A Nonbleachable Rhodopsin Analogue with a Slow Photocycle

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.Rhodopsin is the light receptor molecule in vertebrate rod photoreceptor cells and is involved in vision under dim light conditions. It senses light by its covalently bound chromophore, 11-cis retinal, which undergoes a light-dependent cis to alltrans isomerization to trigger conformational changes of the receptor leading to formation of the activated Meta II state and ultimately to an excitation of the photoreceptor cell (1, 2).In an attempt to create a light-stable rhodopsin, 11-cis-"locked" analogues of retinal were synthesized; the C 13 methyl group was bridged to C 10 of the polyene of retinal to prevent the activating isomerization around the C 11 ϭC 12 double bond. Several different bridges were tested, leading to the synthesis of 11-cis-locked retinals with 5-, 6-, 7-, 8-, and even 9-member ring structures (Ref. 3 and references in Ref. 4). Much attention had been drawn to 11-cis ring-constrained retinals with a 6-member ring, Re 6.10 , as they recombined with opsin to pigments that were capable of at least some rudimentary photochemistry and some marginal activity toward the visual G protein, transducin (5, 6). Re 6.10 exists in four isomeric states, which can be grouped as 9-trans and 9-cis, depending on the isomeric state of the C 9 double bond (Scheme 1). A recent study characterized the overall shape of these four isomers by their affinities for either 11-cis-or all-trans-retinol dehydrogenase (7). ...

Photocyclic behavior of rhodopsin induced by an atypical isomerization mechanism

“…S1) have implied their ability to activate the G protein transducin (G t ). These results suggest an alternative mechanism that can propagate the downstream visual cascades (9)(10)(11)(12)(13)(14). In this study, we provide direct evidence that Rh6mr can activate G t by an atypical cis-to-dicis isomerization that is also photocyclic, undergoing an 11,13-dicis-to-11-cis reisomerization.…”

“…Although Rh6mr G t activation capability has been discussed previously (9)(10)(11)(12)(13)(14), its underlying molecular mechanism is poorly understood. In this study, we found that the 11,13-dicis 6mr isoform is generated as the photoproduct during illumination, and is exclusively responsible for the dynamic structural change in Rh6mr required to attain a Meta-II-like intermediate state that can activate G t .…”

Rhodopsin with 11-cis-Locked Chromophore Is Capable of Forming an Active State Photoproduct

“…Instead, the chromophore composition seems to be dominated largely by the 11-cis-like isomers, and accumulation of alltrans-like isomers seems to be inhibited at neutral to alkaline pH. Possible mechanisms for such an inhibition are discussed in more detail in the accompanying article (24).…”

“…but is superimposed there by the large difference bands of the Meta II-like photoproduct. The same photoequilibration process is described in more detail in the accompanying article (24), where we analyze the thermal relaxation behavior of the Rh 6.10 photoproducts and show that Rh 6.10 can be photolyzed repeatedly to undergo identical excitation/relaxation cycles on the time scale of minutes to hours.…”

“…An active Meta II-like intermediate with a protonated Schiff base is produced, particularly at low pH, while at high pH, presumably photoequilibration processes between the 11-cis-like inactive isomers dominate the photochemistry. In an accompanying article (24) we further investigate the thermal decay behavior of the photoproducts of Rh 6.10 , revealing an interesting slow photocycling behavior of this visual pigment analog. 6.10 )-The mixture of locked retinal isomers was synthesized as reported previously (21,25).…”