Olfactory receptors expressed in the cell membranes of olfactory receptor neurons are responsible for the detection of odor molecules. Activated olfactory receptors are the initial player in a signal transduction cascade which ultimately produces a nerve impulse which is transmitted to the brain. These receptors are members of the class A rhodopsin-like family of G protein-coupled receptors (GPCRs).[1][2] The olfactory receptors form a multigene family consisting of over 900 genes in humans and 1500 genes in mice.[3]
Contents
- 1 Expression
- 2 Mechanism
- 3 Diversity
- 4 Families
- 5 Evolution
- 6 Discovery
- 7 See also
- 8 References
- 9 External links
Expression[edit]
In vertebrates, the olfactory receptors are located in both the cilia and synapses of the olfactory sensory neurons.[4] In insects, olfactory receptors are located on the antennae and other chemosensory organs.[5] Sperm cells also express odor receptors, which are thought to be involved in chemotaxis to find the egg cell.[6]
Mechanism[edit]
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.[7] 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)[8] 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 (ORs) are known from the genomes of more than a dozen organisms: they are seven-helix transmembrane proteins, but there are (as of July 2011) no known structures of any OR. There is a highly conserved sequence in roughly three quarters of all ORs that is a tripodal metal ion binding site,[9] 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.[10] 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.[11]
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.[12] 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).[13] 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.[14]
Diversity[edit]
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.[15]
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.[16][17] 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 almost limitless, the olfactory receptor system is capable of detecting and distinguishing between a practically infinite number of odorant molecules.
Families[edit]
A nomenclature system has been devised for the olfactory receptor family[18] 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.[19]
Two major classes of olfactory receptors have been identified in humans:[20]
- class I (fish-like receptors) OR families 51-56
- class II (tetrapod specific receptors) OR families 1-13
Evolution[edit]
The olfactory receptor gene family in vertebrates has been shown to evolve through genomic events such as gene duplication or gene conversion.[21] 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.[22] Such birth-and-death evolution also involves the elimination of genes from multigene family repertoires by mutations that create nonfunctional pseudogenes. The color 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.[23]
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.[24]
Discovery[edit]
In 2004 Linda B. Buck and Richard Axel won the Nobel Prize in Physiology or Medicine for their work[25] on olfactory receptors.[26] In 2006 it was shown that another class of odorant receptors exist for volatile amines.[27] This class of receptors consists of the trace amine-associated receptors (TAAR) with the exception of TAAR1 which is a receptor for thyronamines.
Unfortunately, as with many other GPCRs, there is still a lack of experimental structures at atomic level for olfactory receptors and structural information is based on homology modeling methods.[28]
See also[edit]
- Phantosmia
- Receptor
- Trace amine-associated receptor
- Odorant
- Pseudogenes
- Gene family
References[edit]
- ^ Gaillard I, Rouquier S, Giorgi D (2004). "Olfactory receptors". Cell. Mol. Life Sci. 61 (4): 456–69. doi:10.1007/s00018-003-3273-7. PMID 14999405.
- ^ Hussain A, Saraiva LR, Korsching SI (2009). "Positive Darwinian selection and the birth of an olfactory receptor clade in teleosts". PNAS 106 (11): 4313–8. Bibcode:2009PNAS..106.4313H. doi:10.1073/pnas.0803229106. PMC 2657432. PMID 19237578.
- ^ Niimura Y and Nei M (2003). "Evolution of olfactory receptor genes in the human genome". PNAS 100 (21): 12235–40. Bibcode:2003PNAS..10012235N. doi:10.1073/pnas.1635157100. PMC 218742. PMID 14507991.
- ^ Rinaldi A (2007). "The scent of life. The exquisite complexity of the sense of smell in animals and humans". EMBO Rep. 8 (7): 629–33. doi:10.1038/sj.embor.7401029. PMC 1905909. PMID 17603536.
- ^ Hallem EA, Dahanukar A, Carlson JR (2006). "Insect odor and taste receptors". Annu. Rev. Entomol. 51: 113–35. doi:10.1146/annurev.ento.51.051705.113646. PMID 16332206.
- ^ Spehr M, Schwane K, Riffell JA, Zimmer RK, Hatt H (2006). "Odorant receptors and olfactory-like signaling mechanisms in mammalian sperm". Mol. Cell. Endocrinol. 250 (1–2): 128–36. doi:10.1016/j.mce.2005.12.035. PMID 16413109.
- ^ Buck LB (2004). "Olfactory receptors and odor coding in mammals". Nutr. Rev. 62 (11 Pt 2): S184–8; discussion S224–41. doi:10.1301/nr.2004.nov.S184-S188. PMID 15630933.
- ^ Jones DT, Reed RR (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.
- ^ Wang J., Luthey-Schulten Z., Suslick K. S. (2003). "Is the Olfactory Receptor A Metalloprotein?". Proc. Natl. Acad. Sci. U.S.A 100: 3035–3039. Bibcode:2003PNAS..100.3035W. doi:10.1073/pnas.262792899.
- ^ 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 (February 2012). "Crucial role of copper in detection of metal-coordinating odorants". Proc. Natl. Acad. Sci. U.S.A. 109 (9): 3492–7. Bibcode:2012PNAS..109.3492D. doi:10.1073/pnas.1111297109. PMC 3295281. PMID 22328155.
- ^ Brookes JC (2007). "Could Humans Recognize Odor by Phonon Assisted Tunneling?". Phys. Rev. Lett. 98. arXiv:physics/0611205. Bibcode:2007PhRvL..98c8101B. doi:10.1103/PhysRevLett.98.038101. PMID 17358733.
- ^ Franco MI, Turin L, Mershin A, Skoulakis EM. (2011). "Molecular vibration-sensing component in Drosophila melanogaster olfaction". PNAS 108 (9): 3797–802. Bibcode:2011PNAS..108.3797F. doi:10.1073/pnas.1012293108. PMC 3048096. PMID 21321219.
- ^ Schramm, V. L. Binding isotope effects: boon and bane, Curr. Opin. Chem. Biol. 2007, 11, 529-536..
- ^ Gilad Y, Lancet D (2003). "Population differences in the human functional olfactory repertoire". Mol. Biol. Evol. 20 (3): 307–14. doi:10.1093/molbev/msg013. PMID 12644552.
- ^ Malnic B, Hirono J, Sato T, Buck LB (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 (2004). "A pharmacological profile of the aldehyde receptor repertoire in rat olfactory epithelium". J. Physiol. (Lond.) 555 (Pt 3): 743–56. doi:10.1113/jphysiol.2003.058040. PMC 1664868. PMID 14724183.
- ^ Glusman G, Bahar A, Sharon D, Pilpel Y, White J, Lancet D (2000). "The olfactory receptor gene superfamily: data mining, classification, and nomenclature". Mamm. Genome 11 (11): 1016–23. doi:10.1007/s003350010196. PMID 11063259.
- ^ Malnic B, Godfrey PA, Buck LB (2004). "The human olfactory receptor gene family". PNAS 101 (8): 2584–9. Bibcode:2004PNAS..101.2584M. doi:10.1073/pnas.0307882100. PMC 356993. PMID 14983052.
- ^ Glusman G, Yanai I, Rubin I, Lancet D (2001). "The complete human olfactory subgenome". Genome Res. 11 (5): 685–702. doi:10.1101/gr.171001. PMID 11337468.
- ^ Nei M and Rooney AP (2005). "Concerted and birth-and-death evolution of multigene families". Annu Rev Genet. 39: 121–152. doi:10.1146/annurev.genet.39.073003.112240. PMC 1464479. PMID 16285855.
- ^ Niimura Y and Nei M (2006). "Evolutionary dynamics of olfactory and other chemosensory receptor genes in vertebrates". J Hum Genet. 51 (6): 505–517. doi:10.1007/s10038-006-0391-8. PMC 1850483. PMID 16607462.
- ^ Gilad Y, Wiebe V, Przeworski M, Lancet D, Paabo S (2004). "Loss of Olfactory Receptor Genes Coincides with the Acquisition of Full Trichromatic Vision in Primates". PLOS Biology 2 (1): 120–125. doi:10.1371/journal.pbio.0020005. PMC 314465. PMID 14737185.
- ^ Pierron D, Cortés NG, Letellier T, Grossman LI. (2013). "Current relaxation of selection on the human genome: Tolerance of deleterious mutations on olfactory receptors.". Mol Phylogenet Evol.. 66 (2): 558–564. PMC 314465. PMID 22906809.
- ^ Buck L, Axel R (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 (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.
- ^ Khafizov K, Anselmi C, Menini A, Carloni P (2007). "Ligand specificity of odorant receptors". J Mol Model 13 (3): 401–9. doi:10.1007/s00894-006-0160-9. PMID 17120078.
External links[edit]
- Olfactory Receptor Database
- Human Olfactory Receptor Data Exploratorium (HORDE)
- Olfactory Receptor Protein at the US National Library of Medicine Medical Subject Headings (MeSH)
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Sensory system: Olfactory system / Olfaction / Rhinencephalon (TA 15.1, GA 10.992)
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| Olfactory epithelium |
Olfactory glands · Olfactory mucosa · Sustentacular cell
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| Olfactory nerve: 1° neuron |
Olfactory receptor neurons (Olfactory receptor) → Olfactory bulb (Glomeruli)
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| Olfactory nerve: 2° neuron |
Mitral cells → Olfactory tract → Olfactory trigone
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Lateral olfactory stria/
Primary olfactory cortex |
Piriform cortex · EC-hippocampus system (Entorhinal cortex, Hippocampal formation) · Prepyriform area · Periamygdaloid cortex
Stria medullaris → Habenular nuclei
Amygdala → Stria terminalis → Hypothalamus
Medial forebrain bundle → Hypothalamus
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| Medial olfactory stria |
Anterior olfactory nucleus
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G protein-coupled receptors: olfactory receptors
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Class I
(fish-like specific receptors) |
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Family 51
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- A2
- A4
- A7
- B2
- B4
- B5
- B6
- D1
- E1
- E2
- F1
- F2
- G1
- G2
- H1
- I1
- I2
- L1
- M1
- Q1
- S1
- T1
- V1
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Family 52
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- A1
- A4
- A5
- B2
- B4
- B6
- D1
- E1
- E2
- E4
- E5
- E6
- E8
- H1
- I1
- I2
- J3
- K1
- K2
- L1
- L2
- M1
- N1
- N2
- N4
- N5
- P1
- R1
- W1
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Family 56
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Class II
(tetrapod specific receptors) |
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Cell surface receptor: G protein-coupled receptors
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Class A:
Rhodopsin like |
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| Class B: Secretin like |
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Orphan
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- GPR (56
- 64
- 97
- 98
- 110
- 111
- 112
- 113
- 114
- 115
- 116
- 123
- 124
- 125
- 126
- 128
- 133
- 143
- 144
- 155
- 157)
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Other
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- Brain-specific angiogenesis inhibitor (1
- 2
- 3)
- Cadherin (1
- 2
- 3)
- Calcitonin
- CALCRL
- CD97
- Corticotropin-releasing hormone (1
- 2)
- EMR (1
- 2
- 3)
- Glucagon (GR
- GIPR
- GLP1R
- GLP2R)
- Growth hormone releasing hormone
- PACAPR1
- GPR
- Latrophilin (1
- 2
- 3
- ELTD1)
- Methuselah-like proteins
- Parathyroid hormone (1
- 2)
- Secretin
- Vasoactive intestinal peptide (1
- 2)
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Class C: Metabotropic
glutamate / pheromone |
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Taste
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- TAS1R (1
- 2
- 3)
- TAS2R (1
- 3
- 4
- 5
- 7
- 8
- 9
- 10
- 13
- 14
- 16
- 19
- 20
- 30
- 31
- 38
- 39
- 40
- 41
- 42
- 43
- 45
- 46
- 50
- 60)
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Other
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- Calcium-sensing receptor
- GABA B (1
- 2)
- Glutamate receptor (Metabotropic glutamate (1
- 2
- 3
- 4
- 5
- 6
- 7
- 8))
- GPRC6A
- GPR (156
- 158
- 179)
- RAIG (1
- 2
- 3
- 4)
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Class F:
Frizzled / Smoothened |
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Frizzled
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- Frizzled (1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
- 9
- 10)
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Smoothened
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B trdu: iter (nrpl/grfl/cytl/horl), csrc (lgic, enzr, gprc, igsr, intg, nrpr/grfr/cytr), itra (adap, gbpr, mapk), calc, lipd; path (hedp, wntp, tgfp+mapp, notp, jakp, fsap, hipp, tlrp)
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Membrane proteins, receptors: cell surface receptors
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| G protein-coupled receptor |
| Class A |
- Eicosanoid receptor (Prostaglandin receptor)
- Protease-activated receptor
- Neurotransmitter receptor
- Purinergic receptor
- Biogenic amine receptor
- Olfactory receptor
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| Class B |
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| Class C |
- Metabotropic glutamate receptor
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| Class D |
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| Class E |
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| Class F |
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| Ligand-gated ion channel |
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| Enzyme-linked receptor |
- Serine/threonine-specific protein kinase
- Receptor tyrosine kinase
- Guanylate cyclase
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| Other/ungrouped |
- Asialoglycoprotein receptor
- Tumor necrosis factor receptor
- Immunoglobulin superfamily
- N-Acetylglucosamine receptor
- Neuropilins
- Transferrin receptor
- EDAR
- Lipoprotein receptor-related protein
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- See also
- cell surface receptor deficiencies
B trdu: iter (nrpl/grfl/cytl/horl), csrc (lgic, enzr, gprc, igsr, intg, nrpr/grfr/cytr), itra (adap, gbpr, mapk), calc, lipd; path (hedp, wntp, tgfp+mapp, notp, jakp, fsap, hipp, tlrp)
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