title: Entropic drive in the sarcoplasmic reticulum ATPase interaction with Mg2+ and Pi.
abstract: Thermodynamic quantities for the binding of Mg2+ (in the presence of Ca2+) and Pi (in the presence of Mg2+ and absence of Ca2+) to sarcoplasmic reticulum ATPase were determined from isothermal titration calorimetry measurements at 25 degrees C. Mg2+ and Pi are involved in reversal of the ATPase hydrolytic reaction, and their interactions with the ATPase are conveniently studied under equilibrium conditions. We found that the Mg2+ binding reaction is endothermic with a binding constant (Kb) = 142 +/- 4 M(-1), a binding enthalpy of 180 +/- 3 kJ mol(-1), and an entropy contribution (TdeltaSb) = 192 +/- 3 kJ mol(-1). Similarly, Pi binding is also an endothermic reaction with Kb = 167 +/- 17 M(-1), deltaHb = 65.3 +/- 5.4 kJ mol(-1), and TdeltaSb = 77.9 +/- 5.4 kJ mol(-1). Our measurements demonstrate that the ATPase can absorb heat from the environment upon ligand binding, and emphasize the important role of entropic mechanisms in energy transduction by this enzyme.
authors.affiliation: Center for Advanced Research in Biotechnology, National Institute of Standards and Technology, Rockville, Maryland 20850, USA.
authors.forename: F P
authors.initials: FP
authors.lastname: Schwarz
authors.forename: G
authors.initials: G
authors.lastname: Inesi
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D002118:Calcium / Q000378:metabolism; D002151:Calorimetry; D019277:Entropy; D066298:In Vitro Techniques; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000201:enzymology; D010710:Phosphates / Q000378:metabolism*; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*
chemical_list: D010710:Phosphates; D000251:Adenosine Triphosphatases; D008274:Magnesium; D002118:Calcium