Rhodopsin

Rhodopsin (also known as visual purple) is a type of photoreceptor cell in the eye that is responsible for the detection of light. It is a biological pigment found in the rods of the retina and is a G-protein-coupled receptor (GPCR).

Rhodopsin is extremely sensitive to light, and thus enables vision in low-light conditions. When exposed to light, the pigment immediately photobleaches, and it takes about 30 minutes to regenerate fully in humans.

Structure

Rhodopsin is composed of a protein moiety called opsin and a covalently bound cofactor called retinal, a photoreactive chromophore. Opsin is a seven-transmembrane domain protein, and its structure is primarily determined by its seven alpha helices.

Function

The primary function of rhodopsin is to detect light, which it does through a process called phototransduction. This process involves a series of steps:

1. Absorption of a photon by the retinal component of rhodopsin.
2. Isomerization of 11-cis-retinal to all-trans-retinal.
3. Activation of the G protein transducin by the opsin component of rhodopsin.
4. Activation of phosphodiesterase, which hydrolyzes cyclic GMP.
5. Closure of cyclic GMP-gated ion channels in the plasma membrane, leading to hyperpolarization of the cell and transmission of the signal to the brain.

Clinical significance

Mutations in the gene encoding rhodopsin can lead to various forms of retinitis pigmentosa, a group of hereditary diseases that result in progressive vision loss. These mutations can affect the stability of the rhodopsin molecule, its ability to bind to 11-cis-retinal, or its ability to activate transducin.

See also

• Photoreceptor cell
• Retina
• Retinitis pigmentosa
• Phototransduction
• Transducin

References

• 
  Bovine rhodopsin
• 
  Visual cycle