Trans-cis isomerization of retinal and a mechanism for ion translocation in halorhodopsin

D. Oesterhelt, P. Hegemann, P. Tavan, K. Schulten

Research output: Contribution to journalArticlepeer-review


The chromophore in halorhodopsin (HR) which acts as a light-driven chloride pump in halobacteria shares many properties with its counterpart in bacteriorhodopsin (BR): (i) a similar retinal protein interaction, (ii) trans to cis isomerization and (iii) similar intermediates of its photocycle. One major difference between the two chromoproteins is that the HR chromophore does not become deprotonated during its photocycle. A mechanism for the photocycle of HR is presented, which, in close analogy to an earlier proposed mechanism for BR, involves the sequence of all-trans → 13-cis, 14 s-cis → 13-cis → all-trans isomerizations of the chromophore, a Schiff base of retinal. In contrast to the situation in BR the 13-cis, 14 s-cis→13-cis isomerization is induced not by deprotonation of the retinal Schiff base chromophore but rather by the movement of an anion (Cl-) towards the protonated nitrogen of the Schiff's base. The suggested mechanism involves the Schiff base directly in the chloride translocation in halorhodopsin.

Original languageEnglish (US)
Pages (from-to)123-129
Number of pages7
JournalEuropean Biophysics Journal
Issue number2
StatePublished - Dec 1986


  • Chloride pump
  • cis-trans isomerization
  • halorhodopsin
  • ion translocation
  • theoretical model

ASJC Scopus subject areas

  • Biophysics


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