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

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.