Proton paths in the sarcoplasmic reticulum Ca2+-ATPase

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Abstract

The sarcoplasmic reticulum Ca2+-ATPase (SERCA1a) pumps Ca2+ and countertransport protons. Proton pathways in the Ca2+ bound and Ca2+-free states are suggested based on an analysis of crystal structures to which water molecules were added. The pathways are indicated by chains of water molecules that interact favorably with the protein. In the Ca2+ bound state Ca2E1, one of the proposed Ca2+ entry paths is suggested to operate additionally or alternatively as proton pathway. In analogs of the ADP-insensitive phosphoenzyme E2P and in the Ca2+-free state E2, the proton path leads between transmembrane helices M5 to M8 from the lumenal side of the protein to the Ca2+ binding residues Glu-771, Asp-800 and Glu-908. The proton path is different from suggested Ca2+ dissociation pathways. We suggest that separate proton and Ca2+ pathways enable rapid (partial) neutralization of the empty cation binding sites. For this reason, transient protonation of empty cation binding sites and separate pathways for different ions are advantageous for P-type ATPases in general.

Abbreviations

BHQ
2,5-di-tert-butyl-1,4-dihydroxybenzene
Ca2E1P
ADP-sensitive phosphoenzyme
E2(TG)
Ca2+-free ATPase with thapsigargin bound
E2(TG + BHQ)
Ca2+-free ATPase with thapsigargin and 2,5-di-tert-butyl-1,4-dihydroxybenzene bound
E2P
ADP-insensitive phosphoenzyme
E2(TG + MgF42−)
E2 complex with thapsigargin and MgF42−
E2(TG + AlF4)
E2 complex with thapsigargin and AlF4
MCCE
multiconformation continuum electrostatics
SR
sarcoplasmic reticulum
TG
thapsigargin

Keywords

SERCA1a
Ca2+ pump
Na,K-ATPase
Proton countertransport
Proton pathway

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