Protein-Controlled Isomerization in Rhodopsins

Abstract

Rhodopsins are photoactive proteins containing a retinal chromophore in animals and microbes. In animal and microbial rhodopsins, 11-cis and all-trans retinal, respectively, forms a Schiff base linkage with a Lys of the 7th helix, which is mostly protonated. Upon light absorption, ultrafast photoisomerization takes place from the 11-cis to the all-trans form in animal rhodopsins and from the all-trans to the 13-cis form in microbial rhodopsins. Retinal isomerization in the restricted protein environment causes light energy to be stored in the primary intermediate states, leading to each function in a late timescale. π-Electrons along the retinal chromophore play crucial roles in the functions of rhodopsins through their specific interactions with the protein moiety. First, a specific chromophore-protein interaction determines the energy gap of light absorption, i.e., the color of rhodopsins. Second, the isomerization reaction is fast and efficient in protein, even though the retinal-binding pocket is structurally restricted. Third, light energy is efficiently stored by the chromophore-protein interaction, to which steric effect and hydrogen-bonding alteration contribute. Finally, the relaxation from such a high-energy state leads to each function in much slower timescales. In this chapter, the ultrafast isomerization processes and their energy storages in animal and microbial rhodopsins are reviewed. The molecular mechanism of highly efficient photoisomerization will also be discussed.