title: Calorimetric studies of the interaction of magnesium and phosphate with Na+, K+) ATPase: evidence for a ligand-induced conformational change in the enzyme.
abstract: The phosphorylation of (Na+, K+)ATPase from the electric organ of the electric eel is dependent on Mg2+. The amount of phosphoenzyme formed was increased by K+ and decreased by Na+. Kinetic analyses indicate that a ternary complex of ATPase, Pi and Mg2+ is formed prior to phosphorylation of the protein. Calorimetric studies revealed extraordinarily large enthalpy changes associated with the binding of Mg2+ (-49 kcal/mol) and of Pi (-42 kcal/mol), indicating a thermodynamically significant conformational change in the enzyme. The dissociation constant for the binding of Mg2+ and Pi derived from calorimetric measurements is in good agreement with the value obtained from the kinetic studies. These results indicate that ion binding induces a conformational change in the enzyme which is a prerequisite for phosphorylation by Pi.
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites; D002151:Calorimetry; D004557:Electric Organ / Q000201:enzymology; D004593:Electrophorus; D004789:Enzyme Activation; D007700:Kinetics; D008024:Ligands; D008274:Magnesium / Q000494:pharmacology; D010710:Phosphates / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D011485:Protein Binding; D011487:Protein Conformation; D012964:Sodium / Q000494:pharmacology
chemical_list: D008024:Ligands; D010710:Phosphates; D012964:Sodium; D000251:Adenosine Triphosphatases; D008274:Magnesium; D011188:Potassium