Sandbox 173: Difference between revisions

Rhodopsin, a homodimeric protein, is a highly characterized G protein-coupled receptor found in membranous disks of the outer segments of rod and cone cells, though rhodopsin is more concentrated in rod cells. It is part of the superfamily of G protein-coupled receptors that mediate responses to visual, olfactory, hormonal, and neurotransmitter signals among others<ref>Article 1</ref>. Rhodopsin is involved in visual signal transduction and the visual system in classic G protein-coupled receptor mechanisms<ref>Article 12</ref>.

The 11-''cis'' retinal (retinylidene) Schiff base functions as an inverse agonist and is prominently involved in the activation of rhodopsin. The primary step in rhodopsin photoactivation occurs in the photoisomeration of rhodopsin, as light energy absorbed from a photon is converted into chemical energy, As a photon is absorbed by the retina, the 11-''cis'' retinylidene ligand is switched into an all-''trans'' retinal configuration<ref>Article 2</ref>. In this extremely efficient <200 fs process, the protein-binding pocket, initially fitted to accommodate the 11-''cis'' conformation of the chromophore, is preserved, which restrains the relaxation of the chromophore. The strained relaxation of conformational energy changes the protein state into the active form<ref>Article 2</ref>.

Opsin is the apoprotein component of rhodopsin. Its ability to activate transducin is modulated by both 11-''cis'' retinal and the all-''trans'' retinal; the 11-''cis'' retinal reduces its activity while the all-''trans'' retinal enhances it through non-covalent interactions <ref>Article Opsin 1</ref>. This may give insight on the ability of all-''trans'' retinal, in combination with opsin, to alter the photoreceptor sensitivities<ref>Article Opsin 1</ref>.