title: Determination and functional significance of low affinity nucleotide sites of Ca2+ + Mg2+ -dependent ATPase of sarcoplasmic reticulum.
abstract: The protective effect of ATP, ADP and GTP against the inactivation of Ca2+ + Mg2+ -dependent ATPase by the thiol reagent NBD-chloride is used to calculate the apparent dissociation constants (K'D) of nucleotide enzyme complexes on the basis of a simple kinetic model. The K'D-values of the complexes with Mg-ATP (80 micrometer) and Mg-GTP (500 micrometer) are found to be rather close to their Km-values in the high concentration range supporting maximum activity. The requirement of the occupancy of the low affinity site by Mg ATP for a high rate of the Ca2+ transport system is explained in terms of the flip-flop mechanism established earlier for the analogous Na+ + K+-transporting ATPase system.
mesh_terms: D009327:4-Chloro-7-nitrobenzofurazan / Q000494:pharmacology; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D004213:Dissociative Disorders; D004794:Enzyme Repression / Q000187:drug effects; D006801:Humans; D008274:Magnesium / Q000378:metabolism*; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*
chemical_list: D000251:Adenosine Triphosphatases; D009327:4-Chloro-7-nitrobenzofurazan; D008274:Magnesium; D002118:Calcium