Olfactory receptors (ORs), also known as odorant receptors, are expressed in the cell membranes of olfactory receptor neurons and are responsible for the detection of odorants (i.e., compounds that have an odor) which give rise to the sense of smell. Activated olfactory receptors trigger nerve impulses which transmit information about odor to the brain. These receptors are members of the class A rhodopsin-like family of G protein-coupled receptors (GPCRs). The olfactory receptors form a multigene family consisting of around 800 genes in humans and 1400 genes in mice.
In vertebrates, the olfactory receptors are located in both the cilia and synapses of the olfactory sensory neurons and in the epithelium of the human airway. In insects, olfactory receptors are located on the antennae and other chemosensory organs. Sperm cells also express odor receptors, which are thought to be involved in chemotaxis to find the egg cell.
Rather than binding specific ligands, olfactory receptors display affinity for a range of odor molecules, and conversely a single odorant molecule may bind to a number of olfactory receptors with varying affinities, which depend on physio-chemical properties of molecules like their molecular volumes . Once the odorant has bound to the odor receptor, the receptor undergoes structural changes and it binds and activates the olfactory-type G protein on the inside of the olfactory receptor neuron. The G protein (Golf and/or Gs) in turn activates the lyase - adenylate cyclase - which converts ATP into cyclic AMP (cAMP). The cAMP opens cyclic nucleotide-gated ion channels which allow calcium and sodium ions to enter into the cell, depolarizing the olfactory receptor neuron and beginning an action potential which carries the information to the brain.
The primary sequences of thousands of olfactory receptors are known from the genomes of more than a dozen organisms: they are seven-helix transmembrane proteins, but there are (as of May 2016) no known structures of any OR. Their sequences exhibit typical class A GPCR motifs, useful for building their structures with molecular modeling. Golebiowski, Ma and Matsunami showed that the mechanism of ligand recognition, although similar to other non-olfactory class A GPCRs, involves residues specific to olfactory receptors, notably in the sixth helix. There is a highly conserved sequence in roughly three quarters of all ORs that is a tripodal metal ion binding site, and Suslick has proposed that the ORs are in fact metalloproteins (mostly likely with zinc, copper and possibly manganese ions) that serve as a Lewis acid site for binding of many odorant molecules. Crabtree, in 1978, had previously suggested that Cu(I) is "the most likely candidate for a metallo-receptor site in olfaction" for strong-smelling volatiles which are also good metal-coordinating ligands, such as thiols. Zhuang, Matsunami and Block, in 2012, confirmed the Crabtree/Suslick proposal for the specific case of a mouse OR, MOR244-3, showing that copper is essential for detection of certain thiols and other sulfur-containing compounds. Thus, by using a chemical that binds to copper in the mouse nose, so that copper wasn’t available to the receptors, the authors showed that the mice couldn't detect the thiols. However, these authors also found that MOR244-3 lacks the specific metal ion binding site suggested by Suslick, instead showing a different motif in the EC2 domain.
In a recent but highly controversial interpretation, it has also been speculated that olfactory receptors might really sense various vibrational energy-levels of a molecule rather than structural motifs via quantum coherence mechanisms. As evidence it has been shown that flies can differentiate between two odor molecules which only differ in hydrogen isotope (which will drastically change vibrational energy levels of the molecule). Not only could the flies distinguish between the deuterated and non-deuterated forms of an odorant, they could generalise the property of "deuteratedness" to other novel molecules. In addition, they generalised the learned avoidance behaviour to molecules which were not deuterated but did share a significant vibration stretch with the deuterated molecules, a fact which the differential physics of deuteration (below) has difficulty in accounting for.
It should be noted, however, that deuteration changes the heats of adsorption and the boiling and freezing points of molecules (boiling points: 100.0 °C for H2O vs. 101.42 °C for D2O; melting points: 0.0 °C for H2O, 3.82 °C for D2O), pKa (i.e., dissociation constant: 9.71x10−15 for H20 vs. 1.95x10−15 for D2O, cf. heavy water) and the strength of hydrogen bonding. Such isotope effects are exceedingly common, and so it is well known that deuterium substitution will indeed change the binding constants of molecules to protein receptors.
It has been claimed that human olfactory receptors are capable of distinguishing between deuterated and undeuterated isotopomers of cyclopentadecanone by vibrational energy level sensing. However this claim has been challenged by another report that the human musk-recognizing receptor, OR5AN1 that robustly responds to cyclopentadecanone and muscone, fails to distinguish isotopomers of these compounds in vitro. Furthermore, the mouse (methylthio)methanethiol-recognizing receptor, MOR244-3, as well as other selected human and mouse olfactory receptors, responded similarly to normal, deuterated, and carbon-13 isotopomers of their respective ligands, paralleling results found with the musk receptor OR5AN1. Hence it was concluded that the proposed vibration theory does not apply to the human musk receptor OR5AN1, mouse thiol receptor MOR244-3, or other olfactory receptors examined. In addition, the proposed electron transfer mechanism of the vibrational frequencies of odorants could be easily suppressed by quantum effects of nonodorant molecular vibrational modes. Hence multiple lines of evidence argue against the vibration theory of smell. This later study was criticized since it used "cells in a dish rather than within whole organisms" and that "expressing an olfactory receptor in human embryonic kidney cells doesn't adequately reconstitute the complex nature of olfaction...". In response, the authors of the second study state "Embryonic kidney cells are not identical to the cells in the nose .. but if you are looking at receptors, it's the best system in the world."
There are a large number of different odor receptors, with as many as 1,000 in the mammalian genome which represents approximately 3% of the genes in the genome. However, not all of these potential odor receptor genes are expressed and functional. According to an analysis of data derived from the Human Genome Project, humans have approximately 400 functional genes coding for olfactory receptors, and the remaining 600 candidates are pseudogenes.
The reason for the large number of different odor receptors is to provide a system for discriminating between as many different odors as possible. Even so, each odor receptor does not detect a single odor. Rather each individual odor receptor is broadly tuned to be activated by a number of similar odorant structures. Analogous to the immune system, the diversity that exists within the olfactory receptor family allows molecules that have never been encountered before to be characterized. However, unlike the immune system, which generates diversity through in-situ recombination, every single olfactory receptor is translated from a specific gene; hence the large portion of the genome devoted to encoding OR genes. Furthermore, most odors activate more than one type of odor receptor. Since the number of combinations and permutations of olfactory receptors is very large, the olfactory receptor system is capable of detecting and distinguishing between a very large number of odorant molecules.
Deorphanization of odor receptors can be completed using electrophysiological and imaging techniques to analyze the response profiles of single sensory neurons to odor repertoires. Such data open the way to the deciphering of the combinatorial code of the perception of smells.
Such diversity of OR expression maximizes the capacity of olfaction. Both monoallelic OR expression in a single neuron and maximal diversity of OR expression in the neuron population are essential for specificity and sensitivity of olfactory sensing. Thus, olfactory receptor activation is a dual-objective design problem. Using mathematical modeling and computer simulations, Tian et al proposed an evolutionarily optimized three-layer regulation mechanism, which includes zonal segregation, epigenetic barrier crossing coupled to a negative feedback loop and an enhancer competition step  . This model not only recapitulates monoallelic OR expression but also elucidates how the olfactory system maximizes and maintains the diversity of OR expression.
A nomenclature system has been devised for the olfactory receptor family and is the basis for the official Human Genome Project (HUGO) symbols for the genes that encode these receptors. The names of individual olfactory receptor family members are in the format "ORnXm" where:
- OR is the root name (Olfactory Receptor superfamily)
- n = an integer representing a family (e.g., 1-56) whose members have greater than 40% sequence identity,
- X = a single letter (A, B, C, ...) denoting a subfamily (>60% sequence identity), and
- m = an integer representing an individual family member (isoform).
For example, OR1A1 is the first isoform of subfamily A of olfactory receptor family 1.
Members belonging to the same subfamily of olfactory receptors (>60% sequence identity) are likely to recognize structurally similar odorant molecules.
Two major classes of olfactory receptors have been identified in humans:
- class I (fish-like receptors) OR families 51-56
- class II (tetrapod specific receptors) OR families 1-13
The olfactory receptor gene family in vertebrates has been shown to evolve through genomic events such as gene duplication or gene conversion. Evidence of a role for tandem duplication is provided by the fact that many olfactory receptor genes belonging to the same phylogenetic clade are located in the same gene cluster. To this point, the organization of OR genomic clusters is well conserved between humans and mice, even though the functional OR count is vastly different between these two species. Such birth-and-death evolution has brought together segments from several OR genes to generate and degenerate odorant binding site configurations, creating new functional OR genes as well as pseudogenes.
Compared to many other mammals, primates have a relatively small number of functional OR genes. For instance, since divergence from their most recent common ancestor (MRCA), mice have gained a total of 623 new OR genes, and lost 285 genes, whereas humans have gained only 83 genes, but lost 428 genes. Mice have a total of 1035 protein-coding OR genes, humans have 387 protein-coding OR genes. The vision priority hypothesis states that the evolution of color vision in primates may have decreased primate reliance on olfaction, which explains the relaxation of selective pressure that accounts for the accumulation of olfactory receptor pseudogenes in primates. However, recent evidence has rendered the vision priority hypothesis obsolete, because it was based on misleading data and assumptions. The hypothesis assumed that functional OR genes can be correlated to the olfactory capability of a given animal. In this view, a decrease in the fraction of functional OR genes would cause a reduction in the sense of smell; species with higher pseudogene count would also have a decreased olfactory ability. This assumption is flawed. Dogs, which are reputed to have good sense of smell, do not have the largest number of functional OR genes. Additionally, pseudogenes may be functional; 67% of human OR pseudogenes are expressed in the main olfactory epithelium, where they possibly have regulatory roles in gene expression. More importantly, the vision priority hypothesis assumed a drastic loss of functional OR genes at the branch of the OWMs, but this conclusion was biased by low-resolution data from only 100 OR genes. High-resolution studies instead agree that primates have lost OR genes in every branch from the MRCA to humans, indicating that the degeneration of OR gene repertories in primates cannot simply be explained by the changing capabilities in vision.
It has been shown that negative selection is still relaxed in modern human olfactory receptors, suggesting that no plateau of minimal function has yet been reached in modern humans and therefore that olfactory capability might still be decreasing. This is considered to provide a first clue to the future human genetic evolution.
In 2004 Linda B. Buck and Richard Axel won the Nobel Prize in Physiology or Medicine for their work on olfactory receptors. In 2006 it was shown that another class of odorant receptors exist for volatile amines. Except for TAAR1, all functional TAARs in humans are expressed in the olfactory epithelium.
- Gaillard I, Rouquier S, Giorgi D (Feb 2004). "Olfactory receptors". Cellular and Molecular Life Sciences. 61 (4): 456–69. doi:10.1007/s00018-003-3273-7. PMID 14999405.
- Hussain A, Saraiva LR, Korsching SI (Mar 2009). "Positive Darwinian selection and the birth of an olfactory receptor clade in teleosts". Proceedings of the National Academy of Sciences of the United States of America. 106 (11): 4313–8. Bibcode:2009PNAS..106.4313H. doi:10.1073/pnas.0803229106. PMC . PMID 19237578.
- Niimura Y (Dec 2009). "Evolutionary dynamics of olfactory receptor genes in chordates: interaction between environments and genomic contents". Human Genomics. 4 (2): 107–18. doi:10.1186/1479-7364-4-2-107. PMC . PMID 20038498.
- Rinaldi A (Jul 2007). "The scent of life. The exquisite complexity of the sense of smell in animals and humans". EMBO Reports. 8 (7): 629–33. doi:10.1038/sj.embor.7401029. PMC . PMID 17603536.
- Gu X, Karp PH, Brody SL, Pierce RA, Welsh MJ, Holtzman MJ, Ben-Shahar Y (Mar 2014). "Chemosensory functions for pulmonary neuroendocrine cells". American Journal of Respiratory Cell and Molecular Biology. 50 (3): 637–46. doi:10.1165/rcmb.2013-0199OC. PMID 24134460.
- Hallem EA, Dahanukar A, Carlson JR (2006). "Insect odor and taste receptors". Annual Review of Entomology. 51: 113–35. doi:10.1146/annurev.ento.51.051705.113646. PMID 16332206.
- Spehr M, Schwane K, Riffell JA, Zimmer RK, Hatt H (May 2006). "Odorant receptors and olfactory-like signaling mechanisms in mammalian sperm". Molecular and Cellular Endocrinology. 250 (1-2): 128–36. doi:10.1016/j.mce.2005.12.035. PMID 16413109.
- Buck LB (Nov 2004). "Olfactory receptors and odor coding in mammals". Nutrition Reviews. 62 (11 Pt 2): S184–8; discussion S224–41. doi:10.1301/nr.2004.nov.S184-S188. PMID 15630933.
- Saberi M, Seyed-allaei (2016). "Odorant receptors of Drosophila are sensitive to the molecular volume of odorants". Scientific Reports. doi:10.1038/srep25103.
- Jones DT, Reed RR (May 1989). "Golf: an olfactory neuron specific-G protein involved in odorant signal transduction". Science. 244 (4906): 790–5. Bibcode:1989Sci...244..790J. doi:10.1126/science.2499043. PMID 2499043.
- de March, Claire A.; Kim, Soo-Kyung; Antonczak, Serge; Goddard, William A. III; Golebiowski, Jérôme (September 2015). "G protein-coupled odorant receptors: From sequence to structure". Protein Science. 24 (9): 1543–1548. doi:10.1002/pro.2717.
- de March, Claire A.; Yu, Yiqun; Ni, Mengjue J.; Adipietro, Kaylin A.; Hiroaki, Matsunami; Ma, Minghong; Golebiowski, Jérôme (June 29, 2015). "Conserved Residues Control Activation of Mammalian G Protein-Coupled Odorant Receptors". Journal of the American Chemical Society. 137 (26): 8611–8616. doi:10.1021/jacs.5b04659.
- Wang J, Luthey-Schulten ZA, Suslick KS (Mar 2003). "Is the olfactory receptor a metalloprotein?". Proceedings of the National Academy of Sciences of the United States of America. 100 (6): 3035–9. Bibcode:2003PNAS..100.3035W. doi:10.1073/pnas.262792899. PMC . PMID 12610211.
- Crabtree RH (1978). "Copper (I): A possible olfactory binding site". Journal of Inorganic and Nuclear Chemistry. 40 (7): 1453. doi:10.1016/0022-1902(78)80071-2.
- Duan X, Block E, Li Z, Connelly T, Zhang J, Huang Z, Su X, Pan Y, Wu L, Chi Q, Thomas S, Zhang S, Ma M, Matsunami H, Chen GQ, Zhuang H (Feb 2012). "Crucial role of copper in detection of metal-coordinating odorants". Proceedings of the National Academy of Sciences of the United States of America. 109 (9): 3492–7. Bibcode:2012PNAS..109.3492D. doi:10.1073/pnas.1111297109. PMC . PMID 22328155.
- Brookes JC, Hartoutsiou F, Horsfield AP, Stoneham AM (Jan 2007). "Could humans recognize odor by phonon assisted tunneling?". Physical Review Letters. 98 (3): 038101. arXiv: . Bibcode:2007PhRvL..98c8101B. doi:10.1103/PhysRevLett.98.038101. PMID 17358733.
- Franco MI, Turin L, Mershin A, Skoulakis EM (Mar 2011). "Molecular vibration-sensing component in Drosophila melanogaster olfaction". Proceedings of the National Academy of Sciences of the United States of America. 108 (9): 3797–802. Bibcode:2011PNAS..108.3797F. doi:10.1073/pnas.1012293108. PMC . PMID 21321219.
- Schramm VL (Oct 2007). "Binding isotope effects: boon and bane". Current Opinion in Chemical Biology. 11 (5): 529–36. doi:10.1016/j.cbpa.2007.07.013. PMC . PMID 17869163.
- Gane S, Georganakis D, Maniati K, Vamvakias M, Ragoussis N, Skoulakis EM, Turin L (2013). "Molecular vibration-sensing component in human olfaction". PLOS ONE. 8 (1): e55780. doi:10.1371/journal.pone.0055780. PMC . PMID 23372854.
- Block E, Jang S, Matsunami H, Sekharan S, Dethier B, Ertem MZ, Gundala S, Pan Y, Li S, Li Z, Lodge SN, Ozbil M, Jiang H, Penalba SF, Batista VS, Zhuang H (May 2015). "Implausibility of the vibrational theory of olfaction". Proceedings of the National Academy of Sciences of the United States of America. 112 (21): E2766–74. doi:10.1073/pnas.1503054112. PMID 25901328.
- Vosshall LB (May 2015). "Laying a controversial smell theory to rest". Proceedings of the National Academy of Sciences of the United States of America. 112 (21): 6525–6. doi:10.1073/pnas.1507103112. PMID 26015552.
- Everts S (2015). "Receptor Research Reignites A Smelly Debate". Chemical & Engineering News. 93 (18): 29–30.
- Turin L, Gane S, Georganakis D, Maniati K, Skoulakis EM (Jun 2015). "Plausibility of the vibrational theory of olfaction". Proceedings of the National Academy of Sciences of the United States of America. 112 (25): E3154. doi:10.1073/pnas.1508035112. PMID 26045494.
- Block E, Jang S, Matsunami H, Batista VS, Zhuang H (Jun 2015). "Reply to Turin et al.: Vibrational theory of olfaction is implausible". Proceedings of the National Academy of Sciences of the United States of America. 112 (25): E3155. doi:10.1073/pnas.1508443112. PMID 26045493.
- Gilad Y, Lancet D (Mar 2003). "Population differences in the human functional olfactory repertoire". Molecular Biology and Evolution. 20 (3): 307–14. doi:10.1093/molbev/msg013. PMID 12644552.
- Malnic B, Hirono J, Sato T, Buck LB (Mar 1999). "Combinatorial receptor codes for odors". Cell. 96 (5): 713–23. doi:10.1016/S0092-8674(00)80581-4. PMID 10089886.
- Araneda RC, Peterlin Z, Zhang X, Chesler A, Firestein S (Mar 2004). "A pharmacological profile of the aldehyde receptor repertoire in rat olfactory epithelium". The Journal of Physiology. 555 (Pt 3): 743–56. doi:10.1113/jphysiol.2003.058040. PMC . PMID 14724183.
- Smith R, Peterlin Z, Araneda R (2013). Pharmacology of Mammalian Olfactory Receptors. Olfactory Receptors Methods in Molecular Biology: Humana Press. pp. 203–209. doi:10.1007/978-1-62703-377-0_15. ISBN 978-1-62703-377-0.
- de March, Claire A.; Ryu, SangEun; Sicard, Gilles; Moon, Cheil; Golebiowski, Jérôme (September 2015). "Structure–odour relationships reviewed in the postgenomic era". Flavour and Fragrance Journal. 30 (5): 342–361. doi:10.1002/ffj.3249.
- Tian, Xiao-Jun; Zhang, Hang; Sannerud, Jens; Xing, Jianhua (2016-05-24). "Achieving diverse and monoallelic olfactory receptor selection through dual-objective optimization design". Proceedings of the National Academy of Sciences. 113 (21): E2889–E2898. doi:10.1073/pnas.1601722113. ISSN 0027-8424.
- Glusman G, Bahar A, Sharon D, Pilpel Y, White J, Lancet D (Nov 2000). "The olfactory receptor gene superfamily: data mining, classification, and nomenclature". Mammalian Genome. 11 (11): 1016–23. doi:10.1007/s003350010196. PMID 11063259.
- Malnic B, Godfrey PA, Buck LB (Feb 2004). "The human olfactory receptor gene family". Proceedings of the National Academy of Sciences of the United States of America. 101 (8): 2584–9. Bibcode:2004PNAS..101.2584M. doi:10.1073/pnas.0307882100. PMC . PMID 14983052.
- Glusman G, Yanai I, Rubin I, Lancet D (May 2001). "The complete human olfactory subgenome". Genome Research. 11 (5): 685–702. doi:10.1101/gr.171001. PMID 11337468.
- Nei M, Rooney AP (2005). "Concerted and birth-and-death evolution of multigene families". Annual Review of Genetics. 39: 121–52. doi:10.1146/annurev.genet.39.073003.112240. PMC . PMID 16285855.
- Niimura Y, Nei M (2006). "Evolutionary dynamics of olfactory and other chemosensory receptor genes in vertebrates". Journal of Human Genetics. 51 (6): 505–17. doi:10.1007/s10038-006-0391-8. PMC . PMID 16607462.
- Niimura Y, Nei M (Feb 2005). "Comparative evolutionary analysis of olfactory receptor gene clusters between humans and mice". Gene. 346 (6): 13–21. doi:10.1016/j.gene.2004.09.025. PMID 15716120.
- Nozawa M, Nei M (2008). "Genomic drift and copy number variation of chemosensory receptor genes in humans and mice". Cytogenetic and Genome Research. 123 (1-4): 263–9. doi:10.1159/000184716. PMC . PMID 19287163.
- Niimura Y, Nei M (2007). "Extensive gains and losses of olfactory receptor genes in mammalian evolution". PLOS ONE. 2 (8): e708. doi:10.1371/journal.pone.0000708. PMC . PMID 17684554.
- Gilad Y, Wiebe V, Przeworski M, Lancet D, Pääbo S (Jan 2004). "Loss of olfactory receptor genes coincides with the acquisition of full trichromatic vision in primates". PLoS Biology. 2 (1): E5. doi:10.1371/journal.pbio.0020005. PMC . PMID 14737185.
- Craven BA, Paterson EG, Settles GS (Jun 2010). "The fluid dynamics of canine olfaction: unique nasal airflow patterns as an explanation of macrosmia". Journal of the Royal Society, Interface / the Royal Society. 7 (47): 933–43. doi:10.1098/Rsif.2009.0490. PMID 20007171.
- Zhang X, De la Cruz O, Pinto JM, Nicolae D, Firestein S, Gilad Y (2007). "Characterizing the expression of the human olfactory receptor gene family using a novel DNA microarray". Genome Biology. 8 (5): R86. doi:10.1186/gb-2007-8-5-r86. PMC . PMID 17509148.
- Matsui A, Go Y, Niimura Y (May 2010). "Degeneration of olfactory receptor gene repertories in primates: no direct link to full trichromatic vision". Molecular Biology and Evolution. 27 (5): 1192–200. doi:10.1093/molbev/msq003. PMID 20061342.
- Niimura Y (Apr 2012). "Olfactory receptor multigene family in vertebrates: from the viewpoint of evolutionary genomics". Current Genomics. 13 (2): 103–14. doi:10.2174/138920212799860706. PMC . PMID 23024602.
- Pierron D, Cortés NG, Letellier T, Grossman LI (Feb 2013). "Current relaxation of selection on the human genome: tolerance of deleterious mutations on olfactory receptors". Molecular Phylogenetics and Evolution. 66 (2): 558–64. doi:10.1016/j.ympev.2012.07.032. PMID 22906809.
- Buck L, Axel R (Apr 1991). "A novel multigene family may encode odorant receptors: a molecular basis for odor recognition". Cell. 65 (1): 175–87. doi:10.1016/0092-8674(91)90418-X. PMID 1840504.
- "Press Release: The 2004 Nobel Prize in Physiology or Medicine". Retrieved 2007-06-06.
- Liberles SD, Buck LB (Aug 2006). "A second class of chemosensory receptors in the olfactory epithelium". Nature. 442 (7103): 645–50. Bibcode:2006Natur.442..645L. doi:10.1038/nature05066. PMID 16878137.
- Liberles SD (October 2015). "Trace amine-associated receptors: ligands, neural circuits, and behaviors". Curr. Opin. Neurobiol. 34: 1–7. doi:10.1016/j.conb.2015.01.001. PMID 25616211.
- Khafizov K, Anselmi C, Menini A, Carloni P (Mar 2007). "Ligand specificity of odorant receptors". Journal of Molecular Modeling. 13 (3): 401–9. doi:10.1007/s00894-006-0160-9. PMID 17120078.
- Olfactory Receptor Database
- Human Olfactory Receptor Data Exploratorium (HORDE)
- Olfactory Receptor Protein at the US National Library of Medicine Medical Subject Headings (MeSH)