Kinetics and pH dependence of light-induced deprotonation of the Schiff base of rhodopsin: Possible coupling to proton uptake and formation of the active form of Meta II

O. Kuwata, C. Yuan, S. Misra, R. Govindjee, T. G. Ebrey

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Abstract

In this paper the data on the kinetics of Meta II formation, the role and stoichiometry of protons in Meta II formation, the kinetics of the light-induced changes of proton concentration, and the site of proton uptake are reviewed. The processes that lead to the deprotonation of the Schiff base in bacteriorhodopsin with rhodopsin are compared. It is shown that the similarity of the signs of the light-induced electrical signals from the two kinds of oriented pigment molecules could be explained by bacteriorhodopsin releasing a proton from its extracellular side while rhodopsin taking up a proton on its cytoplasmic side. The pH dependence of both the absorption spectrum of the unphotolyzed state and the amplitude and kinetics of Meta II formation in bovine rhodopsin were examined. It is concluded that the pKa of the counter-ion to the Schiff base of bovine rhodopsin and of a surface residue that takes up a proton upon photolysis are both less than 4 in the unphotolyzed state. The data on pH dependence of Meta II formation indicated that the mechanisms involved are more complicated than just two sequential, isospectral forms of Meta II in the bleaching sequence. Finally, the evidence that, like in bacteriorhodopsin, the protonation of the Schiff bases's counter-ion (Glu113) is coupled to the changing of the pKa of a protonatable surface group was examined. It is concluded that this coupling probably leads to the formation of the active form of Meta II.

Original languageEnglish (US)
Pages (from-to)1588-1608
Number of pages21
JournalBiokhimiya
Volume66
Issue number11
StatePublished - Dec 1 2001

Keywords

  • Light-induced deprotonation
  • Rhodopsin
  • Schiff base
  • pH dependence

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