Vertebrate visual opsin

[2] However, to trigger the phototransduction cascade, the process that underlies the visual signal, the retinal must be bound to an opsin when it is isomerized.

[3][4] Isomerization of 11-cis-retinal into all-trans-retinal by light induces a conformational change in the protein that activates the phototransduction pathway.

Cone opsins are further subdivided according to the spectral sensitivity of their iodopsin, namely the wavelength at which the highest light absorption is observed (λmax).

[8] Compared to cone opsins, the spectral sensitivity of rhodopsin is quite stable, not deviating far from 500 nm in any vertebrate.

These five classes of vertebrate visual opsins emerged through a series of gene duplications beginning with LWS and ending with Rh1, according to the cladogram to the right; this serves as an example of neofunctionalization.

George Wald received the 1967 Nobel Prize in Physiology or Medicine for his experiments in the 1950s that showed the difference in absorbance by these photopsins (see image).

Three-dimensional structure of bovine rhodopsin. The seven transmembrane domains are shown in varying colors. The retinal chromophore is shown in red.
Normalised absorption spectra of the three human photopsins and of human rhodopsin (dashed). Drawn after Bowmaker and Dartnall (1980). [ 5 ] (Absorption curves do not directly reflect sensitivity spectra.) [ 6 ]