Calorimetric studies of ligand-induced modulation of calcium adenosine 5'-triphosphatase from sarcoplasmic reticulum.
The enthalpy change (delta H degrees ') associated with the binding of Mg2+ to the sarcoplasmic reticulum calcium adenosine 5'-triphosphatase [(Ca2+)ATPase] is -76 kcal/mol. The affinity constant for Mg2+ obtained from calorimetric measurements agrees with the Km value for Mg2+ in the phosphorylation of the enzyme by inorganic phosphate (Pi). The delta H degrees ' of binding of Pi to the enzyme is -23.5 kcal/mol, and the affinity constant for Pi obtained from the calorimetry also agrees with the Km value for Pi in the phosphorylation reaction. delta H degrees ' of Mg2+ binding is reduced to -35 kcal/mol in the presence of either 20 mM Pi or 1.2 mM Ca2+ without a significant change in the affinity of the enzyme for Mg2+. delta H degrees ' of Pi binding to the enzyme drops to -8.5 kcal/mol in the presence of 10 mM Mg2+ without a significant change in the affinity of the enzyme for Pi. On the other hand, the presence of Ca2+ does not affect the delta H degrees ' for the binding of the substrate analogue 5'-adenylyl beta,gamma-imidodiphosphate [App(NH)p], and the presence of this analogue does not affect the delta H degrees ' for Ca2+ binding. The results suggest a model in which a conformational change, largely controlled by Mg2+ binding to the enzyme, leads to the formation of the covalent phosphoprotein intermediate.
D000818:Animals; D002118:Calcium; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002151:Calorimetry; D007700:Kinetics; D008024:Ligands; D008274:Magnesium / Q000494:pharmacology; D010710:Phosphates / Q000494:pharmacology; D010766:Phosphorylation; D011485:Protein Binding; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D013816:Thermodynamics
D008024:Ligands; D010710:Phosphates; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium