Calcium-binding and its effect on circular dichroism of plant calmodulin

The Ca(2+)-binding properties of calmodulin purified from zucchini (Cucurbita pepo L.) has been determined. A value of 3.3 mol Ca(2+) per mol of zucchini calmodulin was measured at pH 7.5 by equilibrium chromatography. The far-and near-UV circular-dichroic spectra of the Ca(2+)-and Mg(2+)-saturated as well as from the metal-free forms of zucchini calmodulin reveal that upon Ca(2+)-binding the α-helix content increases. A comparison with the spectra of vertebrate calmodulin indicates that both calmodulin have a similar secondary structure, similar Ca(2+)-induced conformational changes and the same number of Ca(2+)-binding sites.