A redirected proton pathway in the bacteriorhodopsin mutant Tyr-57 → Asp: Evidence for proton translocation without Schiff base deprotonation

Sanjay Sonar, Thomas Marti, Parshuram Rath, Wolfgang Fischer, Matthew A Coleman, Anders Nilsson, H. Gobind Khorana, Kenneth J. Rothschild

Research output: Contribution to journalArticle

27 Scopus citations

Abstract

Light-driven proton pumping in bacteriorhodopsin involves deprotonation of the retinylidene Schiff base during M formation and reprotonation during N formation as key steps. This study reports on the spectroscopic characterization of the bacteriorhodopsin mutant Tyr-57 → Asp (Y57D). The results reveal that although formation of the M intermediate and Schiff base deprotonation is blocked, the mutant still exhibits a significant level of light-driven proton translocation. The photocycle of Y57D involves formation of K and L intermediates accompanied by the normal chromophore isomerization and changes in the hydrogen bonding of Asp-96 and Asp-115. However, an additional Asp residue deprotonates during formation of the L intermediate along with a transmembrane α-helical structural change that normally occurs upon N formation. We postulate that proton transport in Y57D occurs through a redirected pathway that does not involve the deprotonation of the Schiff base. Chromophore isomerization, which normally results in the transfer of a proton from the Schiff base to Asp-85, instead causes the deprotonation of Asp-57 in Y57D, most likely through an interaction involving Asp-212. This deprotonation of Asp-57 causes the release of a proton into the extracellular medium. Reprotonation of Asp-57 occurs through the Schiff base reprotonation pathway, which consists of a hydrogen-bonded network of residues spanning from Asp-96 to Asp-212. The results also indicate that the transmembrane α-helical structural changes observed during N formation (Rothschild, K. J., Marti, T., Sonar, S., He, Y. W., Rath, P., Fischer, W., Bousche, O., and Khorana, H. G. (1993) J. Biol. Chem. 268, 27046-27052) do not require deprotonation of Asp-96 or of the Schiff base.

Original languageEnglish (US)
Pages (from-to)28851-28858
Number of pages8
JournalJournal of Biological Chemistry
Volume269
Issue number46
StatePublished - Nov 18 1994
Externally publishedYes

ASJC Scopus subject areas

  • Biochemistry

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    Sonar, S., Marti, T., Rath, P., Fischer, W., Coleman, M. A., Nilsson, A., Khorana, H. G., & Rothschild, K. J. (1994). A redirected proton pathway in the bacteriorhodopsin mutant Tyr-57 → Asp: Evidence for proton translocation without Schiff base deprotonation. Journal of Biological Chemistry, 269(46), 28851-28858.