What Is an Opsin?

Opsin roughly includes two families of proteins from different sources. The structure of one section is very different. The tertiary structure may be very similar due to convergence and evolution. Both are 7-transmembrane structures. Among them, the most familiar opsins in the public are several visual opsins in the visual system. They also have several homologous non-visual opsins, which are widely distributed in various parts of the body and in the visual system. Among them, Opsin4 regulates the biological rhythm. Apart from the clearer relationship, the specific functions of the remaining non-visual opsins are still being studied. There is also a kind of opsin in bacteria, because it is similar to animal opsin in three-dimensional structure. It is also called opsin. It has no visual function but can be sensitive to light. It may be derived from different ancestors as animal opsin, but due to convergence and evolution. Similar tertiary structure.

Opsin is a membrane protein with a molecular weight of about 30-50 kDa. It includes an extracellular amino terminus, 7 transmembrane regions, and an intracellular carboxyl terminus, and belongs to the Gprotein-coupled receptor (GPCR) superfamily. Opsin is widely distributed in animals and microorganisms. Non-animal opsins such as archaea, bacteria, and fungi have similar three-dimensional structures to animal opsins, but they differ significantly in amino acid sequences. Some studies have confirmed that there are many types of animal opsin proteins, which are widely distributed, and have some non-visual functions such as visual sensitivity, regulating biological circadian rhythms, and participating in pupil reflection on light.

Since the discovery of the first opsin, bovine rhodopsin, the number of animal opsins has reached thousands, forming a large opsin family. According to whether or not they are directly involved in visual imaging, opsin can be roughly divided into two categories: visual opsin in the visual system and non-visualopsin in the non-visual system; if the molecular evolution analysis and function are different ( Based in part on the different types of receptors G proteins are coupled to), the opsin family can be divided into 7 subfamilies: vertebrate visual and non-visual opsin subfamily, encephalopsin / tmt-opsin subfamily, and Gq-coupled optoin Protein / melanopsin subfamily, Go-coupled opsin subfamily, neuropsin subfamily, peropsin subfamily, and retinalphotoisomerase subfamily. Studies have found that during the long process of biological evolution, the same subfamily opsins maintain high homology among different vertebrates, and the amino acid similarity is generally greater than 75%; the opsins of different subfamilies are in the same species. Amino acid similarity is only 25% to 40%; while for different subfamily opsins, the amino acid similarity between different species is less than 25%.

In the past, it has been thought that opsin is mainly distributed in visual tissues such as the retina, and its function is mainly to form opsins with chromophores. As photoreceptors in animals, they participate in light adaptation of animals to the external environment. However, with the continuous expansion and deepening of opsin research, it has been discovered that some opsin can also be distributed in multiple parts outside the retina, and its function is not just visual sensitivity. These important research results make people have to re-examine the function of opsin, so the research on the function of non-visual opsin outside the retina has become a current hot spot.

Generally, there are two types of photoreceptor cells on the retina of vertebrates: rod cells and cone cells. Opsin in rod cells is rhodopsin (Rh), which is related to dark vision, while cones are in cone cells, which is related to bright vision. According to different absorption spectrum ranges, cones can be further divided into long-wavelength-sensitive opsin (LW), middle-wave-length-sensitive opsin (MW) and short-wave sensitive opsin (short-wavelength-sensitiveopsin, SW), and the latter can also be divided into SW1 and SW2. Most of the vertebrates contain the above-mentioned four kinds of cone proteins on the retina, but in most mammals, SW2 and MW opin have not been found. Vertebrate visual opsin includes rhodopsin and cone protein, both of which are combined with 11-cis-retinal to form opsin. When exogenous light stimulates photoreceptor cells, 11-cis-retinaldehyde in retinoids is induced by light to all-trans-retinal and is separated from opsin; subsequently, opsin Activate the coupled G protein, and then activate the cGMP phosphodiesterase through the intracellular phosphorylation cascade, which will reduce the cGMP concentration in the cell, resulting in the closure of the cGMP-gated Na ion channel and hyperpolarization of the photoreceptor cells The formation of hyperpolarized sensory potentials produces vision after being transmitted to the brain. Some lower vertebrate opsins and vertebrate visual opsins are evolutionarily analyzed. Although they belong to the same subfamily, these opsins are not expressed in rod cells and cone cells, nor do they participate in the visual photoreceptor system. It is called "non-visual opsin". Pinopsin is the first non-visual opsin discovered. It is expressed in the pineal glands of chickens and lizards, and may participate in the reaction to the outside world by activating two different light-transmitting enzymes, transducin and G protein G11. Environmental photosensitivity [2, 3]. Researchers also found a non-visual opsin sensitive to ultraviolet light in the pineal eyes of fish, Xenopus, and lamprey, named "parapinopsin." It was found in research on lampreys that parapinopsin is mainly located in the upper or back area of the pinecone eye, which may be related to the most light received here; it can affect the biological rhythm of the lamprey, which is related to the pineal gland in higher animals. Similar effect.

Opsin is the first stop for the eye to perceive light. It amplifies the light signal through the G protein and phosphorylation cascade coupled to it, and transmits information to the brain through a series of signal transduction pathways. Each opsin has its specific peaks and ranges of spectral sensitivity, which excite different degrees of external light, forming a regular 24-hour circadian rhythm of the human body and a colorful world in front of you. Optoprotein gene mutations and structural dysfunctions can cause severe visual disturbances and circadian clock disturbances.

(A) retinal pigment degeneration

Retinitis pigmentosa (RP) is a type of hereditary disease characterized by progressive loss of retinal photoreceptor cells and RPE cells. It is clinically characterized by night blindness and progressive visual field defects, and is a group of blinding diseases that lack effective treatment methods.

(Two) abnormal color vision

As early as the 19th century, a three-color theory was proposed. There are three types of neural pathways on the retina that are sensitive to red, green, and blue. For visible light of a certain wavelength in the spectrum, the three pathways transmit signals to the brain at different levels of excitation. This results in a corresponding color vision. Later generations also confirmed this theory through studies such as microspectrophotometry, fundus reflection spectrophotometry, and ultramicroelectrode method. Correspondingly, it is L-type, M-type, and S-type opsin in photoreceptor cells. Their abnormalities correspond to the abnormalities of red, green, and blue vision, respectively.

(C) other

Further research on non-visual system opsin found that melanopsin plays a vital role in human circadian rhythms and pupils' reflection of light. Every physiological function of the human body shows highly precise and stable circadian rhythm The "failure" of the biological clock can also bring diseases to the body. Further research on melanopsin has also raised objections to the traditional "blind eye", making people more careful about the implementation of eye removal surgery ^[1] .