A comparative study of the binding effects of Mg²⁺, Ca²⁺, Sr²⁺, and Cd²⁺ on calmodulin by fourier‐transform infrared spectroscopy

Fourier‐transform infrared spectroscopy is applied to examine interactions between divalent metal ions (M 2+ ) and the COO − groups in the M 2+ ‐binding sites of calmodulin as well as effects of M 2+ binding on the main chain conformation. Bands due to the symmetric and antisymmetric stretches of the COO − groups in the amino‐acid side chains give information on the coordination of the COO − groups to M 2+ , and the amide‐I' bands are useful for studying changes in the secondary structure of the protein main chain upon M 2+ binding. Infrared deconvolved spectra as well as second‐derivative spectra are sensitive to structural differences among the M 2+ ‐bound forms (M 2+ = Mg 2+ , Ca 2+ , Sr 2+ , and Cd 2+ ). Bands at about 1658, 1553, and 1424 cm −1 are characteristic of the Ca 2+ ‐bound form, and are called, respectively, marker bands I, II, and III of the active‐type protein. The Sr 2+ ‐ and Cd 2+ ‐bound forms show deconvolved and second‐derivative spectra similar to those of the Ca 2+ ‐bound form, whereas the Mg 2+ ‐bound form gives second‐derivative and deconvolved spectra close to those of the inactive M 2+ ‐free form. Broad parallelism is found to exist between the marker‐band intensities of the M 2+ ‐bound forms and the abillities of M 2+ to stimulate calmodulin‐dependent phosphodiesterase activity. Assignments of the marker bands are discussed.