11cisRetinal

11-cis-retinal, also known as 11-cis retinaldehyde, is the 11-cis isomer of retinaldehyde, a vitamin A metabolite. It serves as the photosensitive chromophore in rhodopsin, the light receptor of rod cells, where it forms a Schiff base with a lysine residue in the opsin protein.

In darkness, 11-cis-retinal is bound to opsin, keeping rhodopsin in an inactive state. Absorption of a photon causes isomerization to all-trans-retinal, triggering a conformational change of rhodopsin to the active form metarhodopsin II and initiating the phototransduction cascade that ultimately reduces membrane cGMP and closes cyclic nucleotide–gated channels, leading to hyperpolarization of the cell and signaling light.

The all-trans-retinal must be removed and recycled to regenerate rhodopsin. It is reduced to all-trans-retinol, transported to the retinal pigment epithelium, and converted through the visual cycle back to 11-cis-retinal via a series of enzymes, including LRAT, RPE65, and retinol dehydrogenases, after which it rebinds to opsin to form functional rhodopsin again. The process is aided by retinoid-binding proteins that shuttle retinoids within cells.

11-cis-retinal is derived from dietary vitamin A and retinoids, and its availability is linked to vitamin A intake. Disruptions in the visual cycle can lead to retinal diseases, highlighting the molecule’s essential role in single-photon vision. Its absorption peak is near 500 nm, aligning with the sensitivity of human rod photoreceptors.