Ca2+ release to lumen from ADP-sensitive phosphoenzyme E1PCa2 without bound K+ of sarcoplasmic reticulum Ca2+-ATPase

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

The mechanism whereby events in and around the catalytic site/head of Ca(2+)-ATPase effect Ca(2+) release to the lumen from the transmembrane helices remains elusive. We developed a method to determine deoccluded bound Ca(2+) by taking advantage of its rapid occlusion upon formation of E1PCa2 and of stabilization afforded by a high concentration of Ca(2+). The assay is applicable to minute amounts of Ca(2+)-ATPase expressed in COS-1 cells. It was validated by measuring the Ca(2+) binding properties of unphosphorylated Ca(2+)-ATPase. The method was then applied to the isomerization of the phosphorylated intermediate associated with the Ca(2+) release process E1PCa2 → E2PCa2 → E2P + 2Ca(2+). In the wild type, Ca(2+) release occurs concomitantly with EP isomerization fitting with rate-limiting isomerization (E1PCa2 → E2PCa2) followed by very rapid Ca(2+) release. In contrast, with alanine mutants of Leu(119) and Tyr(122) on the cytoplasmic part of the second transmembrane helix (M2) and Ile(179) on the A domain, Ca(2+) release in 10 μm Ca(2+) lags EP isomerization, indicating the presence of a transient E2P state with bound Ca(2+). The results suggest that these residues function in Ca(2+) affinity reduction in E2P, likely via a structural rearrangement at the cytoplasmic part of M2 and a resulting association with the A and P domains, therefore leading to Ca(2+) release.

Roles of long-range electrostatic domain interactions and K+ in phosphoenzyme transition of Ca2+-ATPase

Sarcoplasmic reticulum Ca(2+)-ATPase couples the motions and rearrangements of three cytoplasmic domains (A, P, and N) with Ca(2+) transport. We explored the role of electrostatic force in the domain dynamics in a rate-limiting phosphoenzyme (EP) transition by a systematic approach combining electrostatic screening with salts, computer analysis of electric fields in crystal structures, and mutations. Low KCl concentration activated and increasing salt above 0.1 m inhibited the EP transition. A plot of the logarithm of the transition rate versus the square of the mean activity coefficient of the protein gave a linear relationship allowing division of the activation energy into an electrostatic component and a non-electrostatic component in which the screenable electrostatic forces are shielded by salt. Results show that the structural change in the transition is sterically restricted, but that strong electrostatic forces, when K(+) is specifically bound at the P domain, come into play to accelerate the reaction. Electric field analysis revealed long-range electrostatic interactions between the N and P domains around their hinge. Mutations of the residues directly involved and other charged residues at the hinge disrupted in parallel the electric field and the structural transition. Favorable electrostatics evidently provides a low energy path for the critical N domain motion toward the P domain, overcoming steric restriction. The systematic approach employed here is, in general, a powerful tool for understanding the structural mechanisms of enzymes.