Vertebrate visual opsins are a subclass of ciliary opsins and mediate

vision Vision, Visions, or The Vision may refer to: Perception Optical perception * Visual perception, the sense of sight * Visual system, the physical mechanism of eyesight * Computer vision, a field dealing with how computers can be made to gain und ...

in vertebrates. They include the opsins in human rod and

cone cell Cone cells or cones are photoreceptor cells in the retina of the vertebrate eye. Cones are active in daylight conditions and enable photopic vision, as opposed to rod cells, which are active in dim light and enable scotopic vision. Most v ...

s. They are often abbreviated to ''opsin'', as they were the first

opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become retinylidene proteins, but are usually still called opsins regardless. Most pro ...

s discovered and are still the most widely studied opsins.

Opsins

Opsin refers strictly to the apoprotein (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 receptor G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large protein family, group of evoluti ...

s (GPCRs) and must bind retinal ⁠— typically 11-''cis''-retinal ⁠— in order to be photosensitive, since the retinal acts as the

chromophore A chromophore is the part of a molecule responsible for its color. The word is derived . The color that is seen by our eyes is that of the light not Absorption (electromagnetic radiation), absorbed by the reflecting object within a certain wavele ...

. 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 Spectral sensitivity is the relative efficiency of detection, of light or other signal, as a function of the frequency or wavelength of the signal. In visual neuroscience, spectral sensitivity is used to describe the different characteristics ...

of 380nm. 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. The retinylidene protein has a spectral sensitivity that differs from that of free retinal and depends on the opsin sequence. 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 ( 11-''cis''-3,4-Dehydroretinal) - a rarer form that is relatively red-shifted 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

s 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
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Function

Isomerization In chemistry, isomerization or isomerisation is the process in which a molecule, polyatomic ion or molecular fragment is transformed into an isomer with a different chemical structure. Enolization is an example of isomerization, as is tautomer ...

of 11-''cis''-retinal into all-''trans''-retinal by

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

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 photopsins when unbound to retinal and

iodopsin Vertebrate visual opsins are a subclass of ciliary opsins and mediate visual system, vision in vertebrates. They include the opsins in human rod cell, rod and cone cells. They are often abbreviated to ''opsin'', as they were the first opsins dis ...

s when bound to retinal. Cone opsins mediate

photopic vision Photopic vision is the vision of the eye under well-lit conditions (luminance levels from 10 to 108 cd/m2). In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher vis ...

(daylight). Cone opsins are further subdivided according to the

of their iodopsin, namely the wavelength at which the highest light absorption is observed (''λ''_max).

Rod opsins

Opsins expressed in rod cells are called rod opsins. The rod opsins are called scotopsins when unbound to retinal and

rhodopsin Rhodopsin, also known as visual purple, is a protein encoded by the ''RHO'' gene and a G-protein-coupled receptor (GPCR). It is a light-sensitive receptor protein that triggers visual phototransduction in rod cells. Rhodopsin mediates dim ...

s or porphyropsins when bound to retinal (1 and 2, respectively). Rod opsins mediate

scotopic vision In the study of visual perception, scotopic vision (or scotopia) is the vision of the eye under low-light conditions. The term comes from the Greek ''skotos'', meaning 'darkness', and ''-opia'', meaning 'a condition of sight'. In the human eye, c ...

(dim light). Compared to cone opsins, the spectral sensitivity of rhodopsin is quite stable, not deviating far from 500 nm in any vertebrate.

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 duplications beginning with LWS and ending with Rh1, according to the

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

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 Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

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 The nocturnal bottleneck hypothesis is an evolutionary biology hypothesis to explain the origin of several mammalian traits. In 1942, Gordon Lynn Walls described this concept which states that placental mammals were mainly or even exclusively ...

. 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 The Nobel Prize in Physiology or Medicine () is awarded yearly by the Nobel Assembly at the Karolinska Institute for outstanding discoveries in physiology or medicine. The Nobel Prize is not a single prize, but five separate prizes that, acco ...

for his experiments in the 1950s that showed the difference in absorbance by these photopsins (see image).

References

{{G protein-coupled receptors G protein-coupled receptors Vision