Opsin
Opsin means the protein content of a visual pigment, which consists entirely of a protein and a chromophore. In the English -speaking world, the term opsin but is often also for the entire visual pigment.
Opsins belong to the superfamily of heptahelical transmembrane proteins or G- protein-coupled receptors. The chromophore is a terpenoid, mostly 11 -cis-retinal (retinal 1) or 11-cis- 3 ,4 -dehydro- retinal ( retinal 2).
There are two different groups of opsins, the Skotopsine or rod- opsins, which are responsible in the rods for vision in twilight and darkness, as well as the Photopsine or pin - opsins, which allow the pin color vision. 11-cis -retinal forms together with the rod opsin, the photosensitive pigment molecule rhodopsin that triggers the process of visual signal transduction in the rods, and along with the various cone opsins, the absorbent in various colors Iodopsine (see below), the the same thing get in the journal.
- 2.4.2.1 XL- opsin
- 2.4.2.2 L- opsin
- 2.4.2.3 M- opsin
- 2.4.2.4 S- opsin
- 2.4.2.5 UV opsin
Localization
Opsin is stored in vertebrates mainly in conjunction with retinal in the outer segment of the photoreceptors. When the rods the rhodopsin sitting in the discs and covers there 90% of the total protein content of the structure. In the journals are located corresponding to the rhodopsin Iodopsine in Membraneinfaltungen. In insects and other animals with photoreceptors of the rhabdomeric type, the protein is incorporated into the membrane of the brush border of the visual cells.
Nomenclature
The most common found in animal retinal pigment molecules called as the protein, each with a prefix to identify their inherent color or complementary color. This heuristic, but this inconsistent approach to naming is the research results later achieved not just, but is up to the present in use, resulting in many misunderstandings arise, which comes to light especially in the secondary literature and interdisciplinary teaching.
So the names of the different visual pigments were initially often from a their own color characterizing prefix followed by the word - opsin formed. For example, was Cyanopsin the blue visual pigment and Chloropsin the yellow-green visual pigment. After it was recognized that the color absorption of the complete opsin little its chromophore, all the more dependent on the spatial arrangement of the opsin protein, were these names gradually changed and now rather call the site or the sensitivity of the pigment, and often the entire visual pigment (and not only its protein content ) than opsin. Accordingly, we recently for example speak only from red - sensitive or LW opsin ( long-wavelength opsin and long wavelength opsin ). Only when the neutral color of the visual pigment of the rod continues to dominate its historical common name rhodopsin ( visual purple ).
The following paragraphs attempt an overview of the current use various nomenclatures of the opsins admit to being pointed out again that these are rather uneven in part:
Opsin in the adaptation predominantly human medical sources
- Rhodopsin (human, rods) is denoted by its own color purple and has a maximum absorption at about 500 nm green.
- Porphyropsin (human, cones ) purple is after his own color called and absorbs maximally at about 535 nm green.
- Iodopsin (human, cones ) Purple is for his own color called and absorbs maximally at about 565 nm yellow.
- Cyanopsin (human, pin ) is designated according to its natural color turquoise to its absorption maximum at about 420 nm and therefore does even red.
Opsin in the general biosensing
- Rhodopsin visual pigment as the animals ( with and without spine) absorbed depending on the species maximum between 492 nm and 502 nm
- Iodopsin visual pigment as the color sensors animal polychromates based on retinal rhodopsin as 1 and depends on absorbing species and the type of pin with a maximum of about 350 nm (UV), about 420 nm ( S ), about 535 nm (M ), about 565 nm (L) or 640 nm ( XL).
- Porphyropsin as visual pigment of the freshwater fish contains two retinal as a chromophore retinal instead of 1, as rhodopsin. Its absorption maximum is at 522 nm
- Cyanopsin as visual pigment of the color sensors of freshwater fish contains analogous to Porphyropsin retinal chromophore as 2 instead of 1 Retinal how iodopsin. It has a maximum absorption at wavelengths that are similar to those of Iodopsins.
- Skotopsin opsin of the rod, which forms with retinal rhodopsin 1 together
- Photopsin opsin, the pin that forms with retinal 1 together iodopsin
Problems using the fuzzy concept of opsin
The name must not be interpreted rigorously as one would expect from smaller molecules from chemistry. To give even small variations in the structure of the opsin protein different absorption spectra of respectively compared, individual pigment variants. The absorption maximum can be a " certain " opsin so only with simultaneous indication at least of the bound retinal and the zoological origin are associated with, and even then this value can only be understood as the default value of the specified representative of this species. In addition, different authors use these terms rather uneven in its publications. For example, " iodopsin " is used as an umbrella term for the various pigments of the cones, although it also refers to a very special these pigments. As a " rhodopsin " is usually the visual purple of the rods referred to, although in the literature sometimes the signal transduction of the pins is attributed to the " rhodopsin ". " Porphyropsin " in turn is usually contrasted with the visual purple of freshwater fish are the " rhodopsin " of land animals, although a pigment of the cones of the Old World monkeys bears this name. The " Porphyropsin " the rods of freshwater fish, however, has two retinal as a chromophore, while " Porphyropsin " of the Old World monkeys, strictly speaking, a variant of the Iodopsine is so out of Retinal 1 is constructed as a chromophore and a pin - opsin as a protein component.
Conventions for the avoidance of conceptual inconsistencies
The term describes opsin in singular and plural in each case the necessary but not sufficient protein component of the visual pigment.
Classification of opsins on the cell type in which they occur
Skotopsin
The opsin visual pigments such that do not produce color sensations in the processing cascade, ie Skotopsin. It occurs in the rods.
Photopsin
The opsin of a visual pigment, the reaction leads to a color sensation, ie Photopsin. It occurs in the pin.
Classification of Photopsine in spectral
Photopsine be roughly divided into five spectral classes currently on the absorption maximum of the visual pigment in each of them with the chromophore formed together.
XL- opsin
Is a Photopsin which forms a visual pigment for stimuli in the near infrared.
L- opsin
Is a Photopsin which forms a visual pigment for stimuli in the long-wavelength VIS.
M- opsin
Is a Photopsin which forms a visual pigment for stimuli in the middle frequency range of the VIS.
S- opsin
Is a Photopsin which forms a visual pigment for stimuli in the short-wavelength VIS.
UV opsin
Is a Photopsin which forms a visual pigment for stimuli in the near ultraviolet.
Exception to the strict convention
The two far more thoroughly studied classes of visual pigments differ significantly in the chromophore and hot for retinal rhodopsin or one types of retinal 2 types Porphyropsin. This naming of visual pigments with terms that end in- opsin, goes back to the fundamental work of George Wald. It is too deeply rooted in the literature in that they could be avoided.
Summary Table
Defects
Defects in the opsin genes of pins may lead to color blindness. Defects in opsin gene of the rods can lead to pigmentary retinitis.