Iodopsine

Normalized absorption spectra of the three human photopsins and human rhodopsin (dashed)

Iodopsin (Jodopsin) is the collective name for the color vision pigments of the cones of animals (including humans). They are photopsins ( proteins ) in a covalent bond with a coloring substance ( chromophore ) of type 1: neoretinin (11-cis-retinol) or retinal (11-cis-retinal). Pigments whose protein component photopsin binds a type 2 chromophore (3,4-dehydro-11-cis-retinal) are not referred to as iodopsin, but rather as porphyropsin . The absorption maxima and photopsins of the three iodopsins in humans are:

• 560 nm: red-sensitive L-iodopsin with L-photopsin, UniProt P04000 . Defects in the OPN1LW gene are the cause of protanopia
• 530 nm: green-sensitive M-iodopsin with M-photopsin, UniProt P04001 . Defects in the OPN1MW gene are the cause of deuteranopia
• 420 nm: blue-sensitive S-iodopsin with S-photopsin, UniProt P03999 . Defects in the OPN1SW gene are the cause of tritanopia

In the animal kingdom, in addition to these visual pigments with retinal 1 as a chromophore, there are also

• 340 nm: ultraviolet-sensitive UV-iodopsin with UV-photopsin and
• 620 nm: infrared-sensitive XL-iodopsin with XL-photopsin

in front.

Iodopsins contain 350 to 360 amino acids and are membrane proteins that belong to the G-protein-coupled receptors .

genetics

Genetic defect	Possible phenotype
Total loss of a green gene	green-blind, dichromate
Addition of a green gene	normal
Hybrid genes	red-blind, dichromate green weak, trichromate red weak , trichromate

The genes for the L- and M-photopsin of the red and green pigment are on the X chromosome , the gene for the S-photopsin of the blue pigment is on chromosome number 7.

The gene for red-sensitive L-photopsin is only available once in each person, whereas the gene for green-sensitive M-photopsin can, individually different, be present in one to three repeats in tandem. It is thus possible that unequal homologous recombinations can also occur during meiosis . Since the two genes are very similar, pairings between the homologous chromosomes also occur during meiosis, in which the very similar base sequences of the genes for L-opsin and for M-opsin are opposite. Depending on the position of the crossing point, the results of the crossing over differ . Whether one of the color ametropia occurs depends on whether the second sex chromosome can compensate for a defect after fertilization. In women, a second, unchanged X chromosome prevents ametropia. However, if the second X chromosome shows the same deviations, women can also be red or green blind or weak. Since the Y chromosome in men does not have any genes homologous to the photopsin genes, a defect occurs with one hundred percent probability.

A duplication of the red gene is very rare because it is at the end of the chain of photopsin genes lined up one behind the other.

Hybrid genes are created by exchanging sections between the red and green genes. Depending on the proportion of the individual gene segments, the red or green gene fails or pigments with deviating absorption maxima are formed. In the case of green photopsin, it is sufficient that the amino acid alanine at position 285 is exchanged for the amino acid threonine , so that the absorption maximum of the pigment formed by this M-photopsin is shifted from 535 nm to 549 nm.

evolution

Hypothetical human opsin family tree based on matches of the base sequences of the opsin genes

The base sequences of the red and green genes agree 98 percent. They are the result of duplication from a single precursor gene. Since New World monkeys only have a single visual pigment on the X chromosome, the duplication must have taken place after the separation of the gene pools of the ancestors of the Old and New World monkeys when Africa and South America drifted apart due to the continental drift about 40 million years ago.

The amino acid sequence of the red photopsin only differs in 15 of the 364 amino acids of the M-photopsin, the S-photopsin and the scotopsin of the rhodopsin differ considerably.

See also

• Opsin : with an overview of the common terms with the ending -opsin
• Photopsin : protein component of the pigments with retinal 1 or retinal 2 for color vision
• Cyanopsin : pigment made from photopsin and retinal 2 for the color vision of freshwater fish
• Rhodopsin : pigment from scotopsin and retinal 1 for scotopic vision
• Porphyropsin : pigment made from scotopsin and retinal 2 for scotopic vision in freshwater fish

Individual evidence

1. ↑ Error in the source?
2. ↑ ^{a b} Jeremy Nathans : The genes for color vision . In: Spectrum of Science . April 1989, p.  68 ff . ( slideplayer.org [ MS PowerPoint ; 1,2 MB ]).
3. ↑ ^{a b} Jürgen Martin: When red is not red . In: Spectrum of Science - Special . Volume 5: Colors , 2005, p. 74 ff .