Determining the selectivity of a tetra-phosphorylated biomimetic peptide towards uranium in the presence of competing cations through the simultaneous coupling of HILIC to ESI-MS and ICP-MS

A cyclic tetra-phosphorylated biomimetic peptide (pS1368) has been proposed as a promising starting structure to design a decorporating agent of uranyl (UO22+) due to its affinity being similar to that of osteopontin (OPN), a target UO22+ protein in vivo. The determination of this peptide's selectivity towards UO22+ in the presence of competing endogenous elements is also crucial to validate this hypothesis. In this context, the selectivity of pS1368 towards UO22+ in the presence of Ca2+, Cu2+ and Zn2+ was determined by applying the simultaneous coupling of hydrophilic interaction chromatography (HILIC) to electrospray ionization (ESI-MS) and inductively coupled plasma (ICP-MS) mass spectrometry. Sr2+ was used as Ca2+ simulant, providing less challenging ICP-MS measurements. The separation of the complexes by HILIC was first set up. The selectivity of pS1368 towards UO22+ was determined in the presence of Sr2+, by adding several proportions of the latter to UO2(pS1368). UO22+ was not displaced from UO2(pS1368) even in the presence of a ten-fold excess of Sr2+. The same approach has been undertaken to demonstrate the selectivity of pS1368 towards UO22+ in the presence of Cu2+, Zn2+ and Sr2+ as competing endogenous cations. Hence, we showed that pS1368 was selective towards UO22+ in the presence of Sr2+, but also in the presence of Cu2+ and Zn2+. This study highlights the performance of HILIC-ESI-MS/ICP-MS simultaneous coupling to assess the potential of molecules as decorporating agents of UO22+.

A Series of Ni Complexes Based on a Versatile ATCUN-Like Tripeptide Scaffold to Decipher Key Parameters for Superoxide Dismutase Activity

The cellular level of reactive oxygen species (ROS) has to be controlled to avoid some pathologies, especially those linked to oxidative stress. One strategy for designing antioxidants consists of modeling natural enzymes involved in ROS degradation. Among them, nickel superoxide dismutase (NiSOD) catalyzes the dismutation of the superoxide radical anion, O₂^•-, into O₂ and H₂O₂. We report here Ni complexes with tripeptides derived from the amino-terminal Cu^II- and Ni^II-binding (ATCUN) motif that mimics some structural features found in the active site of the NiSOD. A series of six mononuclear Ni^II complexes were investigated in water at physiological pH with different first coordination spheres, from compounds with a N3S to N2S2 set, and also complexes that are in equilibrium between the N-coordination (N3S) and S-coordination (N2S2). They were fully characterized by a combination of spectroscopic techniques, including ¹H NMR, UV-vis, circular dichroism, and X-ray absorption spectroscopy, together with theoretical calculations and their redox properties studied by cyclic voltammetry. They all display SOD-like activity, with a k_cat ranging between 0.5 and 2.0 × 10⁶ M^-1 s^-1. The complexes in which the two coordination modes are in equilibrium are the most efficient, suggesting a beneficial effect of a nearby proton relay.

Tristhiolato Pseudopeptides Bind Arsenic(III) in an AsS3 Coordination Environment Imitating Metalloid Binding Sites in Proteins

The AsIII binding of two NTA-based tripodal pseudopeptides, possessing three cysteine (ligand L1) or d-penicillamine residues (ligand L2) as potential coordinating groups for soft semimetals or metal ions, was studied by experimental (UV, CD, NMR, and ESI-MS) and theoretical (DFT) methods. All of the experimental data, obtained with the variation of the AsIII:ligand concentration ratios or pH values in some instances, evidence the exclusive formation of species with an AsS3-type coordination mode. The UV-monitored titration of the ligands with arsenous acid at pH = 7.0 provided an absorbance data set that allowed for the determination of apparent stability constants of the forming species. The obtained stabilities (logK' = 5.26 (AsL1) and logK' = 3.04 (AsL2)) reflect high affinities, especially for the sterically less restricted cysteine derivative. DFT calculated structures correlate well with the spectroscopic results and, in line with the 1H NMR data, indicate a preference for the all-endo conformers resembling the AsIII environment at the semimetal binding sites in various metalloproteins.

The plasma membrane-associated cation-binding protein PCaP1 of Arabidopsis thaliana is a uranyl-binding protein

Uranium (U) is a naturally-occurring radionuclide that is toxic to living organisms. Given that proteins are primary targets of U(VI), their identification is an essential step towards understanding the mechanisms of radionuclide toxicity, and possibly detoxification. Here, we implemented a chromatographic strategy including immobilized metal affinity chromatography to trap protein targets of uranyl in Arabidopsis thaliana. This procedure allowed the identification of 38 uranyl-binding proteins (UraBPs) from root and shoot extracts. Among them, UraBP25, previously identified as plasma membrane-associated cation-binding protein 1 (PCaP1), was further characterized as a protein interacting in vitro with U(VI) and other metals using spectroscopic and structural approaches, and in planta through analyses of the fate of U(VI) in Arabidopsis lines with altered PCaP1 gene expression. Our results showed that recombinant PCaP1 binds U(VI) in vitro with affinity in the nM range, as well as Cu(II) and Fe(III) in high proportions, and that Ca(II) competes with U(VI) for binding. U(VI) induces PCaP1 oligomerization through binding at the monomer interface, at both the N-terminal structured domain and the C-terminal flexible region. Finally, U(VI) translocation in Arabidopsis shoots was affected in pcap1 null-mutant, suggesting a role for this protein in ion trafficking in planta.

Determination of the affinity of biomimetic peptides for uranium through the simultaneous coupling of HILIC to ESI-MS and ICP-MS

Several proteins have been identified in the past decades as targets of uranyl (UO₂²⁺) in vivo. However, the molecular interactions responsible for this affinity are still poorly known which requires the identification of the UO₂²⁺ coordination sites in these proteins. Biomimetic peptides are efficient chemical tools to characterize these sites. In this work, we developed a dedicated analytical method to determine the affinity of biomimetic, synthetic, multi-phosphorylated peptides for UO₂²⁺ and evaluate the effect of several structural parameters of these peptides on this affinity at physiological pH. The analytical strategy was based on the implementation of the simultaneous coupling of hydrophilic interaction chromatography (HILIC) with electrospray ionization mass spectrometry (ESI-MS) and inductively coupled plasma mass spectrometry (ICP-MS). An essential step had been devoted to the definition of the best separation conditions of UO₂²⁺ complexes formed with di-phosphorylated peptide isomers and also with peptides of different structure and degrees of phosphorylation. We performed the first separations of several sets of UO₂²⁺ complexes by HILIC ever reported in the literature. A dedicated method had then been developed for identifying the separated peptide complexes online by ESI-MS and simultaneously quantifying them by ICP-MS, based on uranium quantification using external calibration. Thus, the affinity of the peptides for UO₂²⁺ was determined and made it possible to demonstrate that (i) the increasing number of phosphorylated residues (pSer) promotes the affinity of the peptides for UO₂²⁺, (ii) the position of the pSer in the peptide backbone has very low impact on this affinity (iii) and finally the cyclic structure of the peptide favors the UO₂²⁺ complexation in comparison with the linear structure. These results are in agreement with those previously obtained by spectroscopic techniques, which allowed to validate the method. Through this approach, we obtained essential information to better understand the mechanisms of toxicity of UO₂²⁺ at the molecular level and to further develop selective decorporating agents by chelation.

A Simple Fluorescence Affinity Assay to Decipher Uranyl-Binding to Native Proteins

Determining the affinity of proteins for uranyl is key to understand the toxicity of this cation and to further develop decorporation strategies. However, usual techniques to achieve that goal often require specific equipment and expertise. Here, we propose a simple, efficient, fluorescence‐based method to assess the affinity of proteins and peptides for uranyl, at equilibrium and in buffered solution. We first designed and characterized an original uranyl‐binding fluorescent probe. We then built a reference scale for uranyl affinity in solution, relying on signal quenching of our fluorescent probe in presence of high‐affinity uranyl‐binding peptides. We finally validated our approach by re‐evaluating the uranyl‐binding affinity of four native proteins. We envision that this tool will facilitate the reliable and reproducible assessment of affinities of peptides and proteins for uranyl.

A Bioinspired NiII Superoxide Dismutase Catalyst Designed on an ATCUN-like Binding Motif

Nickel superoxide dismutase (NiSOD) is an enzyme that protects cells against O₂^·-. While the structure of its active site is known, the mechanism of the catalytic cycle is still not elucidated. Its active site displays a square planar Ni^II center with two thiolates, the terminal amine and an amidate. We report here a bioinspired Ni^II complex built on an ATCUN-like binding motif modulated with one cysteine, which demonstrates catalytic SOD activity in water (k_cat = 8.4(2) × 10⁵ M^-1 s^-1 at pH = 8.1). Its reactivity with O₂^·- was also studied in acetonitrile allowing trapping two different short-lived species that were characterized by electron paramagnetic resonance or spectroelectrochemistry and a combination of density functional theory (DFT) and time-dependent DFT calculations. Based on these observations, we propose that O₂^·- interacts first with the complex outer sphere through a H-bond with the peptide scaffold in a [Ni^IIO₂^·-] species. This first species could then evolve into a Ni^III hydroperoxo inner sphere species through a reaction driven by protonation that is thermodynamically highly favored according to DFT calculations.

Separation of multiphosphorylated cyclopeptides and their positional isomers by hydrophilic interaction liquid chromatography (HILIC) coupled to electrospray ionization mass spectrometry (ESI-MS)

Peptides are efficient models used in different fields such as toxicology to study the interactions of several contaminants at the molecular scale, requiring the development of bio-analytical strategies. In this context, Hydrophilic interaction liquid chromatography (HILIC) coupled to electrospray ionization mass spectrometry (ESI-MS) was used to separate synthetic multiphosphorylated cyclopeptides and their positional isomers at physiological pH. We assessed (i) the selectivity of eleven HILIC columns, from different manufacturers and packed with diverse polar sorbents, and (ii) the effect of mobile phase composition on the separation selectivity. The best selectivity and baseline resolution were achieved with the columns grafted by neutral sorbents amide and diol. Furthermore, we investigated the HILIC retention mechanism of these peptides by examining the effect of the number of phosphorylated residues in the peptide scaffold on their retention. The peptide behavior followed the classical hydrophilic partitioning mechanism exclusively on amide and diol columns. This trend was not fully respected on bare and hybrid silica due to the attractive/repulsive interactions of the deprotonated surface silanol groups with the Arginine or Glutamate residues in the peptide scaffold according to the peptide sequence. The position of the phosphorylated amino acid in the peptide backbone also showed to have an impact on the retention, making possible the separation of positional isomers of these multiphosphorylated cyclic peptides using HILIC.

Lectin recognition and hepatocyte endocytosis of GalNAc-decorated nanostructured lipid carriers

Liver is the main organ for metabolism but is also subject to various pathologies, from viral, genetic, cancer or metabolic origin. There is thus a crucial need to develop efficient liver-targeted drug delivery strategies. Asialoglycoprotein receptor (ASGPR) is a C-type lectin expressed in the hepatocyte plasma membrane that efficiently endocytoses glycoproteins exposing galactose (Gal) or N-acetylgalactosamine (GalNAc). Its targeting has been successfully used to drive the uptake of small molecules decorated with three or four GalNAc, thanks to an optimisation of their spatial arrangement. Herein, we assessed the biological properties of highly stable nanostructured lipid carriers (NLC) made of FDA-approved ingredients and formulated with increasing amounts of GalNAc. Cellular studies showed that a high density of GalNAc was required to favour hepatocyte internalisation via the ASGPR pathway. Interaction studies using surface plasmon resonance and the macrophage galactose-lectin as GalNAc-recognising lectin confirmed the need of high GalNAc density for specific recognition of these NLC. This work is the first step for the development of efficient nanocarriers for prolonged liver delivery of active compounds.

Safer-by-design biocides made of tri-thiol bridged silver nanoparticle assemblies

Silver nanoparticles (AgNPs) are efficient biocides increasingly used in consumer products and medical devices. Their activity is due to their capacity to release bioavailable Ag(i) ions making them long-lasting biocides but AgNPs themselves are usually easily released from the product. Besides, AgNPs are highly sensitive to various chemical environments that triggers their transformation, decreasing their activity. Altogether, widespread use of AgNPs leads to bacterial resistance and safety concerns for humans and the environment. There is thus a crucial need for improvement. Herein, a proof of concept for a novel biocide based on AgNP assemblies bridged together by a tri-thiol bioinspired ligand is presented. The final nanomaterial is stable and less sensitive to chemical environments with AgNPs completely covered by organic molecules tightly bound via their thiol functions. Therefore, these AgNP assemblies can be considered as safer-by-design and innovative biocides, since they deliver a sufficient amount of Ag(i) for biocidal activity with no release of AgNPs, which are insensitive to transformations in the nanomaterial.

A liver-targeting Cu(i) chelator relocates Cu in hepatocytes and promotes Cu excretion in a murine model of Wilson's disease

A hepatocyte-targeting chelator promotes Cu biliary excretion, hence restoring the physiological Cu detoxification pathway in a murine Wilson's disease model.

Recent advances in uranyl binding in proteins thanks to biomimetic peptides

Uranium is an element belonging to the actinide series. It is ubiquitous in rock, soil, and water. Uranium is found in the ecosystem due to mining and milling industrial activities and processing to nuclear fuel, but also to the extensive use of phosphate fertilizers. Understanding uranium binding in vivo is critical, first to deepen our knowledge of molecular events leading to chemical toxicity, but also to provide new mechanistic information useful for the development of efficient decorporation treatments to be applied in case of intoxication. The most stable form in physiological conditions is the uranyl cation (UO₂²⁺), in which uranium oxidation state is +VI. This short review presents uranyl coordination properties and chelation, and what is currently known about uranium binding to proteins. Although several target proteins have been identified, the UO₂²⁺ binding sites have barely been identified. Biomimetic approaches using model peptides are good options to shed light on high affinity uranyl binding sites in proteins. A strategy based on constrained cyclodecapeptides allowed recently to propose a tetraphosphate binding site for uranyl that provides an affinity similar to the one measured with the phosphoprotein osteopontin.

Quantification of Surface GalNAc Ligands Decorating Nanostructured Lipid Carriers by UPLC-ELSD

Nanoparticles have been extensively studied for drug delivery and targeting to specific organs. The functionalization of the nanoparticle surface by site-specific ligands (antibodies, peptides, saccharides) can ensure efficient recognition and binding with relevant biological targets. One of the main challenges in the development of these decorated nanocarriers is the accurate quantification of the amount of ligands on the nanoparticle surface. In this study, nanostructured lipid carriers (NLC) were functionalized with N-acetyl-D-galactosamine (GalNAc) units, known to target the asialoglycoprotein receptor (ASGPR). Different molar percentages of GalNAc-functionalized surfactant (0%, 2%, 5%, and 14%) were used in the formulation. Based on ultra-high-performance liquid chromatography separation and evaporative light-scattering detection (UPLC-ELSD), an analytical method was developed to specifically quantify the amount of GalNAc units present at the NLC surface. This method allowed the accurate quantification of GalNAc surfactant and therefore gave some insights into the structural parameters of these multivalent ligand systems. Our data show that the GalNAc decorated NLC possess large numbers of ligands at their surface and suitable distances between them for efficient multivalent interaction with the ASGPR, and therefore promising liver-targeting efficiency.

Mononuclear Ni(II) Complexes with a S3O Coordination Sphere Based on a Tripodal Cysteine-Rich Ligand: pH Tuning of the Superoxide Dismutase Activity

The superoxide dismutase (SOD) activity of mononuclear Ni^II complexes, whose structures are inspired by the NiSOD, has been investigated. They have been designed with a sulfur-rich pseudopeptide ligand, derived from nitrilotriacetic acid (NTA), where the three acid functions are grafted with cysteines (L^3S). Two mononuclear complexes, which exist in pH-dependent proportions, have been fully characterized by a combination of spectroscopic techniques including ¹H NMR, UV-vis, circular dichroism, and X-ray absorption spectroscopy, together with theoretical calculations. They display similar square-planar S3O coordination, with the three thiolates of the three cysteine moieties from L^3S coordinated to the Ni^II ion, together with either a water molecule at physiological pH, as [NiL^3S(OH₂)]^-, or a hydroxo ion in more basic conditions, as [NiL^3S(OH)]^2-. The ¹H NMR study has revealed that contrary to the hydroxo ligand, the bound water molecule is labile. The cyclic voltammogram of both complexes displays an irreversible one-electron oxidation process assigned to the Ni^II/Ni^III redox system with E_pa = 0.48 and 0.31 V versus SCE for NiL^3S(OH₂) and NiL^3S(OH), respectively. The SOD activity of both complexes has been tested. On the basis of the xanthine oxidase assay, an IC₅₀ of about 1 μM has been measured at pH 7.4, where NiL^3S(OH₂) is mainly present (93% of the Ni^II species), while the IC₅₀ is larger than 100 μM at pH 9.6, where NiL^3S(OH) is the major species (92% of the Ni^II species). Interestingly, only NiL^3S(OH₂) displays SOD activity, suggesting that the presence of a labile ligand is required. The SOD activity has been also evaluated under catalytic conditions at pH 7.75, where the ratio between NiL^3S(OH₂)/ NiL^3S(OH) is about (86:14), and a rate constant, k_cat = 1.8 × 10⁵ M^-1 s^-1, has been measured. NiL^3S(OH₂) is thus the first low-molecular weight, synthetic, bioinspired Ni complex that displays catalytic SOD activity in water at physiological pH, although it does not contain any N-donor ligand in its first coordination sphere, as in the NiSOD. Overall, the data show that a key structural feature is the presence of a labile ligand in the coordination sphere of the Ni^II ion.

Citrulline stimulates muscle protein synthesis, by reallocating ATP consumption to muscle protein synthesis

BACKGROUND

Animal studies and clinical data support the interest of citrulline as a promising therapeutic for sarcopenia. Citrulline is known to stimulate muscle protein synthesis, but how it affects energy metabolism to support the highly energy-dependent protein synthesis machinery is poorly understood.

METHODS

Here, we used myotubes derived from primary culture of mouse myoblasts to study the effect of citrulline on both energy metabolism and protein synthesis under different limiting conditions.

RESULTS

When serum/amino acid deficiency or energy stress (mild uncoupling) were applied, citrulline stimulated muscle protein synthesis by +22% and +11%, respectively. Importantly, this increase was not associated with enhanced energy status (ATP/ADP ratio) or mitochondrial respiration. We further analysed the share of mitochondrial respiration and thus of generated ATP allocated to different metabolic pathways by using specific inhibitors. Our results indicate that addition of citrulline allocated an increased share of mitochondrially generated ATP to the protein synthesis machinery under conditions of both serum/amino acid deficiency (+28%) and energy stress (+21%). This reallocation was not because of reduced ATP supply to DNA synthesis or activities of sodium and calcium cycling ion pumps.

CONCLUSIONS

Under certain stress conditions, citrulline increases muscle protein synthesis by specifically reallocating mitochondrial fuel to the protein synthesis machinery. Because ATP/ADP ratios and thus Gibbs free energy of ATP hydrolysis remained globally constant, this reallocation may be linked to decreased activation energies of one or several ATP (and GTP)-consuming reactions involved in muscle protein synthesis.

Phosphate-Rich Biomimetic Peptides Shed Light on High-Affinity Hyperphosphorylated Uranyl Binding Sites in Phosphoproteins

Some phosphoproteins such as osteopontin (OPN) have been identified as high-affinity uranyl targets. However, the binding sites required for interaction with uranyl and therefore involved in its toxicity have not been identified in the whole protein. The biomimetic approach proposed here aimed to decipher the nature of these sites and should help to understand the role of the multiple phosphorylations in UO₂ ²⁺ binding. Two hyperphosphorylated cyclic peptides, pS₁₆₈ and pS₁₃₆₈ containing up to four phosphoserine (pSer) residues over the ten amino acids present in the sequences, were synthesized with all reactions performed in the solid phase, including post-phosphorylation. These β-sheet-structured peptides present four coordinating residues from four amino acid side chains pointing to the metal ion, either three pSer and one glutamate in pS₁₆₈ or four pSer in pS₁₃₆₈ . Significantly, increasing the number of pSer residues up to four in the cyclodecapeptide scaffolds produced molecules with an affinity constant for UO₂ ²⁺ that is as large as that reported for osteopontin at physiological pH. The phosphate-rich pS₁₃₆₈ can thus be considered a relevant model of UO₂ ²⁺ coordination in this intrinsically disordered protein, which wraps around the metal ion to gather four phosphate groups in the UO₂ ²⁺ coordination sphere. These model hyperphosphorylated peptides are highly selective for UO₂ ²⁺ with respect to endogenous Ca²⁺ , which makes them good starting structures for selective UO₂ ²⁺ complexation.

Mercury Trithiolate Binding (HgS3) to a de Novo Designed Cyclic Decapeptide with Three Preoriented Cysteine Side Chains

Mercury(II) is an unphysiological soft ion with high binding affinity for thiolate ligands. Its toxicity lies in the interactions with low molecular weight thiols including glutathione and cysteine-containing proteins that disrupt the thiol balance and alter vital functions. However, mercury can also be detoxified via interactions with Hg(II)-responsive regulatory proteins such as MerR, which coordinates Hg(II) with three cysteine residues in a trigonal planar fashion (HgS₃ coordination). The model cyclodecapeptide P^3C, c(GCTCSGCSRP) was designed to promote Hg(II) chelation in a HgS₃ coordination environment through the parallel orientation of three cysteine side chains. The binding motif is derived from the dicysteine P^2C cyclodecapeptide validated previously as a model for d¹⁰ metal transporters containing the binding sequence CxxC. The formation of the mononuclear HgP^3C complex with a HgS₃ coordination is demonstrated using electrospray ionization mass spectrometry, UV absorption, and ¹⁹⁹Hg NMR. Hg L_III-edge extended X-ray absorption fine structure (EXAFS) spectroscopy indicates that the Hg(II) coordination environment is T-shaped with two short Hg-S distances at 2.45 Å and one longer distance at 2.60 Å. The solution structure of the HgP^3C complex was refined based on ¹H-¹H NMR constraints and EXAFS results. The cyclic peptide scaffold has a rectangular shape with the three binding cysteine side chains pointing toward Hg(II). The HgP^3CH complex has a p K_a of 4.3, indicating that the HgS₃ coordination mode is stable over a large range of pH. This low p K_a value suggests that the preorientation of the three cysteine groups is particularly well-achieved for Hg(II) trithiolate coordination in P^3C.

Short oligopeptides with three cysteine residues as models of sulphur-rich Cu(i)- and Hg(ii)-binding sites in proteins

Peptides mimicking sulphur-rich fragments found in metallothioneins display unexpectedly different behaviours with the two metal ions Hg(ii) and Cu(i).

A Trishistidine Pseudopeptide with Ability to Remove Both CuΙ and CuΙΙ from the Amyloid-β Peptide and to Stop the Associated ROS Formation

The pseudopeptide L, derived from a nitrilotriacetic acid scaffold and functionalized with three histidine moieties, is reminiscent of the amino acid side chains encountered in the Alzheimer's peptide (Aβ). Its synthesis and coordination properties for CuΙ and CuΙΙ are described. L efficiently complex CuΙΙ in a square-planar geometry involving three imidazole nitrogen atoms and an amidate-Cu bond. By contrast, CuΙ is coordinated in a tetrahedral environment. The redox behavior is irreversible and follows an ECEC mechanism in accordance with the very different environments of the two redox states of the Cu center. This is in line with the observed resistance of the CuΙ complex to oxidation by oxygen and the CuΙΙ complex reduction by ascorbate. The affinities of L for CuΙΙ and CuΙ at physiological pH are larger than that reported for the Aβ peptide. Therefore, due to its peculiar Cu coordination properties, the ligand L is able to target both redox states of Cu, redox silence them and prevent reactive oxygen species production by the CuAβ complex. Because reactive oxygen species contribute to the oxidative stress, a key issue in Alzheimer's disease, this ligand thus represents a new strategy in the long route of finding molecular concepts for fighting Alzheimer's disease.

Cyclic Phosphopeptides to Rationalize the Role of Phosphoamino Acids in Uranyl Binding to Biological Targets

The specific molecular interactions responsible for uranium toxicity are not yet understood. The uranyl binding sites in high-affinity target proteins have not been identified yet and the involvement of phosphoamino acids is still an important question. Short cyclic peptide sequences, with three glutamic acids and one phosphoamino acid, are used as simple models to mimic metal binding sites in phosphoproteins and to help understand the mechanisms involved in uranium toxicity. A combination of peptide design and synthesis, analytical chemistry, extended X-ray absorption fine structure (EXAFS) spectroscopy, and DFT calculations demonstrates the involvement of the phosphate group in the uranyl coordination sphere together with the three carboxylates of the glutamate moieties. The affinity constants measured with a reliable analytical competitive approach at physiological pH are significantly enhanced owing to the presence of the phosphorous moiety. These findings corroborate the importance of phosphoamino acids in uranyl binding in proteins and the relevance of considering phosphoproteins as potential uranyl targets in vivo.

Visualization, quantification and coordination of Ag+ ions released from silver nanoparticles in hepatocytes

Silver nanoparticles (AgNPs) can enter eukaryotic cells and exert toxic effects, most probably as a consequence of the release of Ag⁺ ions. Due to the elusive nature of Ag⁺ ionic species, quantitative information concerning AgNP intracellular dissolution is missing. By using a synchrotron nanoprobe, silver is visualized and quantified in hepatocytes (HepG2) exposed to AgNPs; the synergistic use of electron microscopy allows for the discrimination between nanoparticular and ionic forms of silver within a single cell. AgNPs are located in endocytosis vesicles, while the visualized Ag⁺ ions diffuse in the cell. The averaged NP dissolution rates, measured by X-ray absorption spectroscopy, highlight the faster dissolution of citrate-coated AgNPs with respect to the less toxic PVP-coated AgNPs; these results are confirmed at the single-cell level. The released Ag⁺ ions recombine with thiol-bearing biomolecules: the Ag-S distances measured in cellulo, and the quantitative evaluation of gene expression, provide independent evidence of the involvement of glutathione and metallothioneins in Ag⁺ binding. The combined use of cutting-edge imaging techniques, atomic spectroscopy and molecular biology brings insight into the fate of AgNPs in hepatocytes, and more generally into the physicochemical transformations of metallic nanoparticles in biological environments and the resulting disruption of metal homeostasis.

Preorganized Peptide Scaffolds as Mimics of Phosphorylated Proteins Binding Sites with a High Affinity for Uranyl

Cyclic peptides with two phosphoserines and two glutamic acids were developed to mimic high-affinity binding sites for uranyl found in proteins such as osteopontin, which is believed to be a privileged target of this ion in vivo. These peptides adopt a β-sheet structure that allows the coordination of the latter amino acid side chains in the equatorial plane of the dioxo uranyl cation. Complementary spectroscopic and analytical methods revealed that these cyclic peptides are efficient uranyl chelating peptides with a large contribution from the phosphorylated residues. The conditional affinity constants were measured by following fluorescence tryptophan quenching and are larger than 10(10) at physiological pH. These compounds are therefore promising models for understanding uranyl chelation by proteins, which is relevant to this actinide ion toxicity.

Exposure-dependent Ag+ release from silver nanoparticles and its complexation in AgS2 sites in primary murine macrophages

Silver nanoparticle (AgNP) toxicity is related to their dissolution in biological environments and to the binding of the released Ag(+) ions in cellulo; the chemical environment of recombined Ag(+) ions is responsible for their toxicological outcome, moreover it is indicative of the cellular response to AgNP exposure, and can therefore shed light on the mechanisms governing AgNP toxicity. This study probes the chemistry of Ag species in primary murine macrophages exposed to AgNPs by making use of X-ray Absorption Fine Structure spectroscopy under cryogenic conditions: the linear combination analysis of the near-edge region of the spectra provides the fraction of Ag(+) ions released from the AgNPs under a given exposure condition and highlights their complexation with thiolate groups; the ab initio modelling of the extended spectra allows measuring the Ag-S bond length in cellulo. Dissolution rates depend on the exposure scenario, chronicity leading to higher Ag(+) release than acute exposure; Ag-S bond lengths are 2.41 ± 0.03 Å and 2.38 ± 0.01 Å in acute and chronic exposure respectively, compatible with digonal AgS2 coordination. Glutathione is identified as the most likely putative ligand for Ag(+). The proposed method offers a scope for the investigation of metallic nanoparticle dissolution and recombination in cellular models.

Modulating uranium binding affinity in engineered calmodulin EF-hand peptides: effect of phosphorylation

To improve our understanding of uranium toxicity, the determinants of uranyl affinity in proteins must be better characterized. In this work, we analyzed the contribution of a phosphoryl group on uranium binding affinity in a protein binding site, using the site 1 EF-hand motif of calmodulin. The recombinant domain 1 of calmodulin from A. thaliana was engineered to impair metal binding at site 2 and was used as a structured template. Threonine at position 9 of the loop was phosphorylated in vitro, using the recombinant catalytic subunit of protein kinase CK2. Hence, the T₉TKE₁₂ sequence was substituted by the CK2 recognition sequence TAAE. A tyrosine was introduced at position 7, so that uranyl and calcium binding affinities could be determined by following tyrosine fluorescence. Phosphorylation was characterized by ESI-MS spectrometry, and the phosphorylated peptide was purified to homogeneity using ion-exchange chromatography. The binding constants for uranyl were determined by competition experiments with iminodiacetate. At pH 6, phosphorylation increased the affinity for uranyl by a factor of ∼5, from K_d = 25±6 nM to K_d = 5±1 nM. The phosphorylated peptide exhibited a much larger affinity at pH 7, with a dissociation constant in the subnanomolar range (K_d = 0.25±0.06 nM). FTIR analyses showed that the phosphothreonine side chain is partly protonated at pH 6, while it is fully deprotonated at pH 7. Moreover, formation of the uranyl-peptide complex at pH 7 resulted in significant frequency shifts of the ν_as(P-O) and ν_s(P-O) IR modes of phosphothreonine, supporting its direct interaction with uranyl. Accordingly, a bathochromic shift in ν_as(UO₂)²⁺ vibration (from 923 cm⁻¹ to 908 cm⁻¹) was observed upon uranyl coordination to the phosphorylated peptide. Together, our data demonstrate that the phosphoryl group plays a determining role in uranyl binding affinity to proteins at physiological pH.

Chelation therapy in Wilson's disease: from D-penicillamine to the design of selective bioinspired intracellular Cu(I) chelators

Wilson's disease is an orphan disease due to copper homeostasis dysfunction. Mutations of the ATP7B gene induces an impaired functioning of a Cu-ATPase, impaired Cu detoxification in the liver and copper overload in the body. Indeed, even though copper is an essential element, which is used as cofactor by many enzymes playing vital roles, it becomes toxic when in excess as it promotes cytotoxic reactions leading to oxidative stress. In this perspective, human copper homeostasis is first described in order to explain the mechanisms promoting copper overload in Wilson's disease. We will see that the liver is the main organ for copper distribution and detoxification in the body. Nowadays this disease is treated life-long by systemic chelation therapy, which is not satisfactory in many cases. Therefore the design of more selective and efficient drugs is of great interest. A strategy to design more specific chelators to treat localized copper accumulation in the liver will then be presented. In particular we will show how bioinorganic chemistry may help in the design of such novel chelators by taking inspiration from the biological copper cell transporters.

Hepatocyte targeting and intracellular copper chelation by a thiol-containing glycocyclopeptide

Metal overload plays an important role in several diseases or intoxications, like in Wilson's disease, a major genetic disorder of copper metabolism in humans. To efficiently and selectively decrease copper concentration in the liver that is highly damaged, chelators should be targeted at the hepatocytes. In the present work, we synthesized a molecule able to both lower intracellular copper, namely Cu(I), and target hepatocytes, combining within the same structure a chelating unit and a carbohydrate recognition element. A cyclodecapeptide scaffold displaying a controlled conformation with two independent faces was chosen to introduce both units. One face displays a cluster of carbohydrates to ensure an efficient recognition of the asialoglycoprotein receptors, expressed on the surface of hepatocytes. The second face is devoted to metal ion complexation thanks to the thiolate functions of two cysteine side-chains. To obtain a chelator that is active only once inside the cells, the two thiol functions were oxidized in a disulfide bridge to afford the glycopeptide P(3). Two simple cyclodecapeptides modeling the reduced and complexing form of P(3) in cells proved a high affinity for Cu(I) and a high selectivity with respect to Zn(II). As expected, P(3) becomes an efficient Cu(I) chelator in the presence of glutathione that mimics the intracellular reducing environment. Finally, cellular uptake and ability to lower intracellular copper were demonstrated in hepatic cell lines, in particular in WIF-B9, making P(3) a good candidate to fight copper overload in the liver.