The effects of Ca2+ and Cd2+ on the secondary and tertiary structure of bovine testis calmodulin. A circular-dichroism study

Abstract

Near-u.v. and far-u.v. c.d. spectra of bovine testis calmodulin and its tryptic fragments (TR1C, N-terminal half, residues 1-77, and TR2C, C-terminal half, residues 78-148) were recorded in metal-ion-free buffer and in the presence of saturating concentrations of Ca2+ or Cd2+ under a range of different solvent conditions. The results show the following: (1) if there is any interaction between the N-terminal and C-terminal halves of calmodulin, it has no apparent effect on the secondary or tertiary structure of either half; (2) the conformational changes induced by Ca2+ or Cd2+ are substantially greater in TR2C than they are in TR1C; (3) the presence of Ca2+ or Cd2+ confers considerable stability with respect to urea-induced denaturation, both for the whole molecule and for either of the tryptic fragments; (4) a thermally induced transition occurs in whole calmodulin at temperatures substantially below the temperature of major thermal unfolding, both in the presence and in the absence of added metal ion; (5) the effects of Cd2+ are identical with those of Ca2+ under all conditions studied. A