Reversible Modulation of Glycan Recognition in C‐Type Lectins by Calcium/Lanthanide Exchange

C‐type lectins are Ca 2+ ‐dependent glycan‐binding proteins involved in immune recognition and host–pathogen interactions. Previous structural studies suggested that lanthanide ions can replace Ca 2+ while preserving protein structure, raising the possibility of exploiting paramagnetic lanthanides as NMR probes for studying glycan recognition. Here, we investigated Ca 2+ /Ln 3+ exchange in the immune lectins DC‐SIGN and MGL using T 2 ‐filtered 19 F‐NMR binding assays with monofluorinated monosaccharides, together with heteronuclear NMR analysis of 15 N‐labeled proteins. Chemical shift perturbations, pseudocontact shifts, and paramagnetic relaxation enhancements showed preferential replacement of the Ca 2+ ion at the glycan‐binding site, while the remaining Ca 2+ ‐binding sites were less susceptible to exchange. Unexpectedly, substitution with La 3+ or Yb 3+ suppresses carbohydrate binding in both lectins, despite preserving the folded structure of the carbohydrate‐recognition domains. Glycan recognition was restored upon EDTA‐mediated removal of lanthanides in excess Ca 2+ , demonstrating the reversibility of the process. These findings show that lanthanides do not necessarily behave as functional Ca 2+ mimetics in C‐type lectins but instead act as reversible disruptors of glycan recognition. Beyond highlighting the need to assess the functional consequences of Ca 2+ substitution, this work establishes Ca 2+ /Ln 3+ exchange as a strategy for controlled modulation of lectin activity and for developing new tools to investigate C‐type lectin‐mediated biological processes.