Melanopsin is a photopigment that plays a role in non-visual light-driven cellular processes such as modulating circadian rhythms, retinal vascular development, and the pupillary light reflex (PLR). Turtle species have a very slow PLR, taking minutes for the iris to fully constrict compared to the microsecond to second range observed in mammals. This slow response is thought to arise from the presence of melanopsin in both the retina and iris. Recently, mRNA for both mammalian and non-mammalian melanopsin orthologs were identified in freshwater turtle retina and irises, although the expression and potential role of these photopigments remain under investigation. In this work, we use computational methods to understand which chromophores are present in turtle melanopsin. In mammalian melanopsin, the vitamin A derivative 11-cis-retinal acts as the chromophore bound in the opsin cavity. However, it remains unclear whether this retinal chromophore binds to melanopsin in freshwater turtle species. Visual pigments in freshwater vertebrates often contain a different vitamin A derivative, 11-cis-3,4-didehydroretinal, which is more sensitive to red-light than blue-light. The red-light chromophore enables these species to see well in murky freshwater environments compared to, for example, the blue-green of marine environments. In this work, computational homology models of melanopsin in freshwater and marine turtle species were constructed. Quantum mechanics/molecular mechanics (QM/MM) calculations were carried out to determine how retinal derivatives bind to melanopsin in freshwater and marine turtles. The results provide key insights related to chromophore binding and activation in opsin proteins.

Computational investigation of melanopsin photoreception in freshwater and marine turtles

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.