Retinal

Retinal, technically called retinene1 or "retinaldehyde", is a light-sensitive retinene molecule found in the photoreceptor cells of the retina. Retinal is the fundamental chromophore involved in the transduction of light into visual signals, i.e. nerve impulses, in the visual system of the central nervous system.

Overview
The molecule that takes part in the initial step in the vision process, rhodopsin, has two components called 11-cis retinal and opsin. Retinal is a light-sensitive derivative of vitamin A, and opsin is a protein molecule. Rhodopsin is found in the rod cells of the eye. 11-cis retinal is a powerful absorber of light because it is a polyene; its 6 alternating single and double bonds make up a long unsaturated electron network. When no light is present, the 11-cis retinal molecule is found in a "bent (cis) configuration" (fig A), and as such it is attached to the opsin molecule in a stable arrangement:



When light strikes the retina, within 200 femtoseconds, after the retinal molecule absorbs a photon into one of the pi bonds found between the eleventh and twelfth carbon atoms, the 11-cis retinal is transformed into the all-trans retinal (fig B) in a straightened configuration.

The all-trans retinal configuration, subsequently, does not fit into the binding site of the opsin molecule; as a result, upon isomerization, the trans isomer separates from the protein, which triggers a G protein signaling pathway' including transducin, that results in the generation of an electrical impulse, which is transmitted through the optic nerve to the brain for processing.

It takes a minimum of five photons to trigger a nerve impulse. In the absence of light, enzymes mediate the isomerization of all-trans back to the 11-cis configuration, and rhodopsin is regenerated by a new formation of a Schiff base linkage, which actuates the binding of the cis isomer to opsin. This is the basic mechanism of the vision cycle.

All-trans-retinal is also an essential component of type I, or microbial, opsins such as bacteriorhodopsin, channelrhodopsin, and halorhodopsin. In these molecules, light causes the all-trans-retinal to become 13-cis retinal, which then cycles back to all-trans-retinal in the dark state.

History
This photon induced retinal-bending mechanism was discovered in 1958 by the American biochemist George Wald and his co-workers. For his work, Wald won a share of the 1967 Nobel Prize in Physiology or Medicine with Haldan Keffer Hartline and Ragnar Granit.