NECA has stronger inhibitory power for Hsp82 than Grp94. ATPase inhibition curves for Grp94 (A) and Hsp82 (C) were measured with variable background ATP concentrations. The solid lines are fits to Eq. (2). Error bars are the SEM for at least three measurements. ATP/MgCl2 concentrations for Grp94: 250, 500, 1000, and 1500 μM. ATP/MgCl2 concentrations for Hsp82: 500, 1000, 3000, and 5000 μM. (B and D) The NECA concentration at the inhibition midpoint, [NECA]IC50, is linearly related to the background ATP concentration. Buffer conditions: 25 mM Tris pH 7.5, 150 mM KCl, 310 K.