Replacement of leucine-93 by alanine or threonine slows down the decay of the N and O intermediates in the photocycle of bacteriorhodopsin: Implications for proton uptake and 13-cis-retinal → all-trans-retinal reisomerization

Sriram Subramaniam, Duncan A. Greenhalgh, Parshuram Rath, Kenneth J. Rothschild, H. Gobind Khorana

Research output: Contribution to journalArticlepeer-review

Abstract

We report that the replacement of Leu-93 in bacteriorhodopsin by Ala (L93A) or Thr (L93T) slows down the photocycle by ≈100-fold relative to wild-type bacteriorhodopsin. Time-resolved visible absorption spectroscopy and resonance Raman experiments, respectively, show the presence of long-lived O-like and N-like intermediates in the photocycles of the above mutants. We infer the existence of an equilibrium between the N and O intermediates in the photocycles of these mutants. The L93A and L93T mutants exhibit normal proton pumping under continuous illumination, suggesting that the decay of the N and/or O intermediate, and consequently, proton translocation, can be accelerated by the absorption of a second photon. Since the 13-cis → all-trans reisomerization of retinal is completed during the decay of the N and O intermediates, we conclude that the interaction of Leu-93 with retinal is important in this phase of the photocycle. This conclusion is supported by a recent structural model of bacteriorhodopsin that suggests that Leu-93 is near the C-13 methyl group of retinal.

Original languageEnglish (US)
Pages (from-to)6873-6877
Number of pages5
JournalProceedings of the National Academy of Sciences of the United States of America
Volume88
Issue number15
StatePublished - Aug 1 1991
Externally publishedYes

Keywords

  • Conformational change
  • Integral membrane protein
  • Proton pump
  • Resonance Raman spectroscopy
  • Site-specific mutagenesis

ASJC Scopus subject areas

  • General
  • Genetics

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