Molecular mechanism of protein-retinal coupling in bacteriorhodopsin

John K. Delaney, Ulrike Schweiger, Sriram Subramaniam

Research output: Contribution to journalArticlepeer-review

Abstract

Bacteriorhodopsin is a membrane protein that functions as a light-driven proton pump. Each cycle of proton transport is initiated by the light-induced isomerization of retinal from the all-trans to 13-cis configuration and is completed by the protein-driven reisomerization of retinal to the all-trans configuration. Previous studies have shown thai replacement of Leu-93, a residue in close proximity to the 13-methyl group of retinal, by alanine, resulted in n 250-fold increase in the time required to complete each photocycle. Here, we show thai the kinetic defect in the photocycle of the Leu-93 → Ala mutant occurs at a stage after the completion of proton transport and can be overcome in the presence of strong background illumination. Time-resolved retinal-extraction experiments demonstrate the continued presence of a 13-cis intermediate in the photocycle of the Leu-93 → Ala mutant well after the completion of proton release and uptake. These results indicate that retinal reisomerization is kinetically the rate- limiting step in the photocycle of this mutant and that the slow thermal reisomerization can be bypassed by the absorption of a second photon. The effects observed for the Leu-93 → Ala mutant are not observed upon replacement of any other residue in van der Waals contact with retinal or upon replacement of Leu-93 by valine. We conclude that the contact between Leo-93 and the 13-methyl group of retinal plays a key role in controlling the rate of protein conformational changes associated with retinal reisomerization and return of the protein to the initial state.

Original languageEnglish (US)
Pages (from-to)11120-11124
Number of pages5
JournalProceedings of the National Academy of Sciences of the United States of America
Volume92
Issue number24
DOIs
StatePublished - Nov 21 1995

Keywords

  • energy transduction
  • membrane protein
  • proton pump
  • retinal isomerization
  • spectroscopy

ASJC Scopus subject areas

  • Genetics
  • General

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