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. © 1978.