title: pH and ligand binding modulate the strength of protein-protein interactions in the Ca(2+)-ATPase from sarcoplasmic reticulum membranes.
abstract: The Ca(2+)-ATPase from sarcoplasmic reticulum (SR) membranes couples the Ca(2+) transport to ATP hydrolysis through phosphorylation in its cytoplasmic catalytic domain. Interactions between protein domains and the role of monomer-monomer interactions remain unclear. Here, we report a differential scanning calorimetric study of the thermal unfolding of this protein. In the pH range 6-8, thermal unfolding of the Ca(2+)-ATPase in glycogen phosphorylase-free SR membranes shows a major endothermic peak with a critical temperature midpoint ranging between 51 and 55 degrees C, depending on pH, Ca(2+), Mg(2+)-ADP and KCl concentrations. The enthalpy change of the overall unfolding process ranged between 250 and 300 kcal/mol of Ca(2+)-ATPase monomer. Thermal denaturation of the Ca(2+)-ATPase in SR membranes is well fitted to an irreversible process that can be rationalized in terms of a non-two state process, N (native)right harpoon over left harpoon I (intermediate)-->D (denatured). Thermodynamic analysis show that this protein has a compact structure, implying a tight structural interconnection between catalytic and Ca(2+) transport domains. The apparent cooperative unit, defined by the van 't Hoff enthalpy to the overall unfolding enthalpy ratio, increased from 1.1 at pH 6 to 1.8 at pH 8, showing that monomer-monomer interactions are stronger at weakly basic pH than at weakly acidic pH. While micromolar Ca(2+) concentrations had only a weak effect on the cooperativity of the unfolding process, this is clearly increased by millimolar Mg(2+)-ADP. In addition, high ionic strength lowered the apparent cooperative unit to approximately 1.0 in the pH range 6-8. Taken together, these results suggest that protein-protein interactions are altered by variables that modulate the catalytic activity of this enzyme.
authors.affiliation: Departamento de Bioquímica y Biología Molecular, Facultad de Ciencias, Universidad de Extremadura, 06080, Badajoz, Spain.
authors.forename: J M
authors.initials: JM
authors.lastname: Merino
authors.forename: C
authors.initials: C
authors.lastname: Gutiérrez-Merino
mesh_terms: D000818:Animals; D000252:Calcium-Transporting ATPases / Q000737:chemistry* / Q000378:metabolism*; D002152:Calorimetry, Differential Scanning; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007700:Kinetics; D008024:Ligands; D018482:Muscle, Skeletal / Q000201:enzymology; D011485:Protein Binding; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D013816:Thermodynamics
chemical_list: D008024:Ligands; D000252:Calcium-Transporting ATPases