Electron Spin Resonance Studies on Conformational Changes of the Sarcoplasmic Reticulum Ca2+ – ATPase Induced by Synergistic Action of Calcium and ATP

Changes in motional properties of the -SH group environment in sarcoplasmic reticulum ATPase [1] induced by addition of specific ligands to sarcoplasmic reticulum membrane vesicles were investigated systematically by electron spin resonance (e.p.r.) spectroscopy. Alternatively, two kinds of iodoacetamide analog spin labels, 4-(2-iodoacetamido)- 2,2,5,5-tetramethyl-1-pyrrolidinyl-N-oxyl (label I) and 4-(2-iodoacetamido)-2,2,6,6-tetramethyl- 1-piperidinyl-N-oxyl (label II) were used. The labeling conditions were chosen such that less than three moles of -SH groups per mole of ATPase reacted with the spin labels. A marked increase in isotropic motion of either spin label was observed on addition of calcium in millimolar concentrations plus ATP or β-γ-methylene ATP. Qualitatively similar but smaller changes were also observed with inosine 5'-triphosphate (ITP), acetylphosphate, or ADP in the presence of calcium. These effects were independent of added magnesium. The spectral changes induced by β-γ-methylene ATP, which binds to the ATPase but is not hydrolyzed, and those of calcium plus ADP suggest a conformational change due to simultaneous binding of these ligands in the absence of enzyme phosphorylation. These changes in e.p.r. spectra were different in quality and magnitude from those observed upon separate binding of calcium or adenine nucleotides. The two spin labels, differing only in their numbers of heterocyclic ring atoms, were found to reflect environmental changes in different ways. This demonstrates the usefulness of employing different spin labels to detect and interpret structural transitions in macromolecular assemblies.