Ellipticity changes of the sarcoplasmic reticulum Ca(2+)-ATPase induced by cation binding and phosphorylation

The sarcoplasmic reticulum (SR) Ca(2+)-ATPase is a member of the 'P-type' class of cation transport ATPases which form a covalent phosphorylated intermediate. It has been proposed that during ion transport, these proteins cyclically adopt two major enzymatic states E1 and E2, that are related to two essential conformations of the protein. By the use of especially sensitive circular dichroism (CD) instrumentation it is shown here that Ca2+ addition induces 5% or 2.5% increases in Ca(2+)-ATPase ellipticity at 225 nm in the absence or in the presence of Mg2+, respectively. Furthermore, a 2% change in the same direction was observed when the enzyme was phosphorylated with Pi in the absence of Ca2+. These results suggest that the E1----E2 transition and the E2-P formation are associated with structural changes of the polypeptide backbone structure of the calcium pump protein.