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This is the structure of the lectin + EGF domains of E-selectin, bound to calcium and a tetrasaccharide (Protein Data Bank structure 1G1T) . The calcium ion (yellow sphere) binds to Glu-80, Asn-82, Asn-83, Asn-105, and Asp-106, forcing the protein into a conformation that can bind to the carbohydrate. Note that the carbohydrate (magenta sticks) interacts with the calcium ion as well as with Tyr-48, Glu-92, Tyr-94, and Arg-97.