Relationship between protein rotational dynamics and phosphoenzyme decomposition in the sarcoplasmic reticulum Ca-ATPase

We have investigated the role of large-scale protein rotational mobility in the reaction mechanism of the Ca-ATPase in sarcoplasmic reticulum using conditions that have previously been found to inhibit selectively phosphoenzyme decomposition, i.e. 1) partial delipidation (by detergent extraction or phospholipase treatment) and 2) the addition of nonaqueous solvents (dimethyl sulfoxide, glycerol, and sucrose). Using saturation-transfer electron paramagnetic resonance to probe the microsecond rotational motion of the spin-labeled Ca-ATPase, we find that both calcium-dependent ATPase activity and protein rotational mobility decrease in parallel, suggesting that protein mobility is important to the enzymatic step(s) involving phosphoenzyme decomposition. Using conventional EPR to measure the nanosecond rotational dynamics of spin-labeled lipid hydrocarbon chains, we find that neither the removal of lipid nor the addition of nonaqueous solvents significantly affects the lipid dynamics. We propose that the physical mode of inactivation under these conditions is the reduction in protein mobility through enforced protein-protein interactions, the result of which is a reduction in a motion essential for Ca-ATPase activity.