The How and Why of Ca2+ Involvement in Lectin Activity

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 Ca2+ is recruited to the realm of protein-carbohydrate interactions. Intriguingly, not one but different strategies how Ca2+ assists in singling out the correct sugar ligands are operative. This review dissects the different ways Ca2+ functions as an integral component of various lectins' carbohydrate-binding domains. It spans the range from the role of Ca2+ 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 Ca2+ with a galactose moiety is presented.