11cisretinalem

11-cis-retinal, sometimes rendered as retinaldehyde in the 11-cis configuration, is a light-absorbing aldehyde that serves as the chromophore in vertebrate visual pigments. The term 11cisretinalem is not standard in current scientific usage, and the biologically relevant form is typically referred to as 11-cis-retinal.

Chemically, 11-cis-retinal is derived from vitamin A (retinol) and is the cis isomer of retinaldehyde. In photoreceptor cells, 11-cis-retinal is covalently linked to opsin, forming the visual pigment rhodopsin in rods and similar pigments in cones. The aldehyde group forms a Schiff base with a lysine residue on the opsin, creating a light-sensitive chromophore that is essential for photon capture.

When light is absorbed, 11-cis-retinal is isomerized to all-trans-retinal, triggering a conformational change in the opsin protein. This change activates a G-protein signaling cascade via transducin, leading to a neural signal that is processed as vision. The all-trans-retinal is then released from the opsin and must be recycled through the retinal pigment epithelium (RPE) and neighboring cells to regenerate 11-cis-retinal for another cycle of photoreception.

The visual cycle involves several enzymes and carriers, including LRAT, RPE65, RDH5, and cellular binding proteins, which together convert all-trans-retinal to all-trans-retinol, transport it to the RPE, isomerize and oxidize it, and restore 11-cis-retinal in the retina. Defects in this cycle can lead to night blindness and inherited retinal degenerations, underlining the chromophore’s critical role in vertebrate vision.

In summary, 11-cis-retinal is the cis isomer of retinal that serves as the essential light-sensitive component of visual pigments, enabling the detection of light through a tightly regulated regeneration cycle.