Sweet bitter receptor

Fig. 36.—Binding of a Bitter-sweet Molecule to a Sweet Receptor- and a Bitter Receptor-site.

Fig. 37.—Binding of One Molecule of a Bitter-sweet Compound to (i) a Sweet Receptor, and of a Second Molecule of the Same Compound to (ii) a Bitter Receptor.

The experimental data suggest that sweetness and bitterness are recognized at the same receptor. Furthermore, the receptor discriminates between bitter and sweet tastes based upon differences in functional unit combination. A new taste receptor model is proposed and presented. 

The current accepted theory suggests that a bitter compound and a sweet compound bind independently at specitic receptors. This situation will be referred to as "independent" in this report. The data to follow will demonstrate that a bitter compound and a sweet compound bind at the same receptor in a competitive manner. Therefore, this situation will be referred to as "competitive" in this report. Which theory was the functioning mechanism of taste reception should be determinable when one measured the taste intensities of mixed solutions of bitter and sweet tasting compounds. In this experiment the mechanism could be predicted to elicit a considerable difference in taste intensity and response that was varying based on the final concentration of each component. The "independent" receptor mechanism would be expected to yield data in which the intensities of bitter and sweet would be unaffected by mixing the two tastes, no matter what the concentration. On the other hand, with the "competitive" receptor mechanism one would expect both flavors to become altered, i.e., one stronger and the other weaker, as component concentrations varied the latter would occur because of competition of the substances for the same site. 

An experimental difficulty lies in the fact that there are only a few thousand taste buds in the tongue, with only 50-100 cells in a bud. They age rapidly, having a lifespan of only about ten days.924 There may be only 30,000-50,000 hard-to-isolate taste receptor cells on the tongue s surface.923 However, very recently published reports describe a large family of bitter and sweet receptors in mice and humans924-928 and in Drosophila.929 930 The sweet-sour receptors are thought to activate a G protein called gustducin,931/932 which plays a role similar to that of transducin in vision and... 

The general model developed for sweet and bitter compounds leads to a sweet-bitter receptor, which can be given formal representation as a hydrophobic pocket with a bipolar system (Fig. 

Figure 16. Representation of the hydrophobic pocket and the polar contact groups of a formal sweet-bitter receptor...

During the last few years, the field of taste research has been progressing with enormous speed. Current research addresses structural details on receptor agonist interactions for bitter and sweet taste receptors. For the other taste modalities, including salt taste, where definite proof for the proposed roles of candidate sensor molecules is still missing, more questions still need to be answered. A next big step in taste research will be to understand how taste information is processed along its... 

Shi P, Zhang J. Contrasting modes of evolution between vertebrate sweet/umami receptor genes and bitter receptor genes. Mol. Biol. Evol. 2006 23 292-300. 

Just as in olfaction, a number of clues pointed to the involvement of G proteins and, hence, 7TM receptors in the detection of bitter and sweet tastes. The evidence included the isolation of a specific G protein a subunit termed gustducin, which is expressed primarily in taste buds (Figure 32.13). How could the 7TM receptors be identified The ability to detect some compounds depends on specific genetic loci in both human beings and mice. For instance, the ability to taste the bitter compound 6- -propyl-2-thiouracil (PROP) was mapped to a region on human chromosome 5 by comparing DNA markers of persons who vary in sensitivity to this compound. 

Do these diverse compounds give rise to a common perception of sweetness or to qualitatively different sensations Sweetness does indeed appear to be a unitary percept (Breslin et al. 1994,1996). However, some sweeteners may be discriminable on the basis of their activation of other sensory transduction mechanisms or differences in the temporal properties of their sensory action. For example, the sweetener sodium saccharin activates bitter receptors in some people (Kuhn et al. 2004 Pronin et al. 2007), and also inhibits sweet taste at high concentrations (Galindo-Cuspinera et al. 2006). Sweet proteins such as thaumatin and monellin can have a slow onset or evoke a prolonged sweetness compared with sugars (Faus 2000), likely owing to a relatively high affinity for the sweet taste receptor. 

Just as in olfaction, a number of clues pointed to the involvement of G proteins and, hence, 7TM receptors in the detection of bitter and sweet tastes. The eviden ce included the isolation of a specific G-protein a subunit... 

Bitter and sweet sensations are mediated by G proteins coupled to 7TM receptors, leading to millisecond time resolution. Salty and sour sensations are mediated directly by ion channels, which may lead to faster time resolution. 

Your tongue has about 3000 taste buds, each of which is an onion-shaped collection of 50 to 150 taste cells. Each taste bud is specialized for tasting either sweet, sour, salt, or bitter. It has been suggested that the tongue can also perceive another taste, umami, which is a subtle taste most commonly associated with monosodium glutamate, MSG. At the tips of the bitter and sweet taste cells are receptor molecules shaped to fit parts of certain molecules in our food. 

Present the evidence that suggests that the family of sweet receptors is closely related and parallel to the family of bitter receptors. 

Sour and salty taste responses result from the direct action of hydrogen ions or sodium ions on channels these responses therefore have the potential for very rapid time resolution. Taste responses (bitter and sweet) that are likely to require 7TM receptors and second messengers will exhibit slower time resolution. 

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