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
http://tim.saraogtim.com/pictures/show_graphics.php?ext=g&imagename=rhodopsin
Figure2
shows the structure of 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.
a.
b.
c.
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.
http://www.elmhurst.edu/~chm/vchembook/images/533vitAtocisretinal.gif
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.
Figure5 shows the absorption spectra of the opsin, retinal, and the Rhodopsin.
Figure5. UV-Vis absorption
characteristic of retinal: free and bound as Rhodopsin.