[Effect of urea on conformational changes in the active site of native and reconstituted Ca-ATPase of the sarcoplasmic reticulum].

The effect of urea (1-3 M) on conformational changes in the active center of native and reconstituted Ca-ATPase of sarcoplasmic reticulum modified by fluorescein-5-isothiocyanate (FITC) was studied using the method of fluorescence titration by neodymium (Nd3+) ions. Based on the analysis of curves of fluorescence quenching of FITC-labeled Ca-ATPase by Nd3+ ions, the parameters characterizing the structural changes in the Mg-ATP binding center were determined. It was assumed that FITC and Nd3+ ion bind to different polypeptide fragments moving relative to each other, which provides the conformational lability of the nucleotide binding site at some stages of the catalytic cycle. A comparison of structural changes caused by urea at the active site of native and reconstituted Ca-ATPase of sarcoplasmic reticulum indicates that the Nd3+ binding center is localized in the region of contacts of monomers in the oligomer.