DNA binding domain of the prokaryotic zinc finger protein Ros. We measured Ros87-metal ion dissociation constants and monitored the effects on the structure and function of the domain. Interestingly, we found that the protein folds in the presence of Ni(II) with important structural perturbations, while in the presence of Pb(II) and Hg(II) it does not appear to be significantly folded. Accordingly, an overall strong reduction in the DNA binding capability is observed for all of the examined proteins. Our data integrate and complement the information collected in the past few years concerning the functional and structural effects of metal ion substitution in classical zinc fingers in order to contribute to a better comprehension of the toxicity of these metals in biological systems.

High affinity interactions of Pb2+ with synaptotagmin I

Lead (Pb) is a potent neurotoxin that disrupts synaptic neurotransmission. We report that Synaptotagmin I (SytI), a key regulator of Ca2+-evoked neurotransmitter release, has two high-affinity Pb2+ binding sites that belong to its cytosolic C2A and C2B domains. The crystal structures of Pb2+-complexed C2 domains revealed that protein-bound Pb2+ ions have holodirected coordination geometries and all-oxygen coordination spheres. The on-rate constants of Pb2+ binding to the C2 domains of SytI are comparable to those of Ca2+ and are diffusion-limited. In contrast, the off-rate constants are at least two orders of magnitude smaller, indicating that Pb2+ can serve as both a thermodynamic and kinetic trap for the C2 domains. We demonstrate, using NMR spectroscopy, that population of these sites by Pb2+ ions inhibits further Ca2+ binding despite the existing coordination vacancies. Our work offers a unique insight into the bioinorganic chemistry of Pb(ii) and suggests a mechanism by which low concentrations of Pb2+ ions can interfere with the Ca2+-dependent function of SytI in the cell.

Heme bioavailability and signaling in response to stress in yeast cells

Protoheme (hereafter referred to as heme) is an essential cellular cofactor and signaling molecule that is also potentially cytotoxic. To mitigate heme toxicity, heme synthesis and degradation are tightly coupled to heme utilization in order to limit the intracellular concentration of "free" heme. Such a model, however, would suggest that a readily accessible steady-state, bioavailable labile heme (LH) pool is not required for supporting heme-dependent processes. Using the yeast Saccharomyces cerevisiae as a model and fluorescent heme sensors, site-specific heme chelators, and molecular genetic approaches, we found here that 1) yeast cells preferentially use LH in heme-depleted conditions; 2) sequestration of cytosolic LH suppresses heme signaling; and 3) lead (Pb²⁺) stress contributes to a decrease in total heme, but an increase in LH, which correlates with increased heme signaling. We also observed that the proteasome is involved in the regulation of the LH pool and that loss of proteasomal activity sensitizes cells to Pb²⁺ effects on heme homeostasis. Overall, these findings suggest an important role for LH in supporting heme-dependent functions in yeast physiology.

A Bis-Triazacyclononane Tris-Pyridyl N9 -Azacryptand "Beer Can" Receptor for Complexation of Alkali Metal and Lead(II) Cations

A new bis-triazacyclononane tris-pyridyl N₉ -azacryptand ligand is prepared via a convenient one-pot [2+3] condensation reaction between triazacyclononane and 2,6-bis(bromomethyl) pyridine in the presence of M₂ CO₃ (M=Li, Na, K). The proton, lithium, sodium, potassium and lead(II) complexes of the ligand are characterised in the solid state. Preliminary solution-phase competition experiments indicate that the cryptand ligand preferentially binds lead(II) in the presence of sodium, calcium, potassium and zinc cations in methanol solution.

Defining potential roles of Pb(2+) in neurotoxicity from a calciomics approach

Metal ions play crucial roles in numerous biological processes, facilitating biochemical reactions by binding to various proteins. An increasing body of evidence suggests that neurotoxicity associated with exposure to nonessential metals (e.g., Pb(2+)) involves disruption of synaptic activity, and these observed effects are associated with the ability of Pb(2+) to interfere with Zn(2+) and Ca(2+)-dependent functions. However, the molecular mechanism behind Pb(2+) toxicity remains a topic of debate. In this review, we first discuss potential neuronal Ca(2+) binding protein (CaBP) targets for Pb(2+) such as calmodulin (CaM), synaptotagmin, neuronal calcium sensor-1 (NCS-1), N-methyl-d-aspartate receptor (NMDAR) and family C of G-protein coupled receptors (cGPCRs), and their involvement in Ca(2+)-signalling pathways. We then compare metal binding properties between Ca(2+) and Pb(2+) to understand the structural implications of Pb(2+) binding to CaBPs. Statistical and biophysical studies (e.g., NMR and fluorescence spectroscopy) of Pb(2+) binding are discussed to investigate the molecular mechanism behind Pb(2+) toxicity. These studies identify an opportunistic, allosteric binding of Pb(2+) to CaM, which is distinct from ionic displacement. Together, these data suggest three potential modes of Pb(2+) activity related to molecular and/or neural toxicity: (i) Pb(2+) can occupy Ca(2+)-binding sites, inhibiting the activity of the protein by structural modulation, (ii) Pb(2+) can mimic Ca(2+) in the binding sites, falsely activating the protein and perturbing downstream activities, or (iii) Pb(2+) can bind outside of the Ca(2+)-binding sites, resulting in the allosteric modulation of the protein activity. Moreover, the data further suggest that even low concentrations of Pb(2+) can interfere at multiple points within the neuronal Ca(2+) signalling pathways to cause neurotoxicity.

Thermodynamics of Pb(ii) and Zn(ii) binding to MT-3, a neurologically important metallothionein

Isothermal titration calorimetry (ITC) was used to quantify the thermodynamics of Pb(2+) and Zn(2+) binding to metallothionein-3 (MT-3). Pb(2+) binds to zinc-replete Zn7MT-3 displacing each zinc ion with a similar change in free energy (ΔG) and enthalpy (ΔH). EDTA chelation measurements of Zn7MT-3 and Pb7MT-3 reveal that both metal ions are extracted in a tri-phasic process, indicating that they bind to the protein in three populations with different binding thermodynamics. Metal binding is entropically favoured, with an enthalpic penalty that reflects the enthalpic cost of cysteine deprotonation accompanying thiolate ligation of the metal ions. These data indicate that Pb(2+) binding to both apo MT-3 and Zn7MT-3 is thermodynamically favourable, and implicate MT-3 in neuronal lead biochemistry.

Cu(I) Disrupts the Structure and Function of the Nonclassical Zinc Finger Protein Tristetraprolin (TTP)

Tristetraprolin (TTP) is a nonclassical zinc finger (ZF) protein that plays a key role in regulating inflammatory response. TTP regulates cytokines at the mRNA level by binding to AU-rich sequences present at the 3'-untranslated region, forming a complex that is then degraded. TTP contains two conserved CCCH domains with the sequence CysX₈CysX₅CysX₃His that are activated to bind RNA when zinc is coordinated. During inflammation, copper levels are elevated, which is associated with increased inflammatory response. A potential target for Cu(I) during inflammation is TTP. To determine whether Cu(I) binds to TTP and how Cu(I) can affect TTP/RNA binding, two TTP constructs were prepared. One construct contained just the first CCCH domain (TTP-1D) and serves as a peptide model for a CCCH domain; the second construct contains both CCCH domains (TTP-2D) and is functional (binds RNA) when Zn(II) is coordinated. Cu(I) binding to TTP-1D was assessed via electronic absorption spectroscopy titrations, and Cu(I) binding to TTP-2D was assessed via both absorption spectroscopy and a spin filter/inductively coupled plasma mass spectrometry (ICP-MS) assay. Cu(I) binds to TTP-1D with a 1:1 stoichiometry and to TTP-2D with a 3:1 stoichiometry. The CD spectrum of Cu(I)-TTP-2D did not exhibit any secondary structure, matching that of apo-TTP-2D, while Zn(II)-TTP-2D exhibited a secondary structure. Measurement of RNA binding via fluorescence anisotropy revealed that Cu(I)-TTP-2D does not bind to the TTP-2D RNA target sequence UUUAUUUAUUU with any measurable affinity, while Zn(II)-TTP-2D binds to this site with nanomolar affinity. Similarly, addition of Cu(I) to the Zn(II)-TTP-2D/RNA complex resulted in inhibition of RNA binding. Together, these data indicate that, while Cu(I) binds to TTP-2D, it does not result in a folded or functional protein and that Cu(I) inhibits Zn(II)-TTP-2D/RNA binding.

Function of Metallothionein-3 in Neuronal Cells: Do Metal Ions Alter Expression Levels of MT3?

A study of factors proposed to affect metallothionein-3 (MT3) function was carried out to elucidate the opaque role MT3 plays in human metalloneurochemistry. Gene expression of Mt2 and Mt3 was examined in tissues extracted from the dentate gyrus of mouse brains and in human neuronal cell cultures. The whole-genome gene expression analysis identified significant variations in the mRNA levels of genes associated with zinc homeostasis, including Mt2 and Mt3. Mt3 was found to be the most differentially expressed gene in the identified groups, pointing to the existence of a factor, not yet identified, that differentially controls Mt3 expression. To examine the expression of the human metallothioneins in neurons, mRNA levels of MT3 and MT2 were compared in BE(2)C and SH-SY5Y cell cultures treated with lead, zinc, cobalt, and lithium. MT2 was highly upregulated by Zn²⁺ in both cell cultures, while MT3 was not affected, and no other metal had an effect on either MT2 or MT3.

Mammalian Metallothionein-3: New Functional and Structural Insights

Metallothionein-3 (MT-3), a member of the mammalian metallothionein (MT) family, is mainly expressed in the central nervous system (CNS). MT-3 possesses a unique neuronal growth inhibitory activity, and the levels of this intra- and extracellularly occurring metalloprotein are markedly diminished in the brain of patients affected by a number of metal-linked neurodegenerative disorders, including Alzheimer’s disease (AD). In these pathologies, the redox cycling of copper, accompanied by the production of reactive oxygen species (ROS), plays a key role in the neuronal toxicity. Although MT-3 shares the metal-thiolate clusters with the well-characterized MT-1 and MT-2, it shows distinct biological, structural and chemical properties. Owing to its anti-oxidant properties and modulator function not only for Zn, but also for Cu in the extra- and intracellular space, MT-3, but not MT-1/MT-2, protects neuronal cells from the toxicity of various Cu(II)-bound amyloids. In recent years, the roles of zinc dynamics and MT-3 function in neurodegeneration are slowly emerging. This short review focuses on the recent developments regarding the chemistry and biology of MT-3.

Lead(II) Binding in Natural and Artificial Proteins

This article describes recent attempts to understand the biological chemistry of lead using a synthetic biology approach. Lead binds to a variety of different biomolecules ranging from enzymes to regulatory and signaling proteins to bone matrix. We have focused on the interactions of this element in thiolate-rich sites that are found in metalloregulatory proteins such as Pbr, Znt, and CadC and in enzymes such as δ-aminolevulinic acid dehydratase (ALAD). In these proteins, Pb(II) is often found as a homoleptic and hemidirectic Pb(II)(SR)3- complex. Using first principles of biophysics, we have developed relatively short peptides that can associate into three-stranded coiled coils (3SCCs), in which a cysteine group is incorporated into the hydrophobic core to generate a (cysteine)3 binding site. We describe how lead may be sequestered into these sites, the characteristic spectral features may be observed for such systems and we provide crystallographic insight on metal binding. The Pb(II)(SR)3- that is revealed within these α-helical assemblies forms a trigonal pyramidal structure (having an endo orientation) with distinct conformations than are also found in natural proteins (having an exo conformation). This structural insight, combined with 207Pb NMR spectroscopy, suggests that while Pb(II) prefers hemidirected Pb(II)(SR)3- scaffolds regardless of the protein fold, the way this is achieved within α-helical systems is different than in β-sheet or loop regions of proteins. These interactions between metal coordination preference and protein structural preference undoubtedly are exploited in natural systems to allow for protein conformation changes that define function. Thus, using a design approach that separates the numerous factors that lead to stable natural proteins allows us to extract fundamental concepts on how metals behave in biological systems.

Deciphering metal ion preference and primary coordination sphere robustness of a designed zinc finger with high-resolution mass spectrometry

Small zinc finger (ZnF) motifs are promising molecular scaffolds for protein design owing to their structural robustness and versatility. Moreover, their characterization provides important insights into protein folding in general. ZnF motifs usually possess an exceptional specificity and high affinity towards Zn(II) ion to drive folding. While the Zn(II) ion is canonically coordinated by two cysteine and two histidine residues, many other coordination spheres also exist in small ZnFs, all having four amino acid ligands. Here we used high-resolution mass spectrometry to study metal ion binding specificity and primary coordination sphere robustness of a designed zinc finger, named MM1. Based on the results, MM1 possesses high specificity for zinc with sub-micromolar binding affinity. Surprisingly, MM1 retains metal ion binding affinity even in the presence of selective alanine mutations of the primary zinc coordinating amino acid residues.

A Crystallographic Examination of Predisposition versus Preorganization in de Novo Designed Metalloproteins

Preorganization and predisposition are important molecular recognition concepts exploited by nature to obtain site-specific and selective metal binding to proteins. While native structures containing an MS3 core are often unavailable in both apo- and holo-forms, one can use designed three-stranded coiled coils (3SCCs) containing tris-thiolate sites to evaluate these concepts. We show that the preferred metal geometry dictates the degree to which the cysteine rotamers change upon metal complexation. The Cys ligands in the apo-form are preorganized for binding trigonal pyramidal species (Pb(II)S3 and As(III)S3) in an endo conformation oriented toward the 3SCC C-termini, whereas the cysteines are predisposed for trigonal planar Hg(II)S3 and 4-coordinate Zn(II)S3O structures, requiring significant thiol rotation for metal binding. This study allows assessment of the importance of protein fold and side-chain reorientation for achieving metal selectivity in human retrotransposons and metalloregulatory proteins.