Thermodynamic and Structural Investigation of Synthetic Actinide–Peptide Scaffolds

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.

Strong Uranium(VI) Binding onto Bovine Milk Proteins, Selected Protein Sequences, and Model Peptides

Hexavalent uranium is ubiquitous in the environment. In view of the chemical and radiochemical toxicity of uranium(VI), a good knowledge of its possible interactions in the environment is crucial. The aim of this work was to identify typical binding and sorption characteristics of uranium(VI) with both the pure bovine milk protein β-casein and diverse related protein mixtures (caseins, whey proteins). For comparison, selected model peptides representing the amino acid sequence 13-16 of β-casein and dephosphorylated β-casein were also studied. Complexation studies using potentiometric titration and time-resolved laser-induced fluorescence spectroscopy revealed that the phosphoryl-containing proteins form uranium(VI) complexes of higher stability than the structure-analog phosphoryl-free proteins. That is in agreement with the sorption experiments showing a significantly higher affinity of caseins toward uranium(VI) in comparison to whey proteins. On the other hand, the total sorption capacity of caseins is lower than that of whey proteins. The discussed binding behavior of milk proteins to uranium(VI) might open up interesting perspectives for sustainable techniques of uranium(VI) removal from aqueous solutions. This was further demonstrated by batch experiments on the removal of uranium(VI) from mineral water samples.

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.

Theoretical insight into the binding affinity enhancement of serine with the uranyl ion through phosphorylation

Various type complexes of uranyl ion with serine and phosphoserine were investigated which showed phosphorylation induced binding affinity enhancement of serine with the uranyl ion.

Interactions between uranium(vi) and phosphopeptide: experimental and theoretical investigations

A phosphorylated pentapeptide (WpTPpTW, P¹) motif was designed as a model to mimic possible U(vi) coordination sites of genuine phosphorylated proteins.