Shape and thermodynamic parameters of a Ca2+-dependent ATPase. A solution x-ray scattering and sedimentation equilibrium study.

Solution X-ray scattering and sedimentation equilibrium experiments in solvents of variable density were used to determine some thermodynamic parameters of deoxycholate-solubilized ATPase monomers. Molecular weight and partial specific volume of the unsolvated deoxycholate-ATPase complexes were determined from X-ray scattering and/or sedimentation equilibrium data in H2OH2 18O mixtures. Volume, mean density (electron density) and hydration were determined by analysis of both solution X-ray scattering and sedimentation equilibrium data recorded in solvents containing various concentrations of sucrose. Protein shape was investigated further with the help of the autocorrelation function corresponding to the scattering curve recorded in the absence of sucrose. This curve was shown to represent mainly the particle shape. The autocorrelation function is fitted adequately by a model formed by two adjacent and coaxial cylinders of different heights and diameters. One cylinder is assumed to bind deoxycholate, while the other one, which is larger, is considered to represent the portion of the ATPase molecule that, in sarcoplasmic reticulum membranes, is exposed to the outer medium.