title: A kinetic model for Ca2+ efflux mediated by the Ca2+ + Mg2+-activated ATPase of sarcoplasmic reticulum.
abstract: We present a model for Ca2+ efflux from vesicles of sarcoplasmic reticulum (SR). It is proposed that efflux is mediated by the Ca2+ + Mg2+-activated ATPase that is responsible for Ca2+ uptake in this system. In the normal ATPase cycle of the ATPase, phosphorylation of the ATPase is followed by a conformational change in which the Ca2+-binding sites change from being outward-facing and of high affinity to being inward-facing and of low affinity. To mediate Ca2+ efflux, it is proposed that the ATPase can adopt a conformation in which the Ca2+-binding sites are of low affinity but still outward-facing. It is shown that experimental data on the rates of Ca2+ efflux can be simulated in terms of this model, with Ca2+-binding-site affinities previously proposed to explain ATPase activity [Gould, East, Froud, McWhirter, Stefanova & Lee (1986) Biochem. J. 237, 217-227]. Effects of Mg2+ and adenine nucleotides on efflux rates are explained. It is suggested that Ca2+ efflux from SR mediated by the ATPase could be important in excitation-contraction coupling in skeletal muscle.
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D001665:Binding Sites; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008954:Models, Biological; D011487:Protein Conformation; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000201:enzymology*
chemical_list: D000255:Adenosine Triphosphate; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium