Luminal dissociation of Ca2+ from the phosphorylated Ca2+-ATPase is sequential and gated by Mg2+.
Transport of Ca2+ across the membrane by the Ca2+-ATPase of skeletal muscle sarcoplasmic reticulum involves the transfer of two Ca2+ ions from a pair of cytoplasmic sites to a pair of luminal sites, driven by phosphorylation of the ATPase. The ATPase is inhibited by Mg2+ at alkaline pH values. Inhibition follows from a decrease in the rate of release of Ca2+ from the phosphorylated ATPase. Phosphorylation-induced release of Ca2+ from the ATPase is biphasic at alkaline pH, which is consistent with sequential release of Ca2+ from the phosphorylated ATPase; the rates of both components decrease with increasing Mg concentration. The effect of Mg2+ on the slow phase of release follows from the binding of Mg2+ at the empty outer luminal site, vacated by the release of the first Ca2+ ion. The effect of Mg2+ on the rate of release of the first Ca2+ ion could follow from binding to a gating site also affecting the binding of Ca2+ to the cytoplasmic sites.
Duggleby
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Division of Biochemistry and Molecular Biology, School of Biological Sciences, University of Southampton, Bassett Crescent East, Southampton, Hants. SO16 7PX, UK.
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Lee
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D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*; D017136:Ion Transport; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism*; D008969:Molecular Sequence Data; D018482:Muscle, Skeletal / Q000201:enzymology; D010766:Phosphorylation; D011817:Rabbits
D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium