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Volume 272, Number 41, Issue of October 10, 1997 pp. 25507-25510
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

Ca²⁺ Release from the Phosphorylated and the Unphosphorylated Sarcoplasmic Reticulum Ca²⁺ ATPase Results in Parallel Structural Changes
AN INFRARED SPECTROSCOPIC STUDY

(Received for publication, May 15, 1997, and in revised form, July 1, 1997)

From the Institut für Biophysik und Strahlenbiologie der Universität Freiburg, Albertstrasse 23, D-79104 Freiburg, Germany

Structural changes of the sarcoplasmic reticulum Ca²⁺-ATPase occurring in the reaction step involving phosphoenzyme conversion and Ca²⁺ release (Ca₂E₁-P  E₂-P) were followed using time-resolved infrared spectroscopy in H₂O and ²H₂O. The difference spectra measured between 1800 and 1500 cm¹ were almost identical to those of Ca²⁺ release from the unphosphorylated ATPase (Ca₂E₁  E), implying that parallel structural changes occur in both steps. This suggests that characteristic structural features of the high affinity Ca²⁺ binding sites of Ca₂E₁ are still present in the ADP-sensitive phosphoenzyme Ca₂E₁-P. In both Ca²⁺ release steps at least two carboxyl groups become protonated, each of them experiencing the same strength of hydrogen bonding irrespective of whether or not the Ca²⁺ free ATPase is phosphorylated. This suggests that the same amino acid residues are involved and that they are most likely those that participate in high affinity Ca²⁺ binding and H⁺ countertransport. We propose that during Ca²⁺ release from the phosphoenzyme protons from the lumenal side have access to these residues. Our results are consistent with only one pair of Ca²⁺ binding sites on the ATPase that serves both Ca²⁺ translocation and H⁺ countertransport.

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