Interaction of potassium and magnesium with the high affinity calcium-binding sites of the sarcoplasmic reticulum calcium-ATPase.
The sarcoplasmic reticulum Ca2(+)-ATPase of skeletal muscle has two high affinity calcium sites, one of fast access ("f" site) and one of slow access ("s" site). In addition to Ca2+ these sites are able to interact with other cations like Mg2+ or K+. We have studied with a stopped-flow method the modifications produced by Mg2+ and K+ on the kinetics of the intrinsic fluorescence changes produced by Ca2+ binding to and dissociation from the Ca2(+)-ATPase of sarcoplasmic reticulum. The presence of Mg2+ ions (K1/2 = 0.5 mM at pH 7.2) leads to the appearance of a rapid phase in the Ca2+ binding, which represents half of the signal amplitude at optimal Mg2+. The presence of K+ greatly accelerates both the Ca2+ binding and the Ca2+ dissociation reactions, giving, respectively, a 4- and 8-fold increase of the rate constant of the induced fluorescence change. K+ ions also increase the rate of the 45Ca/40Ca exchange reaction at the s site measured by rapid filtration. These results lead us to build up a model for the Ca2(+)-binding mechanism of the sarcoplasmic reticulum Ca2(+)-ATPase in which Mg2+ and K+ participate at particular steps of the reaction. Moreover, we propose that, in the absence of Ca2+, this enzyme may be the pathway for monovalent ion fluxes across the sarcoplasmic reticulum membrane.
Moutin
M J
MJ
Unite Associeé 520 du Centre National de la Recherche Scientifique, Departement de Biologie Moléculaire et Structurale, Centre d'Etudes Nucléaires de Grenoble, France.
Dupont
Y
Y
D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism*; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D013050:Spectrometry, Fluorescence
D000252:Calcium-Transporting ATPases; D008274:Magnesium; D011188:Potassium; D002118:Calcium