Rhodopsins are light-sensitive, membrane-embedded proteins which bind a retinal as a chromophore. They have different functions and are found in various organisms. However, they all have the initial chemical reaction in common, namely the photoisomerization of the retinal chromophore. This process involves an isomerization of a double bond upon illumination with light. The initial and the product conformations of the retinal depend on the rhodopsin. In visual rhodopsin (also known as type II) the protein binds 11-cis retinal which isomerizes to the all-trans conformer. However, in microbial rhodopsin (also known as type I) the protein binds retinal in the all-trans conformation, which isomerizes to 13-cis isomer.
However, this isomerization categorization was recently challenged with the discovery of the fusion of bestrophin and rhodopsin, the so-called bestrhodopsin.[1] It was reported to isomerize all-trans retinal to 11-cis instead of the typical 13-cis isomer. In contrast, a recent ultrafast Raman experiment has assigned the initial product to 13-cis conformer.[2]
In this contribution we will present results from the computational studies which use the cryo-EM structure as a starting point and utilize non-adiabatic dynamics with the hybrid quantum mechanics/molecular mechanics (QM/MM) method. These results provide atomistic insight into the mechanism and reveal the critical role of the protein environment.