Why? Well, calcium ions have a charge of positive two. And we know from Coulomb's law that the greater the charge is, the greater that interaction is, the greater that electrostatic force is. And so these calcium ions can basically locate the negatively charged side chain groups of proteins, or they can even bind to the oxygen of the carbonyl groups found on the backbone of the protein. And that can form strong interactions. And once those interactions form, the calcium can induce conformational changes in the structure of the protein, and that can bring the different domains in the protein together.