Crystallographic determination of lanthanide ion binding to troponin C.

X-ray intensity data to 2.8 A resolution were collected from each of three crystals of turkey skeletal troponin C (TnC) that had been soaked individually in solutions containing the lanthanide ions europium, thulium and lutetium. Each of the resulting difference electron density maps computed for the three data sets showed three lanthanide ion-binding sites on the TnC molecule. Two of the sites are approximately coincident with the Ca2+ positions in binding loops III and IV and the third site is near the Ca2+-free loop I. The mode of ion binding in loop I is different from that commonly observed for Ca2+ in the Ca2+-binding proteins.