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

This is the authors' abstract. We don't add key points for this paper.

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 ( E P) 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 E P 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. Background: Motions of cytoplasm domains in phosphoenzyme transition of Ca 2+ -ATPase are critical for function. Results: Rate analysis of electrostatic salt screening and mutagenesis on N and P domains combined with electric field analysis suggest electrostatics accelerate the transition. Conclusion: The long-range electrostatics likely guide N and P domain motions. Significance: P-type ion pumps may harness thermal energy through electrostatics.