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 --> E2-P) were followed using time-resolved infrared spectroscopy in H2O and 2H2O. 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.
Barth
A
A
Institut für Biophysik und Strahlenbiologie der Universität Freiburg, Albertstrasse 23, D-79104 Freiburg, Germany. barth@athena.chemie.uni-erlangen.de
Kreutz
W
W
Mäntele
W
W
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D008956:Models, Chemical; D010766:Phosphorylation; D011487:Protein Conformation; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D013055:Spectrophotometry, Infrared; D017550:Spectroscopy, Fourier Transform Infrared
D000252:Calcium-Transporting ATPases; D002118:Calcium