Opsiinien

Opsins are a group of light-sensitive G protein-coupled receptors (GPCRs) located in photoreceptor cells of the retina, including rods and cones in vertebrates. They bind a retinal chromophore derived from vitamin A. When they absorb light, they initiate a phototransduction cascade that converts light into electrical signals.

Structure and mechanism: Opsins have seven transmembrane helices characteristic of GPCRs. In the dark, 11-cis-retinal is bound to the opsin. Photon absorption converts it to all-trans-retinal, activating the opsin to an active form (metarhodopsin II) that stimulates the G protein transducin (Gt). This triggers a phosphodiesterase cascade that lowers cGMP, closes cGMP-gated channels, and hyperpolarizes the photoreceptor, producing a neural signal transmitted to downstream neurons.

Diversity: Vertebrates possess rod opsin (rhodopsin) in rods for low-light vision, and cone opsins for color: long-, medium-, and short-wavelength sensitivity (LWS, MWS, SWS). In humans, three cone opsins are encoded by OPN1LW, OPN1MW, and OPN1SW. Non-visual opsins such as melanopsin (OPN4) contribute to circadian regulation. Invertebrates have distinct sets of opsins that support their specific visual systems.

Genetics and function: Opsin genes vary across species and arise through gene duplication and spectral tuning via amino acid changes, enabling adaptation to different light environments. The retinoid cycle in the retinal pigment epithelium regenerates 11-cis-retinal after bleaching, maintaining vision.

Applications: Opsins are central to vision science and medicine. Their study informs understanding of color vision deficiencies and retinal diseases, and related proteins are widely used in optogenetics to control neuronal activity.