Cell surface glycans are biochemical signals for communication with the environment. They are translated into biological responses by first binding to lectins, this recognition process thereupon triggering ensuing post-binding signaling. The specificity of this interaction must necessarily be exquisite to preclude errors. Toward this end Ca
2+ is recruited to the realm of protein-carbohydrate interactions. Intriguingly, not one but different strategies how Ca
2+ assists in singling out the correct sugar ligands are operative. This review dissects the different ways Ca
2+ functions as an integral component of various lectins’ carbohydrate-binding domains. It spans the range from the role of Ca
2+ in structurally organizing the contact site without direct interaction to the ligand to interplays with charged and even neutral ligands. Finally, a case of an intricate network of up to four coordination bonds of Ca
2+ with a galactose moiety is presented.
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