Vertebrate visual opsins are a subclass of and mediate visual system in vertebrates. They include the opsins in human rod cell and . They are often abbreviated to opsin, as they were the first discovered and are still the most widely studied opsins.
Opsins
Opsin refers strictly to the
apoenzyme (without bound retinal). When an opsin binds retinal to form a
holoprotein, it is referred to as Retinylidene protein. However, the distinction is often ignored, and opsin may refer loosely to both (regardless of whether retinal is bound).
Opsins are G-protein-coupled receptors (GPCRs) and must bind retinal — typically 11- cis-retinal — in order to be photosensitive, since the retinal acts as the chromophore. When the Retinylidene protein absorbs a photon, the retinal isomerizes and is released by the opsin. The process that follows the isomerization and renewal of retinal is known as the visual cycle. Free 11- cis-retinal is photosensitive and carries its own spectral sensitivity of 380nm.
While opsins can only bind retinal, there are two forms of retinal that can act as the chromophore for vertebrate visual opsins:
-
Retinal 1 (11- cis-Retinal) - the common form present in most opsins
-
Retinal 2 (dehydroretinal) - a rarer form that is relatively red shift compared to retinal 1.
Animals living on land and marine fish form their visual pigments exclusively with retinal 1. However, many freshwater fish and amphibians can also form visual pigments with retinal 2, depending on the activation of the enzyme retinal-3,4-desaturase (GO:0061899). Many of these species can switch between these during their life cycle, to adapt to a changing habitat.[George Wald (1939): The Porphyropsin Visual System. In: The Journal of General Physiology. Bd. 22, S. 775–794. PDF][Andrew T. C. Tsin & Janie M. Flores (1985): The in vivo Regeneration of Goldfish Rhodopsin and Porphyropsin. In: J. Exp. Biol. Bd. 122, S. 269–275. PMID 3723071 PDF]
Function
Isomerization of 11-
cis-retinal into all-
trans-retinal by
light induces a conformational change in the protein that activates the phototransduction pathway.
Subclasses
There are two classes of vertebrate visual opsin, differentiated by whether they are expressed in rod or cone photoreceptors.
Cone opsins
Opsins expressed in cone cells are called cone opsins.
The cone opsins are called
when unbound to retinal and
when bound to retinal.
Cone opsins mediate
photopic vision (daylight). 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).
|
|
| Long-wave sensitive | LWS | Cone | 500–570 | OPN1LW "red" erythrolabe (564nm)
OPN1MW "green" chlorolabe (534nm) |
| Short-wave sensitive 1 | SWS1 | Cone | 355–445 | OPN1SW "blue" cyanolabe (420nm)
(extinct in ) |
| Short-wave sensitive 2 | SWS2 | Cone | 400–470 | (extinct in theria) |
| Rhodopsin-like 2 | Rh2 | Cone | 480–530 | (Extinct in mammals) |
Rod opsins
Opsins expressed in rod cells are called rod opsins. The rod opsins are called
when unbound to retinal and
or
when bound to retinal (1 and 2, respectively). Rod opsins mediate
scotopic vision (dim light).
Compared to cone opsins, the spectral sensitivity of rhodopsin is quite stable, not deviating far from 500 nm in any vertebrate.
|
|
| Scotopsin | Rh1 | Rod | Rhodopsin: ~500
Porphyropsin: ~522 | Rhodopsin human rhodopsin (498nm) |
Evolution
Extant vertebrates typically have four cone opsin classes (LWS, SWS1, SWS2, and Rh2) as well as one rod opsin class (rhodopsin, Rh1), all of which were inherited from early vertebrate ancestors. These five classes of vertebrate visual opsins emerged through a series of
gene duplication beginning with LWS and ending with Rh1, according to the
cladogram to the right; this serves as an example of neofunctionalization. Each class has since evolved into numerous variants.
Evolutionary relationships, deduced using the
amino acid sequence of the opsins, are frequently used to categorize cone opsins into their respective class.
Mammals lost Rh2 and SWS2 classes during the nocturnal bottleneck. Primate ancestors later developed two LWS opsins (LWS and MWS), leaving humans with 4 visual opsins in 3 classes.
History
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).
See also
