Opsins are a group of light-sensitive 35-55 kDa membrane-bound
G protein-coupled receptorG protein-coupled receptors , also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors , comprise a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal...
s of the
retinylidene proteinRetinylidene proteins are a family of proteins that use retinal as chromophore for light reception. Proteins of this family are also called opsins...
family found in photoreceptor cells of the
retinaThe vertebrate retina is a light sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade. Another opsin found in the mammalian retina,
melanopsinMelanopsin is a photopigment found in specialized photosensitive ganglion cells of the retina that are involved in the regulation of circadian rhythms, pupillary light reflex, and other non-visual responses to light. In structure, melanopsin is an opsin, a retinylidene protein variety of...
, is involved in
circadian rhythmA circadian rhythm is a roughly-24-hour cycle in the biochemical, physiological or behavioral processes of living entities, including plants, animals, fungi and cyanobacteria...
s and pupillary reflex but not in image-forming.
Classification
There are two types of opsins, type 1 and type 2. Though similar in structure and function, evidence suggests that they
evolved independentlyConvergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, birds and bats do, and are capable of powered flight. The wings are similar in...
of one another in
bacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and animals.
Prokaryotic opsins (type 1)
Like eukaryotic opsins, prokaryotic opsins have a seven transmembrane domain structure similar to that found in eukaryotic G-protein coupled receptors. Despite this similarity, there is no evidence that they are evolutionarily related, suggesting that they evolved independently of one another.
Several type 1 opsins, such as
proteo-Proteorhodopsin is a photoactive retinylidene protein in marine bacterioplanktons. Just like the homologous pigment bacteriorhodopsin found in some archaea, it consists of a transmembrane protein bound to a retinal molecule and functions as a light-driven proton pump. Some members of the family ...
, halo- and
bacteriorhodopsinBacteriorhodopsin is a protein used by archaea, most notably halobacteria. It acts as a proton pump, i.e. it captures light energy and uses it to move protons across the membrane out of the cell...
, are used by various bacterial groups to harvest energy from light to
fix carbonCarbon fixation refers to any process through which gaseous carbon dioxide is converted into a solid compound. It mostly refers to the processes found in autotrophs , usually driven by photosynthesis, whereby carbon dioxide is changed into sugars...
using a non-
chlorophyllChlorophyll is a green pigment found in most plants, algae, and cyanobacteria. Its name is derived from the Greek χλωρός and φύλλον...
-based pathway. Additionally, sensory rhodopsins exist in
HalobacteriaIn taxonomy, the Halobacteria are a class of the Euryarchaeota, found in water saturated or nearly saturated with salt. They are also called halophiles, though this name is also used for other organisms which live in somewhat less concentrated salt water...
that induce a
phototacticPhototaxis is a kind of taxis that occurs when a whole organism moves in response to the stimulus light. This is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive light for photosynthesis...
response by interacting with
transducerIn biology, signal transduction refers to any process by which a cell converts one kind of signal or stimulus into another. Most processes of signal transduction involve ordered sequences of biochemical reactions inside the cell, which are carried out by enzymes and activated by second messengers,...
membrane-embedded proteins that have no relation to G proteins.
Classical type 2 opsin groups
Two families of vertebrate opsins are generally recognized due to different spatial expression and
evolutionIn biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a...
ary histories.
RhodopsinRhodopsin, also known as visual purple, is a pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light, enabling vision...
s, which are used in night vision, are high-sensitivity, low-acuity opsins found in the
rod photoreceptor cellsRod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells. Because they are more light sensitive, rods are responsible for night vision. Named for their cylindrical shape, rods are concentrated...
. Cone opsins, employed in color vision, are low-sensitivity, high-acuity opsins located in the
cone photoreceptor cellsCone cells, or cones, are photoreceptor cells in the retina of the eye that function best in relatively bright light. The cone cells gradually become sparser towards the periphery of the retina....
. Cone opsins are further subdivided according to their absorption maxima (
λmax), the wavelength at which the highest light absorption is observed. Evolutionary relationships, deduced using the
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
sequence of the opsins, are also frequently used to categorize cone opsins into their respective group. Both methods predict four general cone opsin groups in addition to rhodopsin: For example, humans have the following set of photoreceptor proteins responsible for vision:
- Rhodopsin
Rhodopsin, also known as visual purple, is a pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light, enabling vision...
(Rh1, OPN2, RHORho may refer to:*Rho , a letter of the Greek alphabet, upper case Ρ, lower case ρ-Symbols:in mathematics* Spectral radius in a spherical coordinate systemin physics* Density of a material* Volume charge density* Resistivity of a material...
) – expressed in rod cellRod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells. Because they are more light sensitive, rods are responsible for night vision. Named for their cylindrical shape, rods are concentrated...
s, used in night vision
- Four cone opsins (also known as photopsin
Photopsins are the photoreceptor proteins found in the cone cells of the retina that are the basis of color vision. Photopsins are very close analogs of the visual purple rhodopsin that is used in night vision...
s) – expressed in cone cellCone cells, or cones, are photoreceptor cells in the retina of the eye that function best in relatively bright light. The cone cells gradually become sparser towards the periphery of the retina....
s, used in color visionColor vision is the capacity of an organism or machine to distinguish objects based on the wavelengths of the light they reflect or emit. The nervous system derives color by comparing the responses to light from the several types of cone photoreceptors in the eye. These cone photoreceptors are...
- Long Wavelength Sensitive (LWS, OPN1LW
Red-sensitive opsin is a protein that in humans is encoded by the OPN1LW gene.-Further reading:...
) Opsin – λmax in the red region of the electromagnetic spectrum
- Middle Wavelength Sensitive (RH2 or MWS) Opsin – λmax in the green region of the electromagnetic spectrum
- Short Wavelength Sensitive 2 (SWS2) Opsin – λmax in the blue region of the electromagnetic spectrum
- Short Wavelength Sensitive 1 (SWS1) Opsin – λmax in the violet/UV region of the electromagnetic spectrum
The human genes for these last three are
OPN1MWGreen-sensitive opsin is a protein that in humans is encoded by the OPN1MW gene.-Further reading:...
,
OPN1MW2OPN1MW2 is an opsin....
, and
OPN1SWBlue-sensitive opsin is a protein that in humans is encoded by the OPN1SW gene.-Further reading:...
, with the first two being "medium-wave" and the third being "short-wave".
Other animals may have different number of photoreceptor proteins, whose optical absorption spectra are shifted compared to those in human photoreceptors. This leads to a different
perceptionIn philosophy, psychology, and the cognitive sciences, perception is the process of attaining awareness or understanding of sensory information. It is a task far more complex than was imagined in the 1950s and 1960s, when it was predicted that building perceiving machines would take about a decade,...
of light and visual images in humans and other animals. Some insects, for example, can see ultraviolet light, whereas animals with only one type of opsins can see the world only in black and white colors.
Novel type 2 opsin groups
Over the last decade, several novel opsin groups have been discovered that are not involved in vision and that do not group with the five classical groups described above. Much of the research is still ongoing, with the function of many novel opsins unknown. The five classical opsins above are expressed solely in the retina, whereas the new novel opsins have a wide range of expression patterns. Phylogenetic studies have been undertaken to categorize these new opsins and determine their evolutionary relationship to the classical opsins.
{| class="wikitable"
|-
! Name
! Gene
! Notes
|-
|
MelanopsinMelanopsin is a photopigment found in specialized photosensitive ganglion cells of the retina that are involved in the regulation of circadian rhythms, pupillary light reflex, and other non-visual responses to light. In structure, melanopsin is an opsin, a retinylidene protein variety of...
| OPN4
| best studied novel opsin involved in circadian rhythms and pupillary reflex
|-
| Pineal Opsin (Pinopsin)
|
|wide range of expression in the brain, most notably in the
pineal regionThe pineal gland is a small endocrine gland in the vertebrate brain. It produces melatonin, a hormone that affects the modulation of wake/sleep patterns and photoperiodic functions...
|-
| Vertebrate Ancient (VA) opsin
|
| has three isoforms VA short (VAS), VA medium (VAM), and VA long (VAL). It is expressed in the inner retina, within the horizontal and amacrine cells, as well as the pineal organ and habenular region of the brain
|-
| Parapinopsin (PP) Opsin
|
|
|-
| Extraretinal (or extra-ocular) Rhodopsin-Like Opsins (Exo-Rh)
|
| Rhodopsin-like protein expressed in the pineal region
|-
| Encephalopsin or Panopsin
|
OPN3Opsin-3 is a protein that in humans is encoded by the OPN3 gene.-Further reading:...
| originally found in human and mice tissue with a very wide range of expression (brain, testes, heart, liver, kidney, skeletal muscle, lung, pancreas and retina)
|-
| Teleost Multiple Tissue (TMT) Opsin
|
| Teleost fish opsin with a wide range of expression
|-
| Peropsin or "Retinal pigment epithelium-derived rhodopsin homolog"
|
RRHVisual pigment-like receptor peropsin is a protein that in humans is encoded by the RRH gene.-Further reading:...
| expressed in the retinal pigment epithelium (RPE) cells
|-
|
Retinal G protein coupled receptorRPE-retinal G protein-coupled receptor is a protein that in humans is encoded by the RGR gene.-Interactions:Retinal G protein coupled receptor has been shown to interact with KIAA1279.-Further reading:...
| RGR
| expressed in the retinal pigment epithelium (RPE) and Müller cells
|-
| Neuropsin
|
OPN5Opsin-5 is a protein that in humans is encoded by the OPN5 gene.-Further reading:...
|-
{|
Structure and Function
Opsin proteins covalently bind to a
vitamin AVitamin A is linked to a family of similarly shaped molecules, the retinoids, which complete the remainder of the vitamin sequence. Its important part is the retinyl group, which can be found in several forms. In foods of animal origin, the major form of vitamin A is an ester, primarily retinyl...
-based retinaldehyde
chromophoreA chromophore is part of a molecule responsible for its color.When a molecule absorbs certain wavelengths of visible light and transmits or reflects others, the molecule has a color. A chromophore is a region in a molecule where the energy difference between two different molecular orbitals falls...
through a Schiff base linkage to a
lysineLysine is an α-amino acid with the chemical formula HO
2CCH
4NH
2. This amino acid is an essential amino acid, which means that humans cannot synthesize it. Its codons are AAA and AAG.Lysine is a base, as are arginine and histidine...
residue in the seventh transmembrane
alpha helixA common motif in the secondary structure of proteins, the alpha helix is a right- or left-handed coiled conformation, resembling a spring, in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid four residues earlier...
. In vertebrates, the chromophore is either 11-
cisCis may have the following meanings:* "Cis-" as a prefix of Latin origin, meaning "on the same side [as]" or "on this side [of]", with several derived usages:** In chemistry, cis- refers to cis-trans isomerism...
-retinal (A1) or 11-
cis-3,4-didehydroretinal (A2) and is found in the retinal binding pocket of the opsin. The absorption of a photon of light results in the photoisomerisation of the chromophore from the 11-
cis to an all-
trans conformation. The photoisomerization induces a conformational change in the opsin protein, causing the activation of the phototransduction cascade. The opsin remains insensitive to light in the
trans form. It is regenerated by the replacement of the all-
trans retinal by a newly synthesized 11-
cis-retinal provided from the retinal epithelial cells. Opsins are functional while bound to either chromophore, with A2-bound opsin
λmax being at a longer wavelength than A1-bound opsin.
Opsins contain seven transmembrane α-helical domains connected by three extra-cellular and three
cytoplasmThe cytoplasm is the part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the cytoplasm contains organelles, such as mitochondria, which are filled with liquid that is kept separate from the rest of the cytoplasm by biological membranes. The contents of the cell nucleus...
ic loops. Many
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
residues, termed
functionally conserved residues, are highly conserved between all opsin groups, indicative of important functional roles. All residue positions discussed henceforth are relative to the 348
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
bovine rhodopsin crystallized by Palczewski
et al.. Lys296 is conserved in all known opsins and serves as the site for the Schiff base linkage with the chromophore. Cys138 and Cys110 form a highly conserved disulfide bridge. Glu113 serves as the counterion, stabilizing the protonation of the Schiff linkage between Lys296 and the chromophore. The Glu134-Arg135-Tyr136 is another highly conserved motif, involved in the propagation of the transduction signal once a photon has been absorbed.
Certain
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
residues, termed
spectral tuning sites, have a strong effect on
λmax values. Using
site-directed mutagenesisSite-directed mutagenesis is a molecular biology technique in which a mutation is created at a defined site in a DNA molecule, usually a circular molecule known as a plasmid...
, it is possible to selectively mutate these residues and investigate the resulting changes in light absorption properties of the opsin. It is important to differentiate
spectral tuning sites, residues that affect the wavelength at which the opsin absorbs light, from
functionally conserved sites, residues important for the proper functioning of the opsin. They are not mutually exclusive, but, for practical reasons, it is easier to investigate spectral tuning sites that do not affect opsin functionality. For a comprehensive review of spectral tuning sites see Yokoyama and Deeb. The impact of spectral tuning sites on
λmax differs between different opsin groups and between opsin groups of different species.
External links
- Illustration at Baldwin-Wallace College
Baldwin-Wallace College is a liberal arts college in Berea, Ohio, founded in 1845. It is home to the Riemenschneider-Bach Institute and the Baldwin-Wallace Conservatory of Music, an internationally renowned music school. The college is affiliated with the United Methodist Church. Students receive...