Saturation Transfer Electron Spin Resonance Study on the Rotational Diffusion of Calcium- and Magnesium-Dependent Adenosine Triphosphatase in Sarcoplasmic Reticulum Membranes1

Abstract

The technique of saturation transfer electron spin resonance has been applied to study the rotational diffusion of spin-labeled Ca²⁺,Mg²⁺-dependent ATPase molecules in the membranes of sarcoplasmic reticulum vesicles. Comparison of the present data with those for spin-labeled hemoglobin undergoing isotropic rotation leads to a value of 2×10⁻⁴s for the apparent rotational correlation time at 20°C for the membrane-bound protein. Consideration of the anisotropy of the Brownian rotation of the membrane-bound ATPase suggests that the true correlation time for the expected axial rotation may be somewhat smaller than the apparent value. An Arrhenius plot of the rotational motion shows a break, which is interpreted as indicating the occurrence of a conformational change of the ATPase molecule at about 15°C.