Functional properties of calbindin D9K mutants with exchanged Ca2+ binding sites.

We have constructed three different engineered proteins based on calbindin D9K by either exchanging the two calcium binding sites within the protein or making the amino acid sequence of the two calcium binding sites identical. The individual calcium binding sites of the engineered proteins retain most of their ion binding characteristics as well as the basal structure of their Ca2+ ligand sphere in the new environment. Even the protein with its sites interchanged, a mutation involving 30 amino acids out of a total of 75, still binds calcium with an affinity as high as that of many natural EF-hand proteins.