Sarcoplasmic reticulum CaATPase: product inhibition suggests an allosteric site for ATP activation

Sarcoplasmic reticulum CaATPase hydrolysis of high concentrations of ATP was studied in the presence of ADP. The results obtained were best described as noncompetitive inhibition; added product lowered the Vmax but did not affect the slopes of Eadie-Hofstee plots. At these concentrations (0.5-5 mM), ATP is known to act as both a substrate and as an activator of turnover. The inability of ATP to overcome ADP inhibition suggests that activating ATP binds to an allosteric regulatory site rather than to the phosphorylated active site.