Abstact

Lanthanide-binding tag (LBT) peptides selectively complex lanthanide cations (Ln3+) in their binding pockets and are promising for lanthanide separation. However, designing LBTs that selectively target specific Ln3+ cations remains a challenge due to limited molecular-level understanding and control of interactions within the lanthanide-binding pocket. In this study, we reveal that the N5 asparagine residue acts as a gatekeeper in the binding pocket, resulting in a 100-fold selectivity for smaller Lu3+ over larger La3+ cations. Nuclear magnetic resonance spectroscopy and molecular dynamics simulations show that the N5 residue weakly binds to the larger La3+ cation, permitting H2O molecules inside the pocket. For the smaller Lu3+ cations, the N5 residue forms an inter-arm hydrogen bond with the E14 glutamic acid residue, locking the Lu3+ cation in the pocket and preventing H2O infiltration. Mutating the N5 asparagine to a D5 aspartic acid prevents such a hydrogen bond, eliminating the gatekeeping mechanism and precipitously reducing selectivity. The resulting binding affinity to Ln3+ cations is non-monotonic but generally increases with cation size. These results suggest a molecular design paradigm: the reduced affinity for larger lanthanides is due to open pocket conformations, while the selectivity of smaller Ln3+ cations over larger ones is due to the gatekeeping hydrogen bond.