Rhodopsin: The Basics!

Structure of Rhodopsin

Rhodopsin is unique in that it senses light. It is responsible for the monochromatic vision in the dark; this means it allows the eye to see in black and white in dim light. It most strongly absorbs green-blue light and therefore appears reddish purple which is why it’s called “visual purple”.

Rhodopsin is a photo receptor protein. It is a pigment of the rod cell in the retina that is responsible for both the formation of the photoreceptor cells and the first event in the perception of light.

Rhodopsin is composed of two parts; the first part is the opsin protein called scotopsin, which is the amino acid chain of the protein that spans the cell membrane seven times.

The 7-helix structure is confirmed by x-ray crystallography. The 7-helix receptors belong to the G-protein-coupled receptor (GPCR).
The second part is the retinal (or retinaldehyde).This protonated 11-cis-retinal Schiff base is the chromophore of Rhodopsin which is responsible for the dark/light vision in the retina of the eye. Figure1 below shows the retinal chromophore (in purple) embedded in the pocket formed by the seven so-called transmembrane helices forming Rhodopsin. 


          Figure1. Opsin: 7-helix receptors belong to the GPCR and the
          Retinal (in purple) the conjugated molecule, constituting Rhodopsin

Figure2 shows the structure of 11-cis retinal.

11-cis Retinal

Figure2 11-Cis-Retinal
Description: The 11-cis-retinal chromophore lies in a pocket of rhodopsin and is isomerised to all-trans retinal when light is absorbed. The isomerization for retinal leads to the change of shape of rhodopsin creating a nerve impulse in the the optic nerve which is transmitted to the brain inducing vision.

The retinal can not be manufactured in our body; instead it is derived from vitamin A, and made in the retina. Vitamin A does not occur in plants; instead Beta-carotene which occurs in carrot, sweet potatoes, squash and other yellow orange vegetables is converted to vitamin A and from there vitamin A (retinol: an alcohol) is oxidized to retinal.

 Figure3a.b.c. below show beta-carotene converted into the retinal.




Figure3.Vitamin A does not occur in plants, but many plants contains Beta-
can be converted to vitamin A within the intestine and other tissues.
 Vitamin A or retinol shown in the fig-ure above is the immediate precursor to retinal. 


 Each beta-carotene will produce two retinal molecules as shown below.

Two retinal molecules are oxidized from beta-carotene.

Figure4 below are the spectra showing the absorption of the human rhodopsin in the rod cells at 500nm, and the absorption spectra of the human photopsin (blue, green, red) of the cone cells.

 Absorption Spectra

Figure3. Absorption spectra of human Rhodopsin of the rod cell and the three photopsin of the cone cell

Figure5 shows the absorption spectra of the opsin, retinal, and the Rhodopsin.

Figure5. UV-Vis absorption characteristic of retinal: free and bound as Rhodopsin.