Retention of ellipticity between enzymatic states of the Ca2+-ATPase of sarcoplasmic reticulum.

Circular dichroism spectra in the peptide region were obtained from the Ca2+-ATPase of sarcoplasmic reticulum, to establish whether transitions of intermediate states of the enzyme cycle are accompanied by large changes of secondary structure. Since membrane-bound ATPase was used to avoid denaturation, absolute estimates of secondary structural content could not be obtained, due to light scattering interference. Nevertheless, it was possible to demonstrate unambiguously that nearly constant ellipticity is retained by the enzyme following enzyme transitions produced by calcium binding or phosphorylation, even though conformational changes are revealed by other structural probes under the same conditions. We conclude that the conformational changes involved in the long-range reciprocal influence of calcium and phosphorylation sites are related to ligand-induced displacements of amino acid residues which in turn produce reorientation of whole peptide segments of the ATPase protein. This is contrasted by the behavior of calmodulin which undergoes a definite change in ellipticity upon calcium binding.