Light-cycle of rhodopsin. (a) Rhodopsin and 11-cis-retinal. Rhodopsin consists of a colorless protein moiety (the opsin) and the chromophore, 11-cis-retinylidene, which imparts a red color to rhodopsin. The chromophore, a geometric isomer of vitamin A in aldehyde form, is coupled to opsin via the protonated Schiff base at Lys296, located in the transmembrane domain of the protein. Bovine rhodopsin absorbs at a λmax = 498 nm. (b) Photoactivated rhodopsin. Absorption of light by rhodopsin leads with high probability (~65%) to photoisomerization of the cis C11-C12 chromophore double bond to a trans configuration. The probability of isomerization depends only modestly on the wavelength of the light (). This reaction, one of the fastest photochemical reactions known in biology, produces multiple intermediates that culminate in the formation of the G protein–activating state, termed metarhodopsin II, or Meta II. (c) Opsin without chromophore. Ultimately the photoisomerized chromophore, all-trans-retinylidene, is released from the opsin as all-trans-retinal and reduced to alcohol by short-chain alcohol dehydrogenases, such as prRDH, retSDR, and RDH12. The all-trans chromophore diffuses to the adjacent retinal pigment epithelium, where it undergoes enzymatic transformation back to 11-cis-retinal in a metabolic pathway known as the retinoid cycle. Opsin recombines with replenished 11-cis-retinal to form rhodopsin. (d ) Reaction scheme of rhodopsin photoactivation. Upon absorption of a photon by rhodopsin and electronic excitation, fast isomerization of 11-cis-retinylidene to all-trans-retinylidene takes place. At body temperature, the Meta I and Meta II exist in equilibrium shifted toward Meta II. In vitro, further decay of rhodopsin to both opsin and free all-trans-retinal or to Meta III is possible. In vivo, Meta III is not formed at significant levels because it decomposes in the presence of G protein transducin (). In vitro, prolonged incubation of Meta II involves a thermal isomerization of the chromophore double bond with Lys296 to an all-trans-15-syn configuration. This isomerization step is catalyzed by the opsin itself (). On the left are maximal temperatures at which indicated intermediates can be trapped, and on the right is time required for that particular transformation. In the brackets are λmax of absorption for different intermediates. The reaction scheme is based on Shichida & Imai [(); see also the thermodynamic properties of these reactions ()].
