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.
A kinetic model for Ca2+ efflux mediated by the Ca2+ + Mg2+-activated ATPase of sarcoplasmic reticulum
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J M McWhirter, G W Gould, J M East, A G Lee; A kinetic model for Ca2+ efflux mediated by the Ca2+ + Mg2+-activated ATPase of sarcoplasmic reticulum. Biochem J 1 August 1987; 245 (3): 713–721. doi: https://doi.org/10.1042/bj2450713
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