Odorant receptors characteristics

Odorants are thought to bind to integral membrane receptors on the cilia of the olfactory sensory neurons. The receptors are thought to he specific different olfactory neuron types recognize different odorants that share certain characteristics (Buck, 1993). The odorant receptors transduce signals via interactions with G-proteins (so-called because guanosine trisphosphate is involved in their activation). These G-protein-coupled exhibit seven hydrophobic domains (Fig. 5.6). Variation in the amino acid sequence of the transmembrane domain may account for specificity and selectivity of odor reception. 

T o properly recognize the smell of a particular molecule, each sensory neuron should express a single odorant receptor and respond to an appropriate range of odorants with similar molecular characteristics. This presumption (one receptor-one olfactory neuron rule) was deemed acceptable for quite some time but did not have a firm experimental basis until recently when it was demonstrated in the olfactory system of mice.206... 

Trace amine-associated receptors (TAARs) are related to G protein-coupled aminergic neurotransmitter receptors such as dopamine and serotonine receptors and recognize derivatives of the classical monoamines such as B-phenylethylamine, octopamine, tryptamine, and tyramine. Initially, TAARs had been considered neurotransmitter receptors (Borowsky et al. 2001), but recently an expression in olfactory sensory neurons was shown for several mammalian taar genes, with expression characteristics very similar to odorant receptors (Liberies and Buck 2006). Thus the taar genes were recognized as a fourth GPCR family of olfactory receptors (Buck 2000). 

Odorant receptors are members of the 7TM-receptor family. The green cylinders represent the seven presumed transmembrane helices. Strongly conserved residues characteristic of this protein famiiy are shown in blue, whereas highly variable residues are shown in red. 

A characteristic of a-subunits of the Gs subfamily is that they are inhibited by cholera toxin (see Section 5.5.2). The members of the Gs subfamily are activated by hormone receptors, by odor receptors and by taste receptors. Gs-proteins mediate, e.g., signal transmission by type fl adrenaline receptors and that by glucagon receptors. During perception of taste, the taste receptors are activated, and these then pass the signal on via the olfactory G protein G0if. Perception of sweet taste is also mediated via a Gs-protein. Transmission of the signal further involves an adenylyl cyclase in all cases, the activity of which is stimulated by the Gs-proteins. 

The odors of the two enantiomeric carvones are distinctly different from each other. The presence of one or the other isomer is responsible for the characteristic odors of each oil. The difference in the odors is to be expected because the odor receptors in the nose are chiral (see essay, "Stereochemical Theory of Odor"). This phenomenon, in which a chiral receptor interacts differently with each enantiomer of a chiral compound, is called chiral recognition. 

A dramatic example of such differential complexation is the olfactory response to the two enantiomers of carvone. (R)-(-)-Carvone (3) smells like spearmint and is the principal component of spearmint oil, which also contains minor amounts of limonene (16) and a- and fl-phellandrene (17 and 18, respectively). On the other hand, (S)-(+)-carvone (4), along with limonene, is found in caraway seed and dill seed oils and has been shown to be the compound largely responsible for the characteristic odor of these oils. This remarkable difference in how we sense these two enantiomers is because the odor receptors in our noses are chiral environments that are linked to the nervous system. The type of receptor site that complexes with a particular enantiomer will determine what odor is detected by the brain. 

Sensitive Receptor Indicator a measurable physical, chemical, biological, or social (e.g., odor) characteristic of a sensitive receptor. For example, for the sensitive receptor. Crater Lake, water clarity is a sensitive receptor indicator. 

Geosmin, 2-Methyl isobomeol (MIB) and 2-isopropyl methoxy pyrazine are known to be produced by various types of actinomycete cultures (10-15). Geosmin and MIB are saturated tertiary alcohols and resist oxidation. The steric configuration of the hydroxyl and methyl groups in both compounds are believed to interact with receptors in the nose, imparting their characteristic earthy odour (16). The four compounds itemised as the key osmogenes in this odorous emission have extremely low odour threshold concentrations. Their occasional occurence in drinking water can lead to widespread complaints and are routinely monitored for within this Authority. 

It is not yet possible to design a molecule with specific odor (or taste) characteristics because the relations between sensory properties of flavor compounds and their molecular properties are not well understood. As a consequence, the development of compounds with desired flavor qualities has had to rely on relatively tedious synthetic approaches. Recent advances, however, in computer-based methods developed by the pharmaceutical industry to study QSAR (quantitative structure-activity relationships) may ultimately be helpful in the rational design of new flavor-structures with predictable sensory attributes. Results from QSAR studies may also provide insight into the mechanism of the molecule-receptor interaction. 

ORGANOLEPTIC. A term widely used to describe consumer testing procedures for food products, perfumes, wines, and the like in which samples of various products, flavors, etc. are submitted to groups or panels. Such tests are a valuable aid in determining the acceptance of tlie products and thns may be viewed as a marketing technique, They also serve psychological purposes and are an important means of e valuating the subjective aspects of taste, odor, color, and related factors, The physical and chemical characteristics of foods are stimuli for the eye, ear, skin, nose, and mouth, whose receptors initiate impulses that travel to the brain, where perception occurs. 

As is the case for all sensory pathways, the capacity to perceive and respond to olfactory cues (odorants) is the combined result of events that take place in both peripheral and central processing centers. These steps, which will be discussed in detail below, begin with the molecular transduction of chemical signals in the form of odorants into electrical activity by olfactory receptor neurons (ORNs) in the periphery whose axonal projections form characteristic synaptic connections with elements of the central nervous system (CNS). Within the CNS, complex patterns of olfactory signals are integrated and otherwise processed to afford recognition and ultimately, the behavioral responses to the insect s chemical environment. Within the context of pheromone recognition these responses would likely be centered on various elements of the insect s reproductive cycle. 

Menthol is widely used in pharmaceuticals, confectionery, and toiletry products as a flavoring agent or odor enhancer. In addition to its characteristic peppermint flavor, /-menthol, which occurs naturally, also exerts a cooling or refreshing sensation that is exploited in many topical preparations. Unlike mannitol, which exerts a similar effect due to a negative heat of solution, /-menthol interacts directly with the body s coldness receptors. d-Menthol has no cooling effect, while racemic menthol exerts an effect approximately half that of /-menthol. 

These speculations present a meager collection of precepts about what the nose should do. No less meager is the list of what the nose cannot do. Olfaction cannot rely entirely on emission or absorption of electromagnetic radiation by isolated odorant molecules, because optical isomers can have different odors (Friedman Miller, 1971 Laska Teubner, 1999). Nor can the characteristic stench of organosulfur compounds depend on the reaction depicted in in equation 1, because lavage of the olfactory mucosa of experimental animals with iodoacetamide or with methymercury hydroxide followed by iodoacetamide (which would irreversibly modify the hypothesized receptor site) does not affect their responding to dimethyl disulfide (Mason et al., 1987a). 

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