Ca2+ release from the phosphorylated and the unphosphorylated sarcoplasmic reticulum Ca2+ ATPase results in parallel structural changes - An infrared spectroscopic study

Structural changes of the sarcoplasmic reticulum Ca2+-ATPase occurring in the reaction step involving phosphoenzyme conversion and Ca2+ release (Ca2E1-P --> E-2-P) were followed using time-resolved infrared spectroscopy in H2O and (H2O)-H-2. The difference spectra measured between 1800 and 1500 cm(-1) 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.