RHODOPSINS (Latin pigmentum paint) — light-sensitive pigments of photoreceptors of a retina of an eye. Perceiving energy of a light pulse, 3. items undergo a difficult cycle of photochemical transformations as a result of which the separate visual receptor of a retina of an eye containing 3. the item (a flask or a stick), turns into excited state and on an optic nerve transfers the acquired information to c. N of page. Being the main structurally functional part of a photoreceptor membrane of visual cells of a retina of an eye, 3. items thus play a key role in mechanisms sight (see).
Nomenclature and structure of rhodopsins. All studied 3. items of vertebrate and backboneless animals represent complexes of water insoluble membrane protein of opsin and related chromophore (retinal). The retinal, or aldehyde of vitamin A, can exist in two forms — retinal1 and retinal2.
By the nature of chromophore 3. items divide into two classes — rhodopsins (see), containing retinal1, and the porfiropsina containing retinal2. Rhodopsins contain in a mesh cover of an eye of all overland and marine animals, porfiropsina — in a retina of eyes of fresh-water animals. At some fishes and amphibians 3 are found. the items containing at the same time a retinal! and retinal. There are attempts to classify 3. the item on the basis of distinctions in the opsins specific to sticks or flasks of a retina of an eye. E.g., rhodopsin is a complex retinalya1 with bacillary opsin, iodopsin — retinalya1 with coneal opsin, porfiropsin — retinalya2 with bacillary opsin, the complex a retinal — coneal opsin forms tsianopsinony However to classify 3. the item on the basis of opsins extremely difficult, since various opsins, at least, five.
From all known 3. items are fullestly investigated the rhodopsins emitted from eyes of a bull, frog and squid. Their pier. the weight (weight) about 30 — 40 thousand, each molecule contains apprx. 400 amino acids and one chromophore. Besides, in structure of a molecule 3. the item enters an oligosakharidny chain: 3 radicals of a glycosamine, 2 mannoses, 1 galactoses. Lipids (hl. obr. phospholipids) form with a molecule 3. item strong complex. Keeping the main spectral properties (see. Spectral analysis ), 3. items without lipids lose a row functionally important, napr, ability to recovery.
The pure retinal has yellow color, the maximum of its absorption spectrum lies in the area of 370 nanometers. Opsin is colourless, a maximum of absorption — in ultra-violet area (apprx. 280 nanometers). Color of a molecule of rhodopsin reddish-pink, a maximum of an absorption spectrum apprx. 500 nanometers. The reason of such strong spectral shift at formation of a complex (from 370 to 500 nanometers — so-called bathochromic shift) did not receive still unambiguous explanation.
Maxima of absorption spectrums of rhodopsins and porfiropsin occupy rather wide area of a visible range — from 433 to 562 nanometers at rhodopsins and from 510 to 543 nanometers at porfiropsin. If to carry to porfiropsina also 3. the item of flasks of a tadpole of a frog, a carp and a fresh-water turtle, i.e. cyanodogs with a maximum of an absorption spectrum at 620 nanometers, this area is even wider. Development of methods of a mikrospektrofotometriya allowed to define absorption spectrums of many types of single photoreceptor zooblasts and the person. According to the obtained data, 3. items of a retina of the person have the following maxima of absorption spectrums: sticks 498, blue, green and krasnochuvstvitelny flasks — 440, 535 and 575 nanometers respectively.
Studying 3. with the item it is begun it. the researcher H. Muller who in 1851 described how the pinkish-purple retina taken from an eye of a frog becomes on light at first yellowish, and then whitish. In 1877 F. Boll also described this phenomenon, having drawn a conclusion that in visual cells of a retina there is some red light-sensitizing agent and that decolouration of this substance is connected with the mechanism of sight. A big merit in studying 3. the item belongs Keung (W. Kuhne, 1877), Krom managed to allocate 3. the item and in detail to investigate them. He called taken by it 3. the item rhodopsin, established its proteinaceous nature, investigated some of its spectral properties and phototransformations, found ability 3. the item to recovery in the dark. Big contribution to studying 3. the item was introduced by an amer. biochemist and physiologist J. Wald.
Phototransformations of visual pigments. At action on 3. the item of light in them happens a cycle of photochemical transformations, to-rogo priming photochemical reaction cis-a trance isomerization of a retinal is the cornerstone (see. Isomerism ). At the same time there is disruption of communication of chromophore to protein. Sequence of transformations 3. the item can be submitted as follows: rhodopsin (chromophore is in cis-form) —> prelyumirodopsin —> lyumirodopsin —> metarhodopsin I —> metarhodopsin II —> protein opsin —> chromophore in a transform. Under the influence of enzyme — a retinoldegidrogenaza — the last passes into vitamin A which comes from outside joints of sticks and flasks to cells of a pigmented layer of a retina. At blackout of an eye there is a regeneration 3. the item, for implementation a cut needs availability cis-of isomer of the vitamin A serving as an initial product for formation of chromophore (aldehyde of vitamin A). At a lack or absence of an organism of vitamin A formation of rhodopsin can be broken and develop as a result disorder of twilight sight, a so-called night blindness (see. Gemeralopiya ). In the course of phototransformations of rhodopsin at a stage of transition of a lyumirodopsin to metarhodopsin I in a receptor cell there is in response to bright flash a so-called early (korotkolatentny) receptor potential. At the same time it is not a visual signal, though can serve one of tests for a research of the mechanism of transformations 3. the item in a photoreceptor membrane. Functional value has the so-called late receptor potential, stage of latency to-rogo (5 — 10 ms) is commensurable with time of formation of metarhodopsin II. Assume that reaction of transition to I to metarhodopsin II provides emergence of a visual signal.
As on light 3. items continuously become colourless, there have to be mechanisms of their continuous recovery. One of them extremely bystry (photoregeneration), others rather bystry, (biochemical, regeneration, Dark), the third slow (synthesis 3. the item during continuous updating of a photoreceptor membrane in a visual cell). Photoregeneration has fiziol, value at backboneless animals (e.g., at cephalopod mollusks — squids, octopuses). In the mechanism of biochemical regeneration 3. the item at vertebrata, apparently, plays an important role enzyme isomerase (see), providing an isomerization of a trans-retinal (or trans-vitamin A) again in cis-isomeric form. However final proofs of existence of such enzyme are not available yet. Reaction of formation of a molecule 3. the item in the presence in system 11 - cis-isomer of a retinal and opsin happens easily, without energy consumption. Ability of the decoloured rhodopsin to reaction is found phosphorylations (see); it is supposed that this reaction is one of links of the mechanism of light adaptation of a visual cell.
See also Photochemical reactions .
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M. A. Ostrovsky.