Retinalbinding

Retinalbinding refers to the association of retinal, a vitamin A aldehyde (retinaldehyde), with light-detecting proteins in animals and other organisms. The classic example is the covalent binding of 11-cis-retinal to opsin proteins, forming visual pigments such as rhodopsin in rod cells and cone pigments in cone cells. In this complex, 11-cis-retinal forms a Schiff base linkage with a lysine residue in the opsin protein, creating a light-sensitive chromophore that enables phototransduction.

In the dark state, 11-cis-retinal is tightly bound within the opsin binding pocket. When a photon is absorbed, 11-cis-retinal isomerizes to all-trans-retinal, triggering a conformational change in the opsin that initiates the phototransduction cascade. This cascade ultimately alters ion flux and electrical signals in photoreceptor cells. After activation, all-trans-retinal is released and must be recycled to regenerate a new 11-cis-retinal–opsin complex, restoring the pigment’s light sensitivity.

The visual cycle handles retinal recycling. All-trans-retinal is reduced to all-trans-retinol and transported to the retinal pigment epithelium (RPE) by binding proteins such as IRBP. In the RPE, enzymes including LRAT and RPE65 process retinoids to regenerate 11-cis-retinal, which is then transported back to photoreceptors and bound again by opsin. Cellular retinoid-binding proteins (e.g., CRALBP) facilitate retinoid trafficking and stabilization during this cycle.

Beyond covalent opsin binding, retinalbinding can also describe non-covalent interactions between retinal and various retinoid-binding proteins that support transport, storage, and metabolism of retinoids in the retina and related tissues.