INDEPENDENCE OF 2 CONFORMATIONS OF SARCOPLASMIC-RETICULUM CA2-ATPASE MOLECULES IN HYDROLYZING ACETYL PHOSPHATE - A 2-PAIR MODEL OF THE ATPASE STRUCTURAL UNIT()

The sarcoplasmic reticulum Ca2+-ATPase molecules have been shown to ex ist in two conformations (A and B) that result from intermolecular int eraction of ATPase molecules (Nakamura, J., and Tajima, G. (1995) J. B iol. Chem, 270, 17350-17354). The A form binds two calcium ions noncoo peratively, whereas the B form binds the calcium ions cooperatively, H ere, we examined the in dependence of these two forms in the calcium-a ctivated hydrolysis of acetyl phosphate (AcP) under asynchronous and s ynchronous conditions of their E-1-E-2 transitions at 0-5 and 25 degre es C. Irrespective of their synchronism and temperature, the two forms hydrolyzed AcP due to calcium that was bound to each of the forms, in dicating the independence of the two forms in hydrolyzing AcP. Taking into account the monomer-dimer transition of the ATPase molecules on t he sarcoplasmic reticulum membrane accompanying E-1-E-2 transition of the molecules (Dux, L., Taylor, K. A., Ting-Beall, H. P., and Martonos i, A, (1985) J. Biol, Chem. 260, 11730-11743), the two types of molecu les seem to independently carry out such monomer-dimer transition of t he same type of molecules, Two pairs, each consisting of the same type of molecules, are suggested to be the structural unit of the ATPase m olecules.