Open AccessTwo different forms of metarhodopsin II: Schiff base deprotonation precedes proton uptake and signaling state.
Author(s) -
Sophia Arnis,
Klaus Peter Hofmann
Publication year - 1993
Publication title -
proceedings of the national academy of sciences of the united states of america
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.011
H-Index - 771
eISSN - 1091-6490
pISSN - 0027-8424
DOI - 10.1073/pnas.90.16.7849
Subject(s) - deprotonation , bacteriorhodopsin , schiff base , chemistry , protonation , intramolecular force , photochemistry , aqueous solution , rhodopsin , chromophore , biophysics , proton transport , proton pump , retinal , proton , stereochemistry , crystallography , biochemistry , membrane , organic chemistry , biology , ion , atpase , physics , quantum mechanics , enzyme
Rhodopsin is a retinal protein and a G-protein-coupled receptor; it shares with both of these families the seven helix structure. To generate the G-interacting helix-loop conformation, generally identified with the 380-nm absorbing metarhodopsin II (MII) photoproduct, the retinal Schiff base bond to the apoprotein must be deprotonated. This occurs as a key event also in the related retinal proteins, sensory rhodopsins, and the proton pump bacteriorhodopsin. In MII, proton uptake from the aqueous phase must be involved as well, since its formation increases the pH of the aqueous medium and is accelerated under acidic conditions. In the native membrane, the pH effect matches MII formation kinetically, suggesting that intramolecular and aqueous protonation changes contribute in concert to the protein transformation. We show here, however, that proton uptake, as indicated by bromocresol purple, and Schiff base deprotonation (380-nm absorption change) show different kinetics when the protein is solubilized in suitable detergents. Our data are consistent with a two-step reaction: