Strongly hydrogen-bonded water molecules in the Schiff base region of rhodopsins

In many rhodopsins, a positively charged retinal chromophore is stabilized by a negatively charged carboxylate, and the presence of bound water molecules has been found in the Schiff base region by X-ray crystallography of various rhodopsins. Low-temperature Fourier-transform infrared (FTIR) spectroscopy can directly monitor hydrogen-bonding alterations of internal water molecules of rhodopsins. In particular, we found that a bridged water molecule between the Schiff base and Asp 85 in bacteriorhodopsin (BR), a light-driven proton-pump protein, forms an extremely strong hydrogen bond. It is likely that a hydration switch of the water from Asp 85 to Asp 212 plays an important role in the proton transfer in the Schiff base region of BR. Comprehensive studies of archaeal and visual rhodopsins have revealed that strongly hydrogen-bonded water molecules are only found in the proteins exhibiting proton-pump activities. Strongly hydrogen-bonded water molecules and its transient weakening may be essential for the proton-pump function of rhodopsins.