Olfactory receptor

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Olfactory receptors belong to class A of the G protein-coupled receptor. In vertebrates, the olfactory receptors are located in the cilia of the olfactory sensory neurons. In insects, olfactory receptors are located on the antennae. Sperm cells also express odor receptors, which are thought to be involved in chemotaxis to find the egg cell.

Rather than binding to specific ligands like most receptors, olfactory receptors bind to structures on odor molecules. 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 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.

There are a wide range of different odor receptors, with as many as 1,000 in the mammalian genome. Olfactory receptors may make up as much as 3% of the genome. Only a portion of these potential genes form functional odor receptors. According to an analysis of the Human genome project, humans have 347 functional genes coding for olfactory receptors. The reason for the large number of different odor receptors is to provide a system for detecting as many different odors as possible. Even so, each odor receptor does not correspond to just one odor. Each individual odor receptor is broadly tuned to be activated by a number of similar structures. Like the immune system, this system allows molecules that have never been encountered before to be characterized. Also most odors activate more than one type of odor receptor. This aspect provides for the identification of an almost limitless number of different molecules.

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. This class of receptors consists of the trace amine-associated receptors (TAAR) with the exception of TAAR1 which is a receptor for thyronamines.

Unfortunatelly, there is still a lack of experimental structures at atomic level for olfactory receptors and structural information is essentially based on computational methods.

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Malnic B, Hirono J, Sato T, Buck LB. Combinatorial receptor codes for odors. Cell; Mar 5 (1999). PMID 10089886
Araneda RC, Peterlin Z, Zhang X, Chesler A, Firestein S. A pharmacological profile of the aldehyde receptor repertoire in rat olfactory epithelium. Journal of Physiology; Mar 16 (2004). PMID 14724183
Liberles SD, Buck LB. A second class of chemosensory receptors in the olfactory epithelium. Nature; Aug 10 (2006). PMID 16878137
Khafizov K, Anselmi C, Menini A, Carloni P. Ligand specificity of odorant receptors. J Mol Model; Mar (2007). PMID 17120078