Structural Changes of Water Molecules during the Photoactivation Processes in Bovine Rhodopsin

Abstract

Internal water molecules of rhodopsins play an important role in stabilizing the crucial ion pair comprised by the protonated retinal Schiff base and its counterion. Previous low-temperature FTIR spectroscopy of archaeal rhodopsins observed water O−D stretching vibrations at 2400−2100 cm^-1 in D₂O, corresponding to strong hydrogen bonds. Since a water molecule bridges the protonated Schiff base and an aspartate in archaeal rhodopsins, the observed water molecules presumably hydrate the negative charges in the Schiff base region. In contrast, the FTIR spectroscopy data of bovine rhodopsin presented here revealed that there are no spectral changes of water molecules under strongly hydrogen-bonding conditions (in the range 2500 cm^-1 region that corresponds to weak hydrogen bonding. These results imply that the ion pair state in vertebrate visual rhodopsins is stabilized in a manner different from that in archaeal rhodopsins. In addition, the internal water molecules that hydrate the negative charges do not play important role in the photoactivation processes of rhodopsin that involve proton transfer from the Schiff base to Glu113 upon formation of Meta II. Structural changes of the H−D exchangeable peptide amide of a β-sheet are observed upon formation of metarhodopsin II, suggesting that motion of a β-sheet is coupled to the proton transfer reaction from the Schiff base to its counterion.