(retinal) Retinoid with an aldehyde group.

A diterpene derived from the carotenoid VITAMIN A which functions as the active component of the visual cycle. It is the prosthetic group of RHODOPSIN (i.e., covalently bonded to ROD OPSIN as 11-cis-retinal). When stimulated by visible light, rhodopsin transforms this cis-isomer of retinal to the trans-isomer (11-trans-retinal). This transformation straightens-out the bend of the retinal molecule and causes a change in the shape of rhodopsin triggering the visual process. A series of energy-requiring enzyme-catalyzed reactions convert the 11-trans-retinal back to the cis-isomer.

Biologically active form of vitamin A, which combines with certain proteins (opsins) to form visual pigments (e.g. rhodopsin); light absorption causes cis-trans isomerization of retinal, which is transduced by protein into neural signal and vision.