Characterization of the Ca2+ Binding Sites of Calmodulin from Bovine Testis Using 43Ca and 113Cd NMR

Abstract

The exchange rates of Ca2+ ions to the 2 classes of sites on calmodulin were determined from the temperature dependence of the 43Ca NMR line width. The exchange rates differ by a factor of about 40 at room temperature. The apparent pK value for one of these classes was estimated from the pH dependence of the 43Ca line width. The pK values of the 2-high-affinity sites differ by about 0.1-1. Zn2+ ions bind to calmodulin and affect the exchange rates of Ca2+ and Cd2+ for all 4 sites.