Photochemistry of the Retinal Chromophore in Microbial Rhodopsins

Microbial rhodopsins are photoreceptive membrane proteins of microorganisms that express diverse photobiological functions. All-trans-retinylidene Schiff base, the so-called all-trans-retinal, is a chromophore of microbial rhodopsins, which captures photons. It isomerizes into the 13-cis form upon photoexcitation. Isomerization of retinal leads to sequential conformational changes in the protein, giving rise to active states that exhibit biological functions. Despite the rapidly expanding diversity of microbial rhodopsin functions, the photochemical behaviors of retinal were considered to be common among them. However, the retinal of many recently discovered rhodopsins was found to exhibit new photochemical characteristics, such as highly red-shifted absorption, isomerization to 7-cis and 11-cis forms, and energy transfer from a secondary carotenoid chromophore to the retinal, which is markedly different from that established in canonical microbial rhodopsins. Here, I review new aspects of retinal found in novel microbial rhodopsins and highlight the emerging problems that need to be addressed to understand noncanonical retinal photochemistry.

Automated summary

Microbial rhodopsins are photoreceptive membrane proteins that exhibit diverse photobiological functions. The chromophore of these rhodopsins, all-trans-retinal, undergoes isomerization upon photoexcitation, leading to sequential conformational changes in the protein and the emergence of active states with biological functions. Recent studies have revealed new photochemical characteristics of retinal in some microbial rhodopsins, including red-shifted absorption, isomerization to different forms, and energy transfer from a secondary chromophore. These findings challenge the established understanding of retinal photochemistry in canonical microbial rhodopsins.