Terbium replacement of calcium in parvalbumin

Carp muscle calcium binding parvalbumin, crystallized in 2.9 m-ammonium sulfate, can bind two Tb³⁺ ions, which displace the two Ca²⁺ ions normally present. The Ca²⁺ co-ordinated in the loop between the E and the F α-helices is displaced at low Tb³⁺ concentrations; whereas the Ca²⁺ at the CD site is replaced only at higher Tb³⁺ concentration. There is not a third Tb³⁺ site as had been suggested in interpretations of Tb³⁺ fluorescence experiments performed without ammonium sulfate. A third electron density peak in the difference Fourier maps is tentatively assigned to a sulfate ion co-ordinating the EF site Tb³⁺ ion.