Optimized Coordination of Uranyl in Engineered Calmodulin Site 1 Provides a Subnanomolar Affinity for Uranyl and a Strong Uranyl versus Calcium Selectivity

As an alpha emitter and chemical toxicant, uranium toxicity in living organisms is driven by its molecular interactions. It is therefore essential to identify main determinants of uranium affinity for proteins. Others and we showed that introducing a phosphoryl group in the coordination sphere of uranyl confers a strong affinity of proteins for uranyl. In this work, using calmodulin site 1 as a template, we modulate the structural organization of a metal-binding loop comprising carboxylate and/or carbonyl ligands and reach affinities for uranyl comparable to that provided by introducing a strong phosphoryl ligand. Shortening the metal binding loop of calmodulin site 1 from 12 to 10 amino acids in CaMΔ increases the uranyl-binding affinity by about 2 orders of magnitude to log K_pH7 = 9.55 ± 0.11 (K_dpH7 = 280 ± 60 pM). Structural analysis by FTIR, XAS, and molecular dynamics simulations suggests an optimized coordination of the CaMΔ-uranyl complex involving bidentate and monodentate carboxylate groups in the uranyl equatorial plane. The main role of this coordination sphere in reaching subnanomolar dissociation constants for uranyl is supported by similar uranyl affinities obtained in a cyclic peptide reproducing CaMΔ binding loop. In addition, CaMΔ presents a uranyl/calcium selectivity of 10⁷ that is even higher in the cyclic peptide.