The Ca²⁺-transporting ATPase (EC 126.96.36.199) of sarcoplasmic reticulum alternates between several conformational states during ATP-dependent Ca²⁺ transport. The E₁ conformation is stabilized by 0.1 mM Ca²⁺ and the E₂ conformation by vanadate in a Ca²⁺-free medium. Fourier transform infrared spectroscopy reveals significant differences between the two states that indicate differences in the protein secondary structure. The two states and the corresponding spectra can be interconverted reversibly by changing the Ca²⁺ concentration of the medium. The infrared spectral changes indicate the appearance of a new a-helical substructure connected with the E₁ → E₂ conversion accompanied by small changes in β turns, while the β-sheet content remains essentially unchanged. There are also differences between the E₁ and E₂ states in the C=O stretching vibrations of the ester carbonyl groups of phospholipids in intact sarcoplasmic reticulum that are not observed under identical conditions in isolated sarcoplasmic reticulum lipid dispersions. These observations imply an effect of proteins on the structure of the interfacial regions of the phospholipids that is dependent on the conformational state of the Ca²⁺-ATPase. The CH₂₋ and CH₃₋stretching frequencies of the membrane lipids are not affected significantly by the E₁ → E₂ transition. The Fourier transform infrared spectra of sarcoplasmic reticulum vesicles in the presence of 20 mM Ca²⁺ suggest the stabilization of a protein conformation similar to the E₂ state except for differences in the behavior of COO⁻ and phospholipid ester C=O groups that may reflect charge effects of the bound Ca²⁺.
The American Society for Biochemistry and Molecular Biology, Inc.