Retinylidene protein
Encyclopedia
Retinylidene proteins are a family of proteins that use retinal
as chromophore
for light
reception. Proteins of this family are also called opsin
s. They are the molecular basis for a variety of light-sensing systems from phototaxis
in flagellate
s to eyesight in animal
s.
moiety and a reversibly covalently bound non-protein cofactor
, retinal
(retinaldehyde). The protein structure of rhodopsin consists of a bundle of seven transmembrane helices
that form an internal pocket binding the photoreactive chromophore. They form a superfamily with other membrane-bound receptors containing seven transmembrane domains, for example odor
and chemokine
receptors.
s like their relatives, rhodopsins contain retinal which changes conformation in reaction to light via photoisomerization and thus are activated by light. The retinal molecule can take on several different cis-trans isomeric forms, such as all-trans, 11-cis and 13-cis. Photoisomerization (light
-dependent isomerization) of retinal from cis to trans or vice versa induces a conformational change in the receptor protein. This change acts as a molecular switch to activate a signal transduction
mechanism within the cell. Depending on the type of rhodopsin, it either opens an ion channel
(for example in bacteria) or activates an associated G protein
and triggers a second messenger cascade (for example in animal eyes).
, bacteriorhodopsin
, halorhodopsin
, and proteorhodopsin
. They act as light-gated ion channels and can be further distinguished by the type of ion they channel. Bacteriorhodopsin functions as a proton pump
, whereas halorhodopsin act as a chloride pump. Their functions range from bacterial photosynthesis
(bacteriorhodopsin) to driving phototaxis
(channelrhodopsins in flagellate
s). Signal transduction in phototaxis involves depolarization
of the cell membrane.
s. Vertebrate
s contain five subfamilies of (rhod)opsins and arthropod
s three subfamilies . Opsins belong to the class of G protein-coupled receptor
s and bind an 11-cis isomer of retinal at the ground state that photoisomerizes to an all-trans retinal upon light activation. They are commonly found in the light-sensing organs, for example in the photoreceptor cells of vertebrate retina
where they facilitate eyesight. Animal opsins can also be found in the skin
of amphibians, the pineal gland
s of lizards and birds, the hypothalamus
of toads, and the human brain
. They can be categorized into several distinct classes including:
(opsin-2) of the rod cell
s in the vertebrate retina absorbs green-blue light. The photopsin
s of the cone cell
s of the retina differ in a few amino acid
s resulting in a shift of their light absorption spectra. The three human photopsins absorb yellowish-green (photopsin I), green (photopsin II), and bluish-violet (photopsin III) light and are the basis of color vision
, whereas the more light-sensitive "visual purple" is responsible for the monochromatic vision in the dark. Light signal transduction involves an enzyme cascade of G-proteins (transducin
), cGMP phosphodiesterase, closure of a cation channel and ultimately hyperpolarization
of the visual photoreceptor cell.
The visual rhodopsins of arthropod
s and molluscs differ from the vertebrate proteins in their signal transduction cascade involving G-proteins, phospholipase C
, and ultimately depolarization
of the visual photoreceptor cell.
s found in humans include encephalopsin (or panopsin, opsin-3), melanopsin (opsin-4), neuropsin (opsin-5) and peropsin. Melanopsin
is involved in the light entrainment of the circadian clock in vertebrates. Encephalopsins and neuropsins are highly expressed in nerve cells and brain tissue, but so far their function is unknown. Peropsin binds all-trans retinal (microbial-type chromophore) and might function as a photoisomerase
to return retinal to the 11-cis isomer form needed in visual perception.
Retinal
Retinal, also called retinaldehyde or vitamin A aldehyde, is one of the many forms of vitamin A . Retinal is a polyene chromophore, and bound to proteins called opsins, is the chemical basis of animal vision...
as chromophore
Chromophore
A chromophore is the part of a molecule responsible for its color. The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others. The chromophore is a region in the molecule where the energy difference between two different molecular orbitals falls...
for light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
reception. Proteins of this family are also called opsin
Opsin
Opsins are a group of light-sensitive 35–55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical...
s. They are the molecular basis for a variety of light-sensing systems from phototaxis
Phototaxis
Phototaxis is a kind of taxis, or locomotory movement, that occurs when a whole organism moves in response to the stimulus of light. This is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive light for photosynthesis...
in flagellate
Flagellate
Flagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Flowering plants do not produce flagellate cells, but ferns, mosses, green algae, some gymnosperms and other closely related plants...
s to eyesight in animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s.
Structure
All rhodopsins consist of two building blocks, a proteinProtein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
moiety and a reversibly covalently bound non-protein cofactor
Cofactor (biochemistry)
A cofactor is a non-protein chemical compound that is bound to a protein and is required for the protein's biological activity. These proteins are commonly enzymes, and cofactors can be considered "helper molecules" that assist in biochemical transformations....
, retinal
Retinal
Retinal, also called retinaldehyde or vitamin A aldehyde, is one of the many forms of vitamin A . Retinal is a polyene chromophore, and bound to proteins called opsins, is the chemical basis of animal vision...
(retinaldehyde). The protein structure of rhodopsin consists of a bundle of seven transmembrane helices
Transmembrane helix
Transmembrane domain usually denotes a single transmembrane alpha helix of a transmembrane protein. It is called a "domain" because an alpha-helix in a membrane can fold independently from the rest of the protein, similar to domains of water-soluble proteins...
that form an internal pocket binding the photoreactive chromophore. They form a superfamily with other membrane-bound receptors containing seven transmembrane domains, for example odor
Odor
An odor or odour is caused by one or more volatilized chemical compounds, generally at a very low concentration, that humans or other animals perceive by the sense of olfaction. Odors are also commonly called scents, which can refer to both pleasant and unpleasant odors...
and chemokine
Chemokine
Chemokines are a family of small cytokines, or proteins secreted by cells. Their name is derived from their ability to induce directed chemotaxis in nearby responsive cells; they are chemotactic cytokines...
receptors.
Mechanism of light reception
Instead of being activated by binding chemical ligandLigand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s like their relatives, rhodopsins contain retinal which changes conformation in reaction to light via photoisomerization and thus are activated by light. The retinal molecule can take on several different cis-trans isomeric forms, such as all-trans, 11-cis and 13-cis. Photoisomerization (light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
-dependent isomerization) of retinal from cis to trans or vice versa induces a conformational change in the receptor protein. This change acts as a molecular switch to activate a signal transduction
Signal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
mechanism within the cell. Depending on the type of rhodopsin, it either opens an ion channel
Ion channel
Ion channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of cells by allowing the flow of ions down their electrochemical gradient. They are present in the membranes that surround all biological cells...
(for example in bacteria) or activates an associated G protein
G protein
G proteins are a family of proteins involved in transmitting chemical signals outside the cell, and causing changes inside the cell. They communicate signals from many hormones, neurotransmitters, and other signaling factors. G protein-coupled receptors are transmembrane receptors...
and triggers a second messenger cascade (for example in animal eyes).
Types of rhodopsins
Retinylidene proteins or rhodopsins are present in many species from bacteria to algae and animals. They can be divided into two distinct groups based on their sequence as well as the retinal isomer they contain at the ground state and their signal transduction mechanisms.Ion channels and pumps
Rhodopsins found in prokaryotes and algae commonly contain an all-trans retinal isomer at the ground state that isomerizes to 13-cis upon light activation, also known as microbial-type chromophore. Examples are bacterial sensory rhodopsins, channelrhodopsinChannelrhodopsin
Channelrhodopsins are a subfamily of opsin proteins that function as light-gated ion channels. They serve as sensory photoreceptors in unicellular green algae, controlling phototaxis, i.e. movement in response to light. Expressed in cells of other organisms, they enable the use of light to control...
, bacteriorhodopsin
Bacteriorhodopsin
Bacteriorhodopsin is a protein used by Archaea, the most notable one being Halobacteria. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell...
, halorhodopsin
Halorhodopsin
Halorhodopsin is a light-driven ion pump, specific for chloride ions, and found in phylogenetically ancient archaea, known as halobacteria...
, and proteorhodopsin
Proteorhodopsin
Proteorhodopsin is a photoactive retinylidene protein in marine bacterioplanktons and eukaryotes. 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...
. They act as light-gated ion channels and can be further distinguished by the type of ion they channel. Bacteriorhodopsin functions as a proton pump
Proton pump
A proton pump is an integral membrane protein that is capable of moving protons across a cell membrane, mitochondrion, or other organelle. Mechanisms are based on conformational changes of the protein structure or on the Q cycle.-Function:...
, whereas halorhodopsin act as a chloride pump. Their functions range from bacterial photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
(bacteriorhodopsin) to driving phototaxis
Phototaxis
Phototaxis is a kind of taxis, or locomotory movement, that occurs when a whole organism moves in response to the stimulus of light. This is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive light for photosynthesis...
(channelrhodopsins in flagellate
Flagellate
Flagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Flowering plants do not produce flagellate cells, but ferns, mosses, green algae, some gymnosperms and other closely related plants...
s). Signal transduction in phototaxis involves depolarization
Depolarization
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...
of the cell membrane.
G protein-coupled receptors
The retinylidene proteins of the animal kingdom are also referred to as opsinOpsin
Opsins are a group of light-sensitive 35–55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical...
s. Vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s contain five subfamilies of (rhod)opsins and arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s three subfamilies . Opsins belong to the class of G protein-coupled receptor
G protein-coupled receptor
G 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 and bind an 11-cis isomer of retinal at the ground state that photoisomerizes to an all-trans retinal upon light activation. They are commonly found in the light-sensing organs, for example in the photoreceptor cells of vertebrate retina
Retina
The 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...
where they facilitate eyesight. Animal opsins can also be found in the skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
of amphibians, the pineal gland
Pineal gland
The pineal gland is a small endocrine gland in the vertebrate brain. It produces the serotonin derivative melatonin, a hormone that affects the modulation of wake/sleep patterns and seasonal functions...
s of lizards and birds, the hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
of toads, and the human brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
. They can be categorized into several distinct classes including:
- visual opsins (classical rhodopsinRhodopsinRhodopsin, also known as visual purple, is a biological 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,...
and relatives), - melanopsinMelanopsinMelanopsin 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...
s - peropsins
- neuropsins
- encephalopsins
Visual perception
The "visual purple" rhodopsinRhodopsin
Rhodopsin, also known as visual purple, is a biological 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,...
(opsin-2) of the rod cell
Rod cell
Rod 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 visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...
s in the vertebrate retina absorbs green-blue light. The photopsin
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 of the cone cell
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
s of the retina differ in a few amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s resulting in a shift of their light absorption spectra. The three human photopsins absorb yellowish-green (photopsin I), green (photopsin II), and bluish-violet (photopsin III) light and are the basis of color vision
Color vision
Color vision is the capacity of an organism or machine to distinguish objects based on the wavelengths of the light they reflect, emit, or transmit...
, whereas the more light-sensitive "visual purple" is responsible for the monochromatic vision in the dark. Light signal transduction involves an enzyme cascade of G-proteins (transducin
Transducin
Transducin is a heterotrimeric G protein that is naturally expressed in vertebrate retina rods and cones .- Mechanism of action :...
), cGMP phosphodiesterase, closure of a cation channel and ultimately hyperpolarization
Hyperpolarization (biology)
Hyperpolarization is a change in a cell's membrane potential that makes it more negative. It is the opposite of a depolarization.Hyperpolarization is often caused by efflux of K+ through K+ channels, or influx of Cl– through Cl– channels. On the other hand, influx of cations, e.g...
of the visual photoreceptor cell.
The visual rhodopsins of arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s and molluscs differ from the vertebrate proteins in their signal transduction cascade involving G-proteins, phospholipase C
Phospholipase C
Phosphoinositide phospholipase C is a family of eukaryotic intracellular enzymes that play an important role in signal transduction processes. In general, this enzyme is denoted as Phospholipase C, although three other families of phospholipase C enzymes have been identified in bacteria and in...
, and ultimately depolarization
Depolarization
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...
of the visual photoreceptor cell.
Others
Other opsinOpsin
Opsins are a group of light-sensitive 35–55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical...
s found in humans include encephalopsin (or panopsin, opsin-3), melanopsin (opsin-4), neuropsin (opsin-5) and peropsin. Melanopsin
Melanopsin
Melanopsin 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 the light entrainment of the circadian clock in vertebrates. Encephalopsins and neuropsins are highly expressed in nerve cells and brain tissue, but so far their function is unknown. Peropsin binds all-trans retinal (microbial-type chromophore) and might function as a photoisomerase
Photoisomerase
A photoisomerase is a protein in the eye that is responsible for isomerizing photopigments.In order for the eye to function propertly, a special chemical called a chromophore is bound to a protein known as an opsin. When a photon strikes the chromophore, it changes shape . It is this change in...
to return retinal to the 11-cis isomer form needed in visual perception.
See also
- OpsinOpsinOpsins are a group of light-sensitive 35–55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical...
- RhodopsinRhodopsinRhodopsin, also known as visual purple, is a biological 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,...
- Visual cycle
- Visual phototransductionVisual phototransductionVisual phototransduction is a process by which light is converted into electrical signals in the rod cells, cone cells and photosensitive ganglion cells of the retina of the eye....
- Bacterial rhodopsinsBacterial rhodopsinsBacterial rhodopsins are a family of bacterial opsins. They are retinal-binding proteins that provide light-dependent ion transport and sensory functions to a family of halophilic and other bacteria...