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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 KpH7 = 9.55
± 0.11 (KdpH7 = 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