Vertebrate visual opsins are a subclass of

ciliary 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 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 un ...

in vertebrates. They include the opsins in human rod and

cone cell Cone cells, or cones, are photoreceptor cells in the retinas of vertebrate eyes including the human eye. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cone ...

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 A holoprotein or conjugated protein is an apoprotein combined with its prosthetic group. Some enzymes do not need additional components to show full activity. Others require non-protein molecules called cofactors to be bound for activity. Cofact ...

, it is referred to as

Retinylidene protein Retinylidene proteins, are proteins that use retinal as a chromophore for light reception. They are the molecular basis for a variety of light-sensing systems from phototaxis in flagellates to eyesight in animals. Retinylidene proteins include a ...

. 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 group of evolutionarily-related p ...

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

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 The visual cycle is a process in the retina that replenishes the molecule retinal for its use in vision. Retinal is the chromophore of most visual opsins, meaning it captures the photons to begin the phototransduction cascade. When the proton i ...

. 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 o ...

of 380nm. However, in order to trigger the

phototransduction cascade Visual phototransduction is the sensory transduction process of the visual system by which light is detected to yield nerve impulses in the rod cells and cone cells in the retina of the eye in humans and other vertebrates. It relies on the visua ...

, 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-Dehydrogenase. Many of these species can switch between these chromophores during their life cycle, to adapt to a changing habitat.George Wald (1939): ''The Porphyropsin Visual System.'' In: ''

The Journal of General Physiology ''Journal of General Physiology'' is a peer-reviewed scientific journal published by Rockefeller University Press. The journal covers biological, chemical, or physical mechanisms of broad physiological significance. The major emphasis is on physiol ...

.'' Bd. 22, S. 775–794
PDF
/ref>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 372307
PDF
/ref>

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 tautomeriz ...

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

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...

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

photopsin Vertebrate visual opsins are a subclass of ciliary opsins and mediate vision in vertebrates. They include the opsins in human rod and cone cells. They are often abbreviated to ''opsin'', as they were the first opsins discovered and are still t ...

s when unbound to retinal and

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

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 visua ...

(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

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

s 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 the opsin of the rod cells in the retina and a light-sensitive receptor protein that triggers visual phototransduction ...

s or

porphyropsin Vertebrate visual opsins are a subclass of ciliary opsins and mediate vision in vertebrates. They include the opsins in human rod and cone cells. They are often abbreviated to ''opsin'', as they were the first opsins discovered and are still th ...

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

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

(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 ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to d ...

to the right. 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 hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

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 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, accord ...

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