Rotational motion and evidence for oligomeric structures of sarcoplasmic reticulum Ca2+-activated ATPase.

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Abstract
The rotational motion of the [rabbit muscle] sarcoplasmic reticulum Ca2+-activated ATPase (ATP phosphohydrolase, EC 3.6.1.3) was investigated by measuring the decay of laser flash-induced dichroism with the covalently attached triplet probe eosin isothiocyanate. The Arrhenius plot for rotational mobility indicates 2 discontinuities at .apprxeq. 15.degree. C and .apprxeq. 35.degree. C. The experimental data are rationalized in terms of a sudden conformeric change in the ATPase at 15.degree. C and a temperature-dependent equilibrium existing between the conformationally altered ATPase and oligomeric forms of it in the temperature range 15-35.degree. C. The enzymatic activity, as indicated by a discontinuity in the Arrhenius plot for the rate of ATP hydrolysis, appears to be sensitive only to the change at 15.degree. C. There is a strong correlation between the activation energy below 15.degree. C for rotational motion (33.6 .+-. 2.2 kcal/mol) and enzymatic activity (34 .+-. 4 kcal/mol).