Ca2+ Release to Lumen from ADP-sensitive Phosphoenzyme E1PCa2 without Bound K+ of Sarcoplasmic Reticulum Ca2+-ATPase

Abstract

During

Assembly of a Tyr122 Hydrophobic Cluster in Sarcoplasmic Reticulum Ca2+-ATPase Synchronizes Ca2+ Affinity Reduction and Release with Phosphoenzyme Isomerization

“…This result suggests that the Ca 2ϩ -occluded structure in E1PCa 2 is destabilized in this mutant so that a fraction of the Ca 2ϩ of E1PCa 2 is released rapidly (without affinity reduction because such a rapid release does not occur in 10 M Ca 2ϩ ). Similarly, we previously observed Ca 2ϩ release (escape) from E1PCa 2 without affinity reduction in the wild type in the absence of K ϩ and indicated that the stabilization of E1PCa 2 , in this case by the K ϩ binding on its specific site on the P domain, is crucial for stabilizing the Ca 2ϩ -occluded structure of E1PCa 2 (42 (16,17) (Fig. 8, see the decay after the Ca 2ϩ removal (triangles), although it is still much slower than the wild type (cf.…”

“…2A in Ref. 42). Therefore, EP decay represents EP isomerization (which is followed by rapid E2P hydrolysis).…”

Roles of Long-range Electrostatic Domain Interactions and K+ in Phosphoenzyme Transition of Ca2+-ATPase

“…5, reflecting the electrostatic component of the activation energy, are almost zero in LiCl or rather negative in choline-Cl. Without the K ϩ , the E1PCa 2 and E2P structures are unstable as noted above (17,36), and the transition between them could occur as a loose, functionally inefficient steric change, where the N-P domain electrostatic interactions could actually restrict, or act as a barrier, for the domain motions. At the physiological ϳ0.1 M K ϩ , the bound K ϩ markedly accelerates the EP transition probably by allowing a sterically restricted precise domain motion in the proper direction guided by the N-P domain electrostatic interactions, thereby accomplishing efficient Ca 2ϩ transport.…”