Opsin
Opsin describes the protein content of a visual pigment, which consists of a protein and a chromophore . In the English-speaking world, however, the term opsin is often used for the entire visual pigment.
Opsins belong to the superfamily of heptahelical transmembrane proteins or G-protein-coupled receptors . The chromophore is a terpenoid , usually 11-cis- retinal (retinal 1) or 11-cis-3,4-dehydro-retinal (retinal 2) .
There are two different groups of opsins that Skotopsine or rod-Opsine that the rods are responsible for vision in twilight and darkness, and the Photopsine that or journal Opsine in the journal , the color vision allow. 11-cis-retinal, together with the rod opsin, forms the light-sensitive pigment molecule rhodopsin , which triggers the process of visual signal transduction in the rods, and together with the various cone opsins, the iodopsins , which absorb different colors (see below), the get the same in the cones.
Localization
In vertebrates, opsin is essentially stored in connection with retinal in the outer segment of the photoreceptors . With the rods , the rhodopsin sits in the disks and covers 90% of the total protein content of the structure. The iodopsins corresponding to rhodopsin are located in membrane folds in the cones . In insects and other animals with photoreceptors of the rhabdomer type, the protein is built into the membrane of the microvilli border of the visual cells.
nomenclature
The best-known pigment molecules found in animal retina are called like the protein with a prefix to identify their own color or complementary color . This heuristic but inconsistent approach to naming does not do justice to the research results achieved later, but is still in use today, which gives rise to many misunderstandings, which are particularly evident in secondary literature and interdisciplinary teaching.
The names of the different visual pigments were initially often formed from a prefix characterizing their own color , followed by the word -opsin . For example, cyanopsin was the blue visual pigment and chloropsin was the yellow-green visual pigment . After it was recognized that the color absorption of the complete visual pigment depends only a little on its chromophore, but more on the spatial arrangement of the opsin protein, these terms were gradually changed and now rather describe the place of use or the sensitivity of the pigment, often also the entire visual pigment (and not just its protein content) as opsin . Accordingly, we have recently only spoken of red-sensitive or LW-Opsin ( long-wave opsin or long-wavelength opsin ). Only in the case of the color-neutral visual pigment of the rods does its historical trivial name rhodopsin ( visual purple ) continue to dominate .
The following paragraphs try to give an overview of the various currently used nomenclatures of the opsins, whereby it should be pointed out again that these are in part quite inconsistent:
Opsin in the adaptation of mainly human medical sources
- Rhodopsin
- (human, rod) is named purple after its own color and absorbs green at a maximum of around 500 nm .
- Porphyropsin
- (human, cone) is named after its intrinsic color purple and absorbs a maximum of around 535 nm green.
- Iodopsin
- (human, cone) is named after its own color violet and absorbs yellow at a maximum of around 565 nm .
- Cyanopsin
- (human, cone) is named turquoise after its absorption maximum at around 420 nm and therefore looks red itself .
Opsin in general biosensors
- Rhodopsin
- as the visual pigment of the animals ( with and without spine ) is based on retinal 1 and absorbs between 492 nm and 502 nm, depending on the species.
- Iodopsin
- As the visual pigment of the color sensors of animal polychromats , like rhodopsin, it is based on retinal 1 and, depending on the species and type of the cone, absorbs at a maximum of about 350 nm (UV), about 420 nm (S), about 535 nm (M), about 565 nm ( L) or approx. 640 nm (XL).
- Porphyropsin
- as the visual pigment of freshwater fish, retinal 2 contains as chromophore instead of retinal 1 of rhodopsin. Its absorption maximum is 522 nm.
- Cyanopsin
- As the visual pigment of the color sensors of freshwater fish, it contains retinal 2 as a chromophore instead of retinal 1, like iodopsin, analogous to porphyropsin. It absorbs maximally at wavelengths that are similar to those of iodopsin.
- Scotopsin
- Opsin of the rods, which together with Retinal 1 forms rhodopsin.
- Photopsin
- Opsin the cone that forms iodopsin with Retinal 1.
Problems with using the vague term opsin
The naming must not be strictly understood as one is used to with smaller molecules in chemistry. Even small variations in the structure of the opsin protein result in different absorption spectra of the compared, individual pigment variants. The absorption maximum can therefore only be assigned to a "specific" opsin if at least the bound retinal and the zoological origin are given at the same time, and even then this value is only to be understood as a standard value for the representatives of this specified species. In addition, different authors use these terms inconsistently in their publications. For example, “iodopsin” is used as an umbrella term for the various pigments in the cones, although it also designates a very special one of these pigments. The visual purple of the rods is usually referred to as “rhodopsin”, although the signal transduction of the cones is sometimes ascribed to “rhodopsin” in the specialist literature . “Porphyropsin”, on the other hand, is usually contrasted as the visual purple of freshwater fish with the “rhodopsin” of land animals, although a pigment in the cones of the Old World monkeys also bears this name. The “porphyropsin” of the rods of freshwater fish, however, has retinal 2 as a chromophore, while “porphyropsin” of the Old World monkeys is actually a variant of the iodopsins, ie it is composed of retinal 1 as the chromophore and a cone opsin as the protein component.
Conventions to avoid conceptual incoherence
The term opsin describes in singular and plural the necessary but insufficient protein component of a visual pigment.
Classification of opsins according to the cell type in which they occur
Weak point of this definition: color sensations are possible as soon as two visual pigments with different spectral sensitivity are present. Regardless of their name and chemistry. Color sensations are created by comparing the stimuli of different visual pigments. In principle, even a visual pigment or sensor with various upstream filters is sufficient (examples: MKF-6 , RGB sensor ).
Classification of the photopsins in spectral classes
Photopsins are roughly divided into currently 5 spectral classes according to the absorption maximum of the visual pigment they form together with the chromophore:
- XL opsin
- is a photopsin that forms a visual pigment for stimuli in the near infrared .
- L-opsin
- is a photopsin that forms a visual pigment for stimuli in long-wave VIS .
- M-opsin
- is a photopsin that forms a visual pigment for stimuli in the medium frequency range of the VIS.
- S-opsin
- is a photopsin that forms a visual pigment for stimuli in the short-wave VIS.
- UV opsin
- is a photopsin, which forms a visual pigment for stimuli in the near ultraviolet .
Exception to the strict convention
The two classes of visual pigments that have been particularly thoroughly researched to date differ significantly in terms of the chromophore and are called rhodopsin for retinal 1 types and porphyropsin for retinal 2 types . This naming of visual pigments with terms that end in -opsin goes back to the elementary work of George Wald . It is too deeply anchored in the specialist literature to be avoided.
Overview table
| Chromophore | Opsin type | pigment | Absorption maximum λ max in nm |
Examples of occurrence |
|---|---|---|---|---|
| 11- cis - Retinal (Retinal 1) |
Scotopsin | Rhodopsin | 500 | Humans, vertebrates, arthropods, mollusks |
| 11 cis retinal | UV photopsin | UV iodopsin | 340 | Honey bee |
| 11 cis retinal | S-photopsin | S-iodopsin | 430 | Monkeys |
| 11 cis retinal | M-photopsin | M-iodopsin | 535 | |
| 11 cis retinal | L-photopsin | L-iodopsin | 565 | Old world monkeys |
| 11 cis retinal | XL photopsin | XL-iodopsin | 620 | Birds |
| 11 cis retinal | (Apo-) melanopsin | Holo-melanopsin | 485 | Human (retinal ganglion cells) |
| 11- cis -3,4-dehydro-retinal (retinal 2) |
Scotopsin | Porphyropsin | 520 | Freshwater fish, amphibians |
| 11- cis -3,4-dehydro-retinal | UV photopsin | UV cyanopsin | 360 | |
| 11- cis -3,4-dehydro-retinal | S-photopsin | S-cyanopsin | 420 | |
| 11- cis -3,4-dehydro-retinal | M-photopsin | M-cyanopsin | 530 | |
| 11- cis -3,4-dehydro-retinal | L-photopsin | L- cyanopsine | 580 | |
| 11- cis -3,4-dehydro-retinal | XL photopsin | XL-cyanopsin | 620 | |
| 9- cis -retinal | Scotopsin | Iso-rhodopsin | 485 | |
| 9- cis -retinal | Photopsin | Iso-iodopsin | 515 | |
| 9- cis -3,4-dehydro-retinal | Scotopsin | Iso-porphyropsin | 510 | |
| 9- cis -3,4-dehydro-retinal | Photopsin | Isocyanopsin | 575 | |
| 13 cis retinal | Bacteria opsin |
Bacteriorhodopsin , proteorhodopsin |
560 | Bacteria (light-driven proton pump , see also chemiosmotic coupling ) |
| 13 cis retinal | Halo opsin | Halorhodopsin | 570 | Halobacteria ( light-controlled chloride channel ) |
| 13 cis retinal | Channel opsine | Channelrhodopsins | 460-535 | Algae ( light-controlled cation channel ) |
Defects
Defects in the opsin genes of the cones can lead to color blindness . Defects in the rod opsin gene can lead to retinitis pigmentosa .
See also
Individual evidence
- ↑ University of Augsburg ( Memento of the original from June 24, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ University of Frankfurt (PDF; 4.9 MB).
- ↑ a b c d George Wald: The molecular basis of visual excitation (PDF; 292 kB), Nobel lecture, December 12, 1967.
- ^ A b Iodopsin, George Wald, Paul K. Brown, and Patricia H. Smith: Iodopsin. In: The Journal of General Physiology , Vol 38, 623-681, 1955.
- ↑ Lexicon of Medicine: Iodopsin ( Memento from October 15, 2009 in the Internet Archive ) at imedo.de.
