Structural basis for the negative allostery between Ca2+‐ and Mg2+ ‐binding in the intracellular Ca2+ ‐receptor calbindin D9k

H‐NMR and UV spectroscopic studies at physiological ion concentrations show that the C‐terminal site of the protein is significantly populated by magnesium at resting cell calcium levels, and that there is a negative allosteric interaction between magnesium and calcium binding.