Biochemistry - Chemical Reactions in Living Cells, Volume 3 - D. Metzler 1980

Cell Growth, Differentiation, and Chemical Communication
Neurochemistry
Smell and Taste

We have not yet discussed how sensory Neurons are activated. For biochemists, the functioning mechanism of taste and Olfaction receptors remains particularly enigmatic. While it is quite obvious that different substances possess distinct tastes and odors, establishing a correlation between these characteristics and their chemical structures is far from simple.

It is quite remarkable that Bacteria possess something akin to The ability to distinguish tastes. They are attracted to compounds that can be utilized in metabolic processes; this phenomenon is known as chemotaxis [115—117]. For example, E. coli actively swims toward higher concentrations of L-Serine (but not D-serine) or D-ribose. Conversely, compounds such as phenol act as repellents. By what mechanism does a tiny Introduction/4.html">Prokaryotic Cell sense the direction of a concentration gradient? Available data strongly suggest that The Plasma Membrane contains receptors whose response is somehow coupled with The regulation of flagellar movement, which drives the locomotion of the entire cell. Given the extremely small size of a bacterium, it is hard to conceive that it can perceive a concentration difference between its front and rear ends. Individual bacteria (such as E. coli or Salmonella) typically swim in a straight line, tumble from time to time, and then resume swimming straight in a new random direction (Box 4-B). The chemotactic response presumably arises precisely from this relatively prolonged straight-line movement, during which the bacterium has time to sense Changes in the concentration of the attractant over time. If the bacterium swims in the direction of a decreasing attractant concentration, tumbling occurs sooner. Theoretically, this can be quite plausibly explained as follows: as Membrane Receptors become increasingly occupied by molecules of the attractant, the rate v_t of substance X formation within the membrane or cell increases [equation (16-1)].

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When [X] rises above a certain threshold level, the bacterium begins to tumble. At the same time, substance X is constantly degraded at a rate v_d. Eventually, a balance between the rates v_f and v_d is established such that the concentration of X drops and settles at a steady-state level.

There are numerous known Examples of chemotaxis in lower invertebrates such as Euglena. Interesting instances of chemically regulated feeding behavior can be observed in Coelenterates equipped with nematocysts. For instance, the chemoreceptors of Hydra "sense" Glutathione released by the damaged Tissues of their prey. Other closely related organisms respond to Proline. In sea anemones (Anthopleura), asparagine induces tentacle contraction, while glutathione triggers swallowing [118]. Many other examples could be cited. It is difficult to imagine that the mechanisms of olfaction and gustation in humans could differ fundamentally from these described phenomena.

Regarding human Sensory Organs, it should be noted first and foremost that there is no clear correlation between the physical and Chemical properties of a substance (including its Spatial Structure) and its taste and odor [119, 120]. This is a highly complex issue. It has been hypothesized that There are 20 to 30 primary odors, which likely correspond to 20 to 30 types of receptor Proteins. Apparently, the physical basis of taste and smell involves the binding of individual molecules to proteins, inducing a specific conformational change in the latter; this, in turn, leads to the depolarization of a patch of the sensory Cell Membrane and the generation of an Action Potential, much like what occurs in synaptic transmission. Another fascinating phenomenon has been discovered: certain Peptides possess an extraordinary sweet taste, and chemical stimulant proteins exist. Two sweet-tasting chemostimulatory proteins have been identified. A third protein, found in a tropical fruit, can alter taste perception: after exposure to this protein, acids begin to taste sweet [121]. It has also been noted that for many people, after tasting artichokes, Water tastes sweet [122]. Thus, the response of taste receptors to specific substances can undergo transient modifications As a result of the binding of other substances to adjacent sites on the sensitive membrane.