Allosteric binding of nickel(II) to calmodulin

The binding of Ni(II) to calmodulin (CAM) in the presence and in the absence of Ca(II) was investigated by equilibrium dialysis in order to test the physicochemistry of direct Ni(II)-CAM interactions that might be responsible for the effects of this metal on CAM observed in vivo. Samples containing 5 microM CAM, 5 mM Tris/HCl buffer (pH 7.4), and NaCl to maintain the ionic strength I = 3600 microM, with or without 200 microM CaCl2, were dialyzed at 37 degrees C against 1-300 microM 63NiCl2. In the presence of Ca(II), the CAM molecule has two binding sites for Ni(II) (K1 = 7.25 x 10(5) M-1; K2 = 3.79 x 10(5) M-1) (Hill coefficient = 1.20 +/- 0.03 SE). In the absence of Ca(II), a complicated Ni(II)-binding curve is obtained indicating formation of many mutually interacting complex species. Binding of Ni(II) to CAM in the presence of Ca(II) is inhibited slightly by added MnCl2 (50 microM) and very strongly by CuCl2 and ZnCl2 (10 microM). To elucidate the mechanism of this inhibition, binding of Zn(II) (0.5-50 microM 65ZnCl2) to CAM in the presence of Ca(II) (200 microM) was also studied. The maximum molecular ratio of Zn(II) to CAM in the Zn(II)/Ca(II)/CAM complex approached 0.5. Thus, the observed inhibition by Zn(II) of the Ni(II) binding to Ca(II)/CAM does not involve competition for the same binding sites but is rather caused by a conformational arrangement of CAM in its Ca(II)/Zn(II) complex that is different than the Ca(II) complex.(ABSTRACT TRUNCATED AT 250 WORDS)