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  • Title: Lumenal and cytoplasmic binding sites for calcium on the calcium ATPase of sarcoplasmic reticulum are different and independent.
    Author: Myung J, Jencks WP.
    Journal: Biochemistry; 1994 Jul 26; 33(29):8775-85. PubMed ID: 8038168.
    Abstract:
    The calcium ATPase of sarcoplasmic reticulum reacts with inorganic phosphate (Pi) to form phosphoenzyme that can bind two Ca2+ ions from the lumen of intact vesicles. Therefore, as the concentration of lumenal Ca2+ is increased, the concentration of phosphoenzyme at equilibrium increases; however, it levels off at lower maximal concentrations with decreasing concentrations of Pi. This requires that two Ca2+ ions can bind to lumenal binding sites of both the phosphoenzyme and the unphosphorylated enzyme. If lumenal Ca2+ could bind only to the phosphoenzyme, saturating concentrations of lumenal Ca2+ would drive phosphoenzyme formation to completion even at low concentrations of Pi. Phosphorylation is inhibited by cytoplasmic Ca2+ with K0.5 = 2.1 and 4 microM in the absence and in the presence of 40 mM lumenal Ca2+, respectively. K0.5 = 4 microM is much lower than K0.5 = 70 microM, which is expected if lumenal Ca2+ could bind only to the phosphoenzyme. Occupancy of the lumenal sites on the unphosphorylated enzyme by Ca2+ does not significantly change the rate constants of kp = 220 s-1 for phosphorylation by ATP, kCa = 90 s-1 for dissociation of Ca2+, and kMg = 50 s-1 for dissociation of Mg2+. We conclude that the calcium ATPase has two low-affinity lumenal Ca(2+)-binding sites that are independent of the high-affinity cytoplasmic Ca(2+)-binding sites. The results are consistent with a mechanism of Ca2+ transport in which phosphorylation of the enzyme by ATP drives the translocation of two Ca2+ ions from the high-affinity to the low-affinity sites.
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