Odor Receptor Functioning

In this model, OBPs participate in the selective transport of pheromone and other semiochemicals to their olfactory receptors. The selectivity of the system is likely to be achieved by layers of filters [ 16], i.e., by the participation of compartmentalized OBPs and olfactory receptors. It seems that OBPs transport only a subset of compounds that reach the pore tubules. Some of these compounds may not bind to the receptors compartmentalized in the particular sensilla. The odorant receptors, on the other hand, are activated by a subset of compounds, as indicated by studies in Drosophila, showing that a single OR is activated by multiple compounds [66]. If some potential receptor ligand reaches the pore tubules but are not transported by OBPs, receptor firing is prevented because the receptors are protected by the sensillar lymph. In other words, even if neither OBPs nor odorant receptors (ORs) are extremely specific, the detectors (olfactory system) can show remarkable selectivity if they function in a two-step filter. 

Actnally the human genome contains abont 600 odorant receptor genes. However, abont 250 of these are pseudogenes that have no function. They are remnants of evolution. 

Receptors and their ligands are numerous, varied, and essential to all forms of life. Cell-surface receptors on bacteria detect feeding attractants as well as dangerous molecules. From bacteria to humans seven-helix receptors function to detect light, odors, hormones, and other molecules. Tire numbers of different receptors are impressive. For example, the tiny nematode C. elegans has 650 seven-helix transmembrane receptors and 411 protein kinases, many of which may be associated with receptors.34 Our bodies have thousands. 

Touhara, K., Sengoku, S., Inaki, K., Tsuboi, A., Hirono, I., Sato, T., Sakano, H., and Haga, T., Functional identification and reconstitution of an odorant receptor in single olfactory neurons, Proc. Natl. Acad. Sci. USA, 96, 4040, 1999. 

Wetzel C. H., Behrendt H., Gisselmann G., Storkuhl K. F., Hovemann B. and Hatt H. (2001) Functional expression and characterization of a Drosophila odorant receptor in a heterologous cell system. PNAS 98, 9377-9380. 

Zhao H., Ivic L., Otaki J. M., Hashimoto M., Mikoshiba K. and Firestein S. (1998) Functional expression of a mammalian odorant receptor. Science 279, 237-242. 

This chapter will discuss the isolation of Drosophila odorant receptor (DOR) genes, how these genes have expanded our understanding of the development and functional anatomy of the olfactory system, how the odor response profiles of OSNs respond to odorants, and the mechanisms by which odor-specific activity is relayed to the brain. 

Molecular structure must be implicated as odorants bind specifically with the sensory receptors called odorant receptors (ORs). The olfactory mucus has proteins called odorant binding proteins (OBPs) that dissolve the odorant molecule in the aqueous/lipid interface of the mucus. The OBPs act as binder molecules to assist the transfer of odorant to the receptor and increase its relative concentration in the mucus relative to inhaled air. They also function to remove used odorants for breakdown and free up the receptor to detect other molecules. 

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