A large photolysis-induced pKa increase of the chromophore counterion in bacteriorhodopsin: Implications for ion transport mechanisms of retinal proteins

Mark S. Braiman, Andrei K. Dioumaev, Jennifer R. Lewis

Research output: Contribution to journalArticle

47 Scopus citations

Abstract

The proton-pumping mechanism of bacteriorhodopsin is dependent on a photolysis-induced transfer of a proton from the retinylidene Schiff base chromophore to the aspartate-85 counterion. Up until now, this transfer was ascribed to a >7-unit decrease in the pKa of the protonated Schiff base caused by photoisomerization of the retinal. However, a comparably large increase in the pKa of the Asp-85 acceptor also plays a role, as we show here with infrared measurements. Furthermore, the shifted vibrational frequency of the Asp-85 COOH group indicates a transient drop in the effective dielectric constant around Asp-85 to ∼2 in the M photointermediate. This dielectric decrease would cause a >40 kJ-mol-1 increase in free energy of the anionic form of Asp-85, fully explaining the observed pKα increase. An analogous photolysis-induced destabilization of the Schiff base counterion could initiate anion transport in the related protein, halorhodopsin, in which aspartate-85 is replaced by Cl- and the Schiff base proton is consequently never transferred.

Original languageEnglish (US)
Pages (from-to)939-947
Number of pages9
JournalBiophysical Journal
Volume70
Issue number2 I
DOIs
StatePublished - Feb 1996
Externally publishedYes

ASJC Scopus subject areas

  • Biophysics

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