D. Canet et al., CA2-RETICULUM ATPASE RANDOMIZES THE 2 TRANSPORTED IONS( TRANSLOCATIONACROSS SARCOPLASMIC), The Journal of biological chemistry, 271(34), 1996, pp. 20566-20572
Cytoplasmic Ca2+ dissociation is sequential, and the Ca2+ ions bound t
o the nonphosphorylated ATPase are commonly represented as superimpose
d on each other, so that the superficial Ca2+ is freely exchangeable f
rom the cytoplasm, whereas the deeper Ca2+ is not. Under conditions wh
ere ADP-sensitive phosphoenzyme accumulates (leaky vesicles, 5 degrees
C, pH 8, 300 mM K+), luminal Ca2+ dissociation is sequential as well,
so that the representation of two superimposed Ca2+ ions still holds
on the phosphoenzyme, with the superficial Ca2+ facing the lumen freel
y exchangeable and the deeper Ca2+ blocked by the superficial Ca2(+).
Under the same conditions, we have investigated whether a prebuilt Ca2
+ order is maintained during membrane translocation, Starting from a p
rebuilt order on the cytoplasmic side, we showed that the Ca2+ ions ca
nnot be identified after translocation to the luminal side, The same r
esult was obtained starting from a prebuilt order on the luminal side
and following the luminal to cytoplasmic translocation, We conclude th
at the two Ca2+ ions are mixed during ATP-induced phosphorylation as w
ell as during ADP-induced dephosphorylation.