Similarities and Differences in Photochemistry of Type I and Type II Rhodopsins

Abstract

The diversity of the retinal-containing proteins (rhodopsins) in nature is extremely large. Fundamental similarity of the structure and photochemical properties unites them into one family. However, there is still a debate about the origin of retinal-containing proteins: divergent or convergent evolution? In this review, based on the results of our own and literature data, a comparative analysis of the similarities and differences in the photoconversion of the rhodopsin of types I and II is carried out. The results of experimental studies of the forward and reverse photoreactions of the bacteriorhodopsin (type I) and visual rhodopsin (type II) rhodopsins in the femto- and picosecond time scale, photo-reversible reaction of the octopus rhodopsin (type II), photovoltaic reactions, as well as quantum chemical calculations of the forward photoreactions of bacteriorhodopsin and visual rhodopsin are presented. The issue of probable convergent evolution of type I and type II rhodopsins is discussed.

Fluorescence of the Retinal Chromophore in Microbial and Animal Rhodopsins

“…Fluorescence and photoisomerization of the chromophore in opsin-based photosensitive proteins have been investigated for decades [7,8,98,99]. Recently, the research in this area has been facilitated by application of rhodopsins as tools for optogenetics [100][101][102], which is, along with photopharmacology [103][104][105][106], a widely used approach to control and monitor biological cell activity using light.…”

“…Animal opsin-based photoreceptors perform a variety of roles related to sensation of light, such as vision, control of circadian rhythms, change in body color, etc. In the majority of cases, the light-activated functioning cycle of animal opsin-based photoreceptors involves the formation of the active state, which binds G-protein to launch signal transduction cascades [8,65]. Similarly to microbial rhodopsins, the central role in the photoactivation of animal opsin-based photoreceptors plays a chromophore that captures a photon and undergoes isomerization to start the corresponding photocycle.…”

“…The first path converts the chromophore back to the initial form, and the second path leads to the successful photoisomerization reaction that initiates a series of processes required for a rhodopsin to perform its biological function. In rhodopsins, the protein environment facilitates the photoisomerization of a chromophore, since it is the process that is directly related to rhodopsin functioning, and, as a rule, photoisomerization in rhodopsins occurs very efficiently (e.g., photoisomerization quantum yields in bovine visual rhodopsin and bacteriorhodopsin are 0.65 and 0.64, respectively) [7][8][9]. On the contrary, fluorescence is a side process for rhodopsin functions, and the radiative pathway is suppressed in these proteins.…”