Rhodopsin (G protein coupled receptor that is activated by light)

Rhodopsin (G protein coupled receptor that is activated by light)

Rhodopsin (also known as visual purple): a light sensitive pigment in the rod cells of the retina involved in visual photo transduction coupled to transducin.

The human retina contains two types of photoreceptors: rods and cones (primary recipients of visual stimulation).

  • Cones are involved in color vision while rods are stimulated by weak light like moonlight over a range of wavelengths.

Photoreceptor cells have two parts: the inner segment and the outer segment

  • Outer segments of the rod photoreceptor contain a stack of membrane bound discs that contain the photosensitive pigment rhodopsin.
Figure 1: Enlarged picture of the Disc on the outer segment of rods showing rhodopsin molecules

Rhodopsin consists of two components:

1) Protein molecule called opsin (specifically scotopsin)

  • Light sensitive G protein coupled receptor that embeds in the lipid bilayer of cell membranes using seven protein transmembrane domains.
  • The amino-terminal domain projects into the disk, and the carboxyl-terminal domain faces the cytosol of the outer segment.

2) Covalently bound cofactor called retinal (carotenoid).

  • Form of retinal bound to opsin: 11-cis-retinal.

The light absorbing pigment 11-cis-retinal (Chromophore) is covalently bound to the amino group of a lysine residue in the seventh transmembrane domain of the opsin protein.

Figure 2: Structure of Rhodopsin showing opsin and 11- cis- retinal moity and the linkage

When a photon is absorbed by the retinal component of rhodopsin, the energy causes a photochemical change. 11-cis-retinal (bent form) is converted to all trans– retinal (linear isomer).

Metarhodopsin ӀӀ

  • Opsin is covalently bound to all-trans-retinal is called metarhodopsin II, or activated opsin.

Analogous to other G protein coupled receptors, light-activated form of rhodopsin interacts with and activates an associated G protein (Gt = transducin).

Transducin (Heterotrimeric G protein)

Transducin (Gt): The trimeric G protein coupled to rhodopsin

  • Gt  is found only in rod cells.
  • Transducin present near to rhodopsin on the cytoplasmic face of the disk membrane.
  • Transducin belongs to the same family of heterotrimeric GTP-binding proteins as Gs and Gi.
  • Consists of three subunit: α subunit, β subunit and ϒ subunit.
  • Alpha subunit of transducin (Gt) can bind either GDP or GTP.

When GDP is bound to Gtα (inactive form) : All three subunits stay together

When GTP is bound to Gtα (active form): Alpha subunit dissociate from trimeric complex.

Figure 3: Rhodopsin coupled to transducin protein

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