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Side-Chain Protonation and Mobility in the Sarcoplasmic Reticulum Ca2+-ATPase: Implications for Proton Countertransport and Ca2+ Release.
Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
2007 (English)In: Biophys J, ISSN 0006-3495, Vol. 93, no 9, 3259-70 p.Article in journal (Refereed) Published
Abstract [en]

Protonation of acidic residues in the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA 1a) was studied by multiconformation continuum electrostatic calculations in the Ca(2+)-bound state Ca(2)E1, in the Ca(2+)-free state E2(TG) with bound thapsigargin, and in the E2P (ADP-insensitive phosphoenzyme) analog state with MgF(4)(2-) E2(TG+MgF(4)(2-)). Around physiological pH, all acidic Ca(2+) ligands (Glu(309), Glu(771), Asp(800), and Glu(908)) were unprotonated in Ca(2)E1; in E2(TG) and E2(TG+MgF(4)(2-)) Glu(771), Asp(800), and Glu(908) were protonated. Glu(771) and Glu(908) had calculated pK(a) values larger than 14 in E2(TG) and E2(TG+MgF(4)(2-)), whereas Asp(800) titrated with calculated pK(a) values near 7.5. Glu(309) had very different pK(a) values in the Ca(2+)-free states: 8.4 in E2(TG+MgF(4)(2-)) and 4.7 in E2(TG) because of a different local backbone conformation. This indicates that Glu(309) can switch between a high and a low pK(a) mode, depending on the local backbone conformation. Protonated Glu(309) occupied predominantly two main, very differently orientated side-chain conformations in E2(TG+MgF(4)(2-)): one oriented inward toward the other Ca(2+) ligands and one oriented outward toward a protein channel that seems to be in contact with the cytoplasm. Upon deprotonation, Glu(309) adopted completely the outwardly orientated side-chain conformation. The contact of Glu(309) with the cytoplasm in E2(TG+MgF(4)(2-)) makes this residue unlikely to bind lumenal protons. Instead it might serve as a proton shuttle between Ca(2+)-binding site I and the cytoplasm. Glu(771), Asp(800), and Glu(908) are proposed to take part in proton countertransport.

Place, publisher, year, edition, pages
2007. Vol. 93, no 9, 3259-70 p.
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:su:diva-19428ISI: 000250199300030PubMedID: 17938423OAI: oai:DiVA.org:su-19428DiVA: diva2:185952
Available from: 2009-01-14 Created: 2009-01-14 Last updated: 2011-01-11Bibliographically approved

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