FIGURE 4.

Ca²⁺ binding to derCD23 as seen in three of the four molecules in the asymmetric unit (molecules A, B, and C). Ca²⁺ binding to the principal binding site is as follows: three residues (Glu-249, Thr-251, and Asp-270) provide four oxygen atoms, and three water molecules provide three oxygen atoms (black dashed lines). Asn-269 is not directly involved in the binding of Ca²⁺, but it forms a H-bond with a ligating water molecule (gray dashed line). The auxiliary site residues Ser-252 and Asp-258 are distant from one another.