rodopszin

Rodopsin, also known as rhodopsin, is a light-sensitive G protein-coupled receptor found in the rod photoreceptor cells of the retina. It is a hetero-oligomer composed of the protein opsin bound to the chromophore 11-cis-retinal. When a photon is absorbed, 11-cis-retinal isomerizes to all-trans-retinal, triggering a conformational change in opsin to the active form, metarhodopsin II.

Activation of rod opsin initiates the phototransduction cascade. Metarhodopsin II activates the heterotrimeric G protein transducin (G_t), causing the alpha subunit to exchange GDP for GTP. Activated transducin then stimulates a cGMP phosphodiesterase (PDE6), which lowers intracellular cGMP levels. The fall in cGMP leads to closure of cGMP-gated cation channels in the rod outer segment, resulting in hyperpolarization of the rod cell and a reduction in glutamate release onto bipolar cells. This signal is transmitted through the retinal circuitry to enable vision in low-light conditions.

Rodopsin is regenerated through the retinoid cycle. All-trans-retinal is reduced to all-trans-retinol, transported to the retinal pigment epithelium, and converted back to 11-cis-retinal, which recombines with opsin. This regeneration process allows sustained sensitivity to light and underpins the recovery of vision after exposure to bright illumination.

Rods, and thus rodopsin, provide high sensitivity in dim light but offer limited spatial resolution and no color discrimination. Rods are concentrated in the peripheral retina, while central vision relies more on cones. The peak sensitivity of human rodopsin is around 498 nm. Mutations in the RHO gene, encoding rod opsin, can cause retinal diseases such as retinitis pigmentosa.