Comparison of the isomerization mechanisms of human melanopsin and invertebrate and vertebrate rhodopsins

Abstract

Comparative modeling and ab initio multiconfigurational quantum chemistry are combined to investigate the reactivity of the human nonvisual photoreceptor melanopsin. It is found that both the thermal and photochemical isomerization of the melanopsin 11-cis retinal chromophore occur via a space-saving mechanism involving the unidirectional, counterclockwise twisting of the =C11H-C12H= moiety with respect to its Lys340-linked frame as proposed by Warshel for visual pigments [Warshel A (1976) Nature 260 (5553):679-683]. A comparison with the mechanisms documented for vertebrate (bovine) and invertebrate (squid) visual photoreceptors shows that such a mechanism is not affected by the diversity of the three chromophore cavities. Despite such invariance, trajectory computations indicate that although all receptors display less than 100 fs excited state dynamics, human melanopsin decays from the excited state ∼40 fs earlier than bovine rhodopsin. Some diversity is also found in the energy barriers controlling thermal isomerization. Human melanopsin features the highest computed barrier which appears to be ∼2.5 kcal mol −1 higher than that of bovine rhodopsin. When assuming the validity of both the reaction speed/quantum yield correlation discussed by Warshel, Mathies and coworkers [Weiss RM, Warshel A (1979) J Am Chem Soc 101:6131-6133; Schoenlein RW, Peteanu LA, Mathies RA, Shank CV (1991) Science 254(5030):412-415] and of a relationship between thermal isomerization rate and thermal activation of the photocycle, melanopsin turns out to be a highly sensitive pigment consistent with the low number of melanopsincontaining cells found in the retina and with the extraretina location of melanopsin in nonmammalian vertebrates.F or a long time it was assumed that the human retina contains only two types of photoreceptor cells: the rods and cones responsible for dim-light and daylight vision, respectively. However, recent studies have revealed the existence of a small number of intrinsically photosensitive retinal ganglion cells (ipRGCs) that regulate nonvisual photoresponses (1). ipRGCs express an atypical opsin-like protein named melanopsin (2, 3) which plays a role in the regulation of unconscious visual reflexes and in the synchronization of endogenous physiological responses to the dawn/dusk cycle (circadian rhythms) (4, 5).Melanopsins are unique among vertebrate photoreceptors because their amino acid sequence shares greater similarity to invertebrate than vertebrate rhodopsin (i.e., the photoreceptor of rods) (6, 7). Like rhodopsins, melanopsins feature an up-down bundle architecture of seven transmembrane α-helices incorporating the 11-cis isomer of retinal as a covalently bound protonated Schiff base (PSB11 in Fig. 1A). Light-induced (i.e., photochemical) isomerization of PSB11 to its all-trans isomer (PSBAT) triggers an opsin conformational change that, ultimately, activates the receptor and signaling cascade (8, 9). However, similar to invertebrate and in contrast to vertebrate rhodopsins, melanopsins are bistable ...

Ectopic Expression of Mouse Melanopsin in Drosophila Photoreceptors Reveals Fast Response Kinetics and Persistent Dark Excitation

“…The slow physiological light response of ipRGC expressing melanopsin (a rise time of several seconds and duration of ϳ50 s in mice) is a well documented phenomenon with many implications, which has been attributed to an intrinsic property of melanopsin (25). This claim is consistent with the heterologous expression of melanopsin-producing responses that are similar across cell types, and differing as one would expect for different melanopsins (26,27).…”

“…The Light Response of Opn4-expressing Drosophila Photoreceptors Is Much Faster than the Light Response of ipRGC-The slow physiological light response of ipRGC expressing the native melanopsin has been attributed to an intrinsic property of melanopsin (25). In contrast to the slow light response of ipRGC, the light response of fly photoreceptors is very fast (e.g.…”

Molecular bases for the selection of the chromophore of animal rhodopsins

“…As documented in the SI Appendix [see SI Appendix, section 1 for the testing of alternative protocols ranked for their ability to yield ASR models of comparable quality to the Rh model (31)], the S 0 models of ASR AT and ASR 13C reproduce the vertical excitation energies (ΔE S1-S0 ) associated with the observed absorption maximum ðλ a max Þ value within a few kilocalories per mole (+1.5 and +1.0 kcal·mol −1 for ASR AT and ASR 13C , respectively; see SI Appendix, section 2.1 for all computed and observed excitation energy values). The origin of the λ a max values is investigated by isolating the chromophores from their protein environment and recomputing their ΔE S1-S0 without geometrical relaxation (this quantity is, usually, red-shifted compared with that of the protein due to the lack of effects stabilizing S 0 ).…”

“…We assume that during such an ultrashort time the trajectory describes the average evolution of the corresponding S 1 population. This has been assessed for both Rh (23,24,31) and ASR models (SI Appendix, section 3). As shown in Fig.…”

Communication: Analytical optimal pulse shapes obtained with the aid of genetic algorithms: Controlling the photoisomerization yield of retinal

“…[1][2][3][4][5] Once the photoisomerization is completed, it induces a cascade of biochemical events resulting in the generation of an electric signal. 5 The photoisomerization of 11-cis-retinal is an ultrafast coherent process regularly used to test experimental and theoretical quantum control models in biophysical systems.…”