title: Changes in Ca2+ affinity related to conformational transitions in the phosphorylated state of soluble monomeric Ca2+-ATPase from sarcoplasmic reticulum.
abstract: Changes in Ca2+ binding after phosphorylation of membranous or detergent-solubilized preparations of sarcoplasmic reticulum Ca2+-ATPase with ATP were followed spectrophotometrically by the use of murexide. Distinct Ca2+ release from the two high-affinity translocation sites was observed, particularly at alkaline pH and at low Ca2+/Mg2+ concentration ratios. Phosphorylation also induced additional binding of Ca2+ at a third site in competition with Mg2+. Ca2+ release was increased after solubilization of Ca2+-ATPase in predominantly monomeric form with the nonionic detergent octaethyleneglycol monododecyl ether. At 0 degree C, chemical-quench studies with [32P]ATP indicated that release of Ca2+ is correlated with the level of ADP-insensitive phosphoenzyme (2 mol of Ca2+ released per mol of E2P formed), both for membranous and detergent solubilized Ca2+-ATPase. Ca2+ release was also found to be accompanied by changes in intrinsic fluorescence. Analysis of the data at 20 degrees C, pH 8.0, showed that binding of Ca2+ to transport sites on E2P occurs with a half-saturation constant of 0.7 mM and a Hill coefficient of 1.8. This is consistent with a drastic decrease in Ca2+ affinity following conversion of ADP-sensitive E1P to ADP-insensitive E2P. The similarity between membranous and detergent-solubilized Ca2+-ATPase supports the view that not more than a single Ca2+-ATPase polypeptide chain is required to complete the conformational transitions which are the basis for active transport of Ca2+.
authors.forename: J P
authors.initials: JP
authors.lastname: Andersen
authors.forename: K
authors.initials: K
authors.lastname: Lassen
authors.forename: J V
authors.initials: JV
authors.lastname: Møller
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008433:Mathematics; D008954:Models, Biological; D009132:Muscles / Q000201:enzymology*; D010766:Phosphorylation; D011487:Protein Conformation; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D013050:Spectrometry, Fluorescence; D014364:Tryptophan / Q000032:analysis
chemical_list: D014364:Tryptophan; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium