Olfactory receptor (protein)
Odor receptors or olfactory receptors ( English olfactory receptors ; abbreviated OR ) are receptors for chemical stimuli and, as membrane proteins of chemoreceptors, are particularly involved in the perception of smell (see olfactory perception ). In addition, odor receptors are also found in organs that are not involved in odor perception (e.g. in the liver and testicles ). In vertebrates , the olfactory receptor molecule is a G protein-coupled receptor . The number of different types of olfactory receptors is around 350 in humans, while it is around 1200 different in dogs.
In physiology , the term olfactory receptor is also used for an entire nerve cell which, as a sensory cell of the olfactory system, stores specific olfactory receptor proteins in the membrane of its cilia : the olfactory cell as a receptor cell for the sense of smell.
selectivity
Odor receptors are target molecules for odorous substances that bind to them and can activate the odor receptor. Thereby, odor receptors show a selectivity for different odorous substances. At the extracellular end of the olfactory receptor, which spans the cell membrane seven times, the receptor protein forms a pocket. This pocket represents a docking point for the fragrance molecule, with which it can connect according to the lock and key principle . Due to their molecular structure , only certain molecules can bind to the bag. Therefore, the olfactory receptor is specific for that particular molecule or group of structurally similar molecules. Slight changes in the protein structure lead to changes in the conformation of the docking site and thus vary the specificity of an olfactory receptor.
Diversity
Every olfactory cell produces numerous olfactory receptor molecules of a certain type with the same protein structure and anchors them in the membrane. The olfactory cells of a vertebrate differ from one another by the type of olfactory receptor they have built in. Recent vertebrates have many different types of olfactory receptors, varying in number depending on the species , from around 100 in fish to over 1000 in mice and dogs.
In humans, more than three hundred different olfactory receptors can be formed, the structure of which is individually genetically coded. The approximately 340 genes of the human OR family can be found scattered in over fifty locations in the genome ; only chromosomes 8 , 20 and the Y chromosome do not contain any genes for olfactory receptors according to current knowledge. About 170 subfamilies can be distinguished according to the similarity of their sequences. Often genes with similar sequence motifs are chromosomally adjacent. It is assumed that the diversity of the olfactory receptors was caused by gene duplication and later mutation . In addition to intact genes, there are almost as many pseudogenes in the human genome that are inactive.
Signal transduction
After binding the odorous substance to the receptor protein, there is a conformational change of that protein and an activation of the attached G protein (G olf ). This complex of receptor and G protein is responsible for the transmission of the olfactory stimulus into the cell interior ( signal transduction ). It activates the enzyme adenylyl cyclase , which catalyzes the conversion of ATP to cAMP , and thus increases the concentration of this second messenger in the cilia . This intracellular messenger substance in turn activates protein kinases , which can open ion channels on the cell membrane and thus influence the membrane potential . First, the type of ion channel is opened via cAMP, through which positive sodium and calcium ions can flow into the cell interior. The influx of calcium then indirectly activates a second type of ion channel, which is specific for negative chlorine ions that are now flowing out of the cell. The result is a depolarization that can generate an action potential on the axon hillock of the olfactory cell .
This signal from a sensory cell is transmitted as an action potential series via its axon in the olfactory nerve ( olfactory nerve ) to neurons in the olfactory bulb ( olfactory bulb ). From here there are connections to the primary olfactory cortex or to the further evaluation of the olfactory perception in other regions of the central nervous system via the olfactory tract ( tractus olfactorius ) .
Classification and naming
The olfactory receptors form a superfamily. All previously known olfactory receptors of vertebrates have basically the same structure with a variable recognition region, 7 transmembrane sections and an activation area for the G protein. The following scheme is proposed for naming individual olfactory receptors or their genes:
OR / number / letter / number : for example OR1A1.
OR stands for the superfamily of olfactory receptors and the following number for one of the 56 families. The following capital letter unites a group of olfactory receptors whose genes match at least 60%. The last number classifies the special olfactory receptor. In addition, classes of families can be defined:
- Class I: fish analog receptors, families 51-56
- Class II: tetrapod- specific receptors, families 1–13
Research history
The greatest success in recent years in researching the sense of smell was achieved by the two American researchers Linda Buck and Richard Axel , who were able to identify around 1000 genes coding for olfactory receptors in their investigations and received the 2004 Nobel Prize for Medicine . These genes determine the protein structure of a receptor protein and thus its specificity. So far, only a few of the approximately 340 olfactory receptors in the human body have been investigated in more detail.
Individual evidence
- ↑ Anna Menini: The Neurobiology of Olfaction . In: Frontiers in Neuroscience . CRC, 2010, ISBN 978-1-4200-7199-3 (English).
- ↑ B. Malnic, PA Godfrey, LB Buck: The human olfactory receptor gene family . In: Proceedings of the National Academy of Sciences . tape 101 , no. 8 , February 13, 2004, ISSN 0027-8424 , p. 2584-2589 , doi : 10.1073 / pnas.0307882100 (English).
- ↑ L. Oboti, P. Peretto, SD Marchis, A. Fasolo: From chemical neuroanatomy to an understanding of the olfactory system . In: European journal of histochemistry (EJH) . tape 55 , no. 4 , 2011, ISSN 2038-8306 , p. e35 , PMID 22297441 , PMC 3284237 (free full text) - (English).
- ↑ H. Spors, DF Albeanu, VN Murthy, D. Rinberg, N. Uchida, M. Wachowiak, RW Friedrich: Illuminating vertebrate olfactory processing . In: The Journal of neuroscience. The official journal of the Society for Neuroscience . tape 32 , no. 41 , October 2012, ISSN 1529-2401 , p. 14102–14108 , doi : 10.1523 / JNEUROSCI.3328-12.2012 , PMID 23055479 , PMC 3752119 (free full text) - (English).
- ↑ G. Glusman, A. Bahar, D. Sharon, Y. Pilpel, J. White: The olfactory receptor gene superfamily. Data mining, classification, and nomenclature . In: Mammalian Genome: Official Journal of the International Mammalian Genome Society . tape 11 , no. November 11 , 2000, ISSN 0938-8990 , p. 1016-1023 , doi : 10.1007 / s003350010196 , PMID 11063259 (English).
