Modulation of stoichiometry of the sarcoplasmic reticulum calcium pump may enhance thermodynamic efficiency

The coupling of calcium transport to ATP hydrolysis in rabbit muscle sarcoplasmic reticulum vesicles was determined under steady-state conditions in the presence of 5 mM oxalate and using various concentrations of vesicles to modulate the concentration of free Ca2+ in the medium. This experimental approach takes advantage of the fact that at high concentrations of vesicles the slow rate of liberation of Ca2+ from its oxalate complex becomes rate limiting for pumping, therefore pushing the steady-state levels of this cation to very low values. A reduction in the number of calcium ions transported per ATP cleaved from a value near 2 at a low concentration of vesicles (high medium Ca2+ concentration) to a limiting value of about 1 at a very high concentration of vesicles (low medium Ca2+ concentration) was observed. A marked decrease in the specific ATPase activity was also found to take place as the concentration of the sarcoplasmic reticulum vesicles was increased to high levels and the concentration of medium Ca2+ declined. The data presented indicate that binding of 1 Ca2+ to the sarcoplasmic reticulum ATPase is sufficient to activate the pump. Furthermore, these findings support the existence of a control mechanism for the calcium pump that helps to avoid a futile cycle of ATP cleavage with no net transport of calcium and that increases the pumping capability at low concentrations of free Ca2+.