Rhodopsin is a common visual pigment that is part of the rod-shaped visual retina of the eye retina of vertebrates. This substance has a very high photosensitivity and is a key component of photoreception. Another name for rhodopsin is visual purple.
Currently, rhodopsins include pigments of not only the rods, but also the rhabdomomeric visual arthropod receptors.
General characteristics of the pigment
By its chemical nature, rhodopsin is a membrane protein of animal origin containing a chromophore group in its structure. It is she who determines the ability of the pigment to capture light quanta. The rhodopsin protein has a molecular weight of approximately 40 kDA and contains 348 amino acid units.
The light absorption spectrum of rhodopsin consists of three bands:
- α (500 nm);
- β (350 nm);
- γ (280 nm).
Rays of γ are absorbed by aromatic amino acids in the composition of the polypeptide chain, and β and α are absorbed by the chromophore group.
Rhodopsin is a substance that can decompose under the influence of light, which triggers the electrotonic pathway of signal transmission through nerve fibers. This property is also characteristic of other photoreceptor pigments.
Rhodopsin structure
According to the chemical structure, rhodopsin is a chromoglycoprotein, which consists of 3 components:
- chromophore group;
- 2 oligosaccharide chains;
- water-insoluble protein opsin.
Vitamin A aldehyde (retinal), which is located in 11-cisform, acts as a chromophore group. This means that the long part of the retinal chain is curved and twisted to form an unstable configuration.
In the spatial organization of the rhodopsin molecule, 3 domains are distinguished:
- intramembrane;
- cytoplasmic;
- interdisc.
The chromophore group is located in the intramembrane domain. Her connection with the opsin is through the Schiff base.
Photoconversion Scheme
The mechanism of photoconversion of the rhodopsin pigment under the influence of light is based on the cis-trans isomerization of the retinal, i.e., on the conformational transition of the 11-cis form of the chromophore group into a straightened trans form. This process is carried out with tremendous speed (less than 0.2 picosecond) and activates a number of further transformations of rhodopsin, which occur already without the participation of light (dark phase).
The product formed under the action of a light quantum is called photododopsin. Its peculiarity is that the trans-retinal is still associated with the opsin polypeptide chain.
From the completion of the first reaction to the end of the dark phase, rhodopsin sequentially undergoes the following series of transformations:
- photorodopsin;
- batorodopsin;
- lumorodopsin;
- metarodopsin Ia;
- metarodopsin Ib;
- metarodopsin II;
- opsin and full-trans retinal.
These transformations are accompanied by stabilization obtained from the light quantum of energy and conformational rearrangement of the protein part of rhodopsin. As a result, the chromophore group is finally separated from the opsin and immediately removed from the membrane (the trans form has a toxic effect). After that, the process of regeneration of the pigment in the initial state.
Rhodopsin regeneration occurs due to the fact that outside the membrane, the trans-retinal acquires a cis-shape again, and then returns back, where it again forms a covalent bond with opsin. In vertebrates, recovery has the character of enzymatic resynthesis and occurs with the expenditure of energy, while in invertebrates it is carried out due to photoisomerization.
The mechanism of signal transmission from the pigment to the nervous system
The active component of the start of phototransduction is metarodopsin II. In this state, the pigment is able to interact with the protein transducin, thereby activating it. As a result, the transducine-bound HDF is replaced by GTP. At this stage, there is a simultaneous activation of a huge number of transducine molecules (500-1000). This process is called the first stage of amplification of the light signal.
Molecules of activated transducine then interact with photodiesterase (PDE). This enzyme in the active state is capable of very quickly destroying the cGMP compound necessary to maintain the ion channels in the receptor membrane in the open state. After the activation of PDE molecules caused by transducine, the concentration of cGMP drops to such a level that the channels are closed and sodium ions cease to enter the cell.
A decrease in the concentration of Na + in the cytoplasm of the outer part of the receptor puts the cytoplasmic membrane in a state of hyperpolarization. As a result, a transmembrane potential arises, which extends to the presynaptic end, reducing the release of the mediator. This is precisely the semantic result of the process of all transformations in the visual receptor.