| THE
MOLECULAR BASIS OF TASTE Taste
reception occurs at the apical tip of taste cells that form taste buds. Each onion
shaped taste bud has is composed of 50–100 taste cells that possess microvilli.
Each single taste bud contains 50–100 taste cells representing all 5 taste sensations.
Embedded in the cell membranes of these taste cells are receptor proteins. Each
of the taste receptors are transmembrane proteins which function either by physically
binding to a flavor ingredient (sweet, bitter and umami) or by acting as a channel
to allow ions to flow directly into a taste cell (salty and sour). This interaction
triggers a signaling cascade that culminates with signals to the brain through
a network of taste nerve fibers.
In the case of sweet, umami and bitter tasting it is the size and shape and the
chemical groups on a tastant molecule which determine any taste it will have as
they will determine its ability to dock onto one or more of the different receptor
proteins (See
schematic or full reference).
A single taste cell seems to be restricted to expressing only a single type of
receptor strongly supporting the (labelled-line
model) as evidenced by data using gene-knockout mice (see
data). And further data
here, where targeted expression
of a novel bitter receptor to bitter (T2R-expressing) cells resulted in dose-dependent
aversion to the specific bitter tastant. In marked contrast, directing expression
of the same receptor to sweet cells produces animals that are strongly attracted
to this bitter tastant. RECEPTOR
PROTEINS FOUND IN TASTE CELLS SWEET
-- Sugars and artificial
sweeteners are detected by T1R2
and T1R3 heteromers, (see Margolskee, 2002) UMAMI
--T1R1
and T1R3 GPCRs combine to form a broadly tuned L-amino-acid receptor
BITTER--
Bitter chemicals are
detected by 30
T2R receptor family members. (see Margolskee, 2002)
SOUR
-- Acids are detected
by PKD1L3- and PKD2L1
sour-sensing cells (Ishimaru, et al., 2006; Zuker et.al. 2006).
SALT
-- Not known although approximately 20% effect may be by amiloride-sensitive sodium
channels.
FAT --
Speculative -- CD36 Receptor (see Abumrad, 2005) EXAMPLES
OF TASTANT MOLECULES TEMPERATURE
RECEPTORS Temperature
is an essential element of human taste experience. Food and drink which — within
a given culture — is considered to be properly served hot is often considered
distasteful if cold, and vice versa. Some sugar substitutes have strong heats
of solution, as is the case of sorbitol, erythritol, xylitol, mannitol, lactitol
and maltitol. When they are dry and are allowed to dissolve in saliva, besides
the sweet taste also heat effects can be recognized. The cooling effect upon eating
may be desirable, as in a mint candy made with crystalline sorbitol, or undesirable
if it's not typical for that product
Some substances activate cold trigeminal receptors. One can sense a cool sensation
(also known as "cold", "fresh" or "minty") from, e.g., spearmint, menthol,
ethanol or camphor, which is caused by the food activating the TRP-M8 ion
channel on nerve cells that signal cold. The reactions behind this sense are therefore
analogous to those behind the hot sense. Unlike the actual change in temperature
described for sugar substitutes, coolness is only a perceived phenomena. Spiciness
or (false) heat See also: Scoville scale Substances such as ethanol and capsaicin
cause a burning sensation by inducing a trigeminal nerve reaction together
with normal taste reception. The heat is caused by the food activating a nerve
cell ion channel called TRP-V1, which is also activated by hot temperatures.
References:
Abumrad, N.A.(2005)
CD36
may determine our desire for dietary fats. J Clin Invest. 2005 November 1;
115(11): 2965–2967.
Hiroaki Matsunami1, Jean-Pierre Montmayeur1 and Linda B. Buck (2000) A
family of candidate taste receptors in human and mouse Nature 404, 601-604. Seeing,
Feeling, and Tasting: Molecular Genetics of Sensory Signal Transduction Ryusuke
Yoshida, Keisuke Sanematsu, Noriatsu Shigemura, Keiko Yasumatsu and Yuzo Ninomiya
(2005) Taste
Receptor Cells Responding with Action Potentials to Taste Stimuli and their Molecular
Expression of Taste Related Genes Jayaram
Chandrashekar1, Mark A. Hoon, Nicholas J. P. Ryba and Charles S. Zuker 2006 --The
receptors and cells for mammalian taste Nature 444, 288-294. Robert
F. Margolskee (2002 ) Molecular
Mechanisms of Bitter and Sweet Taste Transduction J. Biol. Chem., Vol. 277,
Issue 1, 1-4, January 4. Yoshiro
Ishimaru, Hitoshi Inada, Momoka Kubota, Hanyi Zhuang, Makoto Tominaga,, and Hiroaki
Matsunami (2006) Transient
receptor potential family members PKD1L3 and PKD2L1 form a candidate sour taste
receptor PNAS August 15, , vol. 103, no. 33, 12569-12574. Angela
L. Huang, Xiaoke Chen, Mark A. Hoon, Jayaram Chandrashekar, Wei Guo, Dimitri Tränkner,
Nicholas J. P. Ryba and Charles S. Zuker (2006) The
cells and logic for mammalian sour taste detection Nature 442, 934-938. | 
