The E1→E2 transition of Ca2+-transporting ATPase in sarcoplasmic reticulum occurs without major changes in secondary structure. A circular-dichroism study

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Abstract
C.d. spectroscopy was used to investigate the structures of Ca2+-ATPase (Ca2+-transporting ATPase) in the E1 and E2 states in native, in fluorescein isothiocyanate (FITC)-labelled and in solubilized sarcoplasmic reticulum (SR) preparations. The E1 state was stabilized by 100 .mu.M-Ca2+ and the E2 state by 0.5 mM-Na3 VO4 and 0.1 mM-EGTA. There were no significant diffeences detected in the c.d. spectra and the calculated secondary structures between the E1 and E2 states in any of the three types of preparations. The FITC-labelled SR did show the characteristic changes in FITC fluorescence on addition of Ca2+ or vanadate, indicating that the preparation was competent for E1 .fwdarw. E2 transitions. Therefore the absence of changes in the c.d. spectra implies that the E1 .fwdarw. E2 transition in the Ca2+-ATPase does not involve a major net rearrangement of the polypeptide backbone conformation.