A Nonbleachable Rhodopsin Analogue with a Slow Photocycle

Reiner Vogel,Gui-Bao Fan,Steffen Lüdeke,Friedrich Siebert,Mordechai Sheves

doi:10.1074/jbc.m205032200

Reiner Vogel, Gui-Bao Fan + Show 3 more

Open Access

https://doi.org/10.1074/jbc.m205032200

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Abstract

Archaeal rhodopsins, e.g. bacteriorhodopsin, all have cyclic photoreactions. Such cycles are achieved by a light-induced isomerization step of their retinal chromophores, which thermally re-isomerize in the dark. Visual pigment rhodopsins, which contain in the dark state an 11-cis retinal Schiff base, do not share such a mechanism, and following light absorption, they experience a bleaching process and a subsequent release of the photo-isomerized all-trans chromophore from the binding pocket. The pigment is eventually regenerated by the rebinding of a new 11-cis retinal. In the artificial visual pigment, Rh(6.10), in which the retinal chromophore is locked in an 11-cis geometry by the introduction of a six-member ring structure, an activated receptor may be formed by light-induced isomerization around other double bonds. We have examined this activation of Rh(6.10) by UV-visible and FTIR spectroscopy and have revealed that Rh(6.10) is a nonbleachable pigment. We could further show that the activated receptor consists of two different subspecies corresponding to 9-trans and 9-cis isomers of the chromophore. Both subspecies relax in the dark via separate pathways back to their respective inactive states by thermal isomerization presumably around the C(13)=C(14) double bond. This nonbleachable pigment can be repeatedly photolyzed to undergo identical activation-relaxation cycles. The rate constants of these photocycles are pH-dependent, and the half-times vary between several hours at acidic pH and about 1.5 min at neutral to alkaline pH, which is several orders of magnitude longer than for bacteriorhodopsin.

Highlights

Rhodopsin is the light receptor molecule in vertebrate rod photoreceptor cells and is involved in vision under dim light conditions
Archaeal rhodopsins, which function as ion pumps or signal transducers, all show photocycles that are initiated by lightdependent isomerization of the covalently bound chromophore all-trans retinal around its C13ϭC14 double bond, thereby triggering subsequent conformational changes required for their particular function [9]
These photocycles are eventually closed in the dark by a thermal back-isomerization of the 13-cis species to the all-trans geometry of the dark states and take from milliseconds to seconds for completion [10, 13, 14]

Summary

A Nonbleachable Rhodopsin Analogue with a Slow Photocycle*

In the artificial visual pigment, Rh6.10, in which the retinal chromophore is locked in an 11-cis geometry by the introduction of a six-member ring structure, an activated receptor may be formed by light-induced isomerization around other double bonds. We have examined this activation of Rh6.10 by UV-visible and FTIR spectroscopy and have revealed that Rh6.10 is a nonbleachable pigment. We could further show that the activated receptor consists of two different subspecies corresponding to 9-trans and 9-cis isomers of the chromophore Both subspecies relax in the dark via separate pathways back to their respective inactive states by thermal isomerization presumably around the C13‫؍‬C14 double bond. Rh6.10 constitutes the first visual pigment for which a slow photocycle could be detected, but it may offer the possibility to apply to rhodopsin those biophysical techniques that require repeated excitation-relaxation cycles

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