Abstact

We describe the photoisomerization of the retinylidene protonated Schiff base in human retinol-binding protein II (hCRBPII) and the role of water molecules in this process. We characterize the photoisomerization of the 15-cis/all-trans retinylidene protonated Schiff base in this system using UV-visible spectroscopy and atomic-resolution X-ray crystallography. We further demonstrate a process where the pK(a) of the protonated Schiff base is substantially altered by light-induced dehydration of the binding pocket, suggesting novel pathways of photoswitching that rely not on isomerization or conformational change of the chromophore but rather on light-induced reorganization of the protein environment.