Effects of phospholipid fatty acyl chain length on phosphorylation and dephosphorylation of the Ca2+-ATPase

Abstract

The kinetics of the Ca2+-ATPase purified from sarcoplasmic reticulum have been studied after reconstitution into bilayers of dimyristoleoylphosphatidylcholine [di(C(14:1))PC], dioleoylphosphatidylcholine [di(C(18:1))PC] and dinervonylphosphatidylcholine [di(C(24:1))PC]. In di(C(24:1))PC the rate of phosphorylation of the ATPase by ATP was comparable with that in di(C(18:1))PC (about 70 s-1), but in di(C(14:1))PC the rate was much lower (21 s-1). Fluorescence responses of the ATPase suggest changes in the phosphoryl-transfer step rather than in the preceding conformational change E1Ca2ATP⇆E1'Ca2ATP. The rate of dephosphorylation of the phosphorylated ATPase was found to decrease in the order di(C(24:1))PC < di(C(14:1))PC < di(C(18:1))PC. For the ATPase in di(C24:1)PC the rate of dephosphorylation (3.3 s-1) was slow enough to be the rate-limiting step for ATP hydrolysis; in di(C(14:1))PC, it is suggested that both phosphorylation and dephosphorylation contribute to rate limitation. Phosphorylation of the ATPase in di(C(24:1))PC by P(i) was normal, but no phosphoenzyme could be detected in di(C(14:1))PC. The rate of the Ca2+-transport step was normal in di(C(24:1))PC, suggesting that the single Ca2+ ion bound to the ATPase in di(C(24:1))PC could be transported.