Zinc binding in S100 proteins. A. Comparison of zinc binding sites between S100A15 and S100A7 (upper panel), S100B (middle panel) and S100A12 (lower panel). Note the unique strategy employed in S100A15 to incorporate an exogenous ligand to complete the distorted coordination geometry of the bound zinc. B. Structural divergence between S100A15 and S100A7 localized to the predicted receptor-binding surface highlighted by the black arrows representing the three divergent residues. Note the decreased acidity in S100A15.