Scents and sensibility: how biology perceives chemistry
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Columbia University, Biological Sciences, United States
The olfactory system plays a leading role in the perception of flavors, even though we experience this perception in our oral cavity as "taste". Without a sense of smell, food loses all but its most elementary flavor qualities.
Smell begins in the periphery with the olfactory epithelium, a thin tissue lining the bones of the upper nasal cavity (1). Residing in this tissue are upwards of several million bipolar shaped cells which are the primary olfactory sensory neurons. At one end of these neurons there are hair-like projections called cilia. Residing within the membranes of these cilia are the proteins that make up the molecular machinery for detecting odor molecules that are breathed into the nasal cavity and become adsorbed to the mucus (2).
These include, first and foremost, the receptor protein or “odor receptor”. This protein, made up of just over 300 amino acids strung together like pearls on a string, has a unique shape, including a pocket that allows for the entry and binding of specific odor molecules, much the way a lock permits certain shaped keys to be inserted. Again in analogy with a lock and key, where the right shaped key can open the lock, when a molecule fits into the binding pocket it is able to activate the receptor. This sets in motion a series of events through "second messenger" proteins which very rapidly cause an electrical change in the state of the sensory neuron. It is this electrical change, encoded in trains of changing voltage impulses, that signals to the brain the presence of a particular odor molecule.
There are nearly 400 different odor receptors expressed by the various neurons of the epithelium in humans (3). However even this large number of receptors is insufficient to explain our ability to detect and identify many thousands of diverse chemicals as odorants (one recent report claims we have the potential to discriminate trillions!). It is hypothesized that there is a combinatorial code in which any given odor can be detected by several receptors and any given receptor can bind any of several presumably related odors. In our analogy, the keys fit very loosely to differing degrees into many locks.
Chemists are particularly interested in those parts of a molecule that are likely to participate in various sorts of reactions and synthetic manipulations. These would include such things as the functional group (aldehyde, acid, ester, etc.) or if there are double bonds or charge carrying atoms. However, what is relevant to the synthetic chemist may not be important to the biological system, and in particular to the odor receptor protein. Thus we should begin by taking a biological approach to odor chemistry. For example the definition of an odorant cannot be made chemically – many chemical compounds that appear nearly identical to a known odorant may have a different smell or none at all. The only definition of an odorant is that it binds to an odor receptor to give rise to a biological response. Precisely what parts of a chemical compound influence that binding is one of the most challenging questions in biology.
The actual perception of an odorant depends on the particular combination of receptors that are activated. In a complex mixture of tens to hundreds of different odor molecules this can quickly become a very complicated matrix of activated receptors with an astronomical number of combinations. An open question is whether evolution has perhaps found a simplified way of performing this apparently incalculable task. One possible solution would be the existence of a few dozen common chemical structures that would serve as primary features from which all other odors are constructed. This would be similar to the way the visual system can perceive thousands of hues of light by combining only three (blue, green and red) primary “colors” or wavelengths. Although the idea of primaries in olfaction has been discussed for several decades it was largely abandoned after the discovery of the large family of receptors. Now we are recognizing that it might be required after all, and with new molecular tools we may be able to understand better the basic mystery of how biological entities encode chemicals.
The perception of the odor code is affected by whether we are activating the olfactory receptor cells by sniffing in ("orthonasal smell") or by breathing out ("retronasal smell"). When breathing in, the odor chemical code is perceived as smell. When breathing out during eating, the code is combined with the input from the other senses to form the merged perception of flavor. This challenges current research in analyzing the specific roles of smell in flavors that lead to healthy or unhealthy eating, as the talks at this conference document.
References
1. Firestein SJ. (2009). Ann. N Y Acad Sci. 1170:161. doi: 10.1111/j.1749-6632.2009.04882.x
2. Niimura Y (2012). Curr. Genomics 13: 103
3. Pifferi S, Menini A, Kurahashi T (2013). In The Neurobiology of Olfaction (ed. Menini A). Boca Raton FL: CRC Press, Chap 8.
Keywords:
olfactory epithelium,
olfactory receptor neuron,
olfactory cilia,
olfactory receptor,
second messenger,
combinatorial code,
orthonasal smell,
retronasal smell
Conference:
Science of Human Flavor Perception, New York, United States, 9 May - 9 May, 2014.
Presentation Type:
Abstract
Topic:
Science of Human Flavor Perception
Citation:
Firestein
S
(2015). Scents and sensibility: how biology perceives chemistry.
Front. Integr. Neurosci.
Conference Abstract:
Science of Human Flavor Perception.
doi: 10.3389/conf.fnint.2015.03.00006
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Received:
28 Jul 2014;
Published Online:
30 Jan 2015.
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Correspondence:
Prof. Stuart Firestein, Columbia University, Biological Sciences, New York, NY, United States, sfirestein@gmail.com