Love for Sweets Isn′t Only Based On Your "Sweet Tooth"

Background: Most humans enjoy eating candy and sweets. Sweet-tasting compounds in foods include large proteins (brazzein), as well as smaller carbohydrates (glucose) and artificial sweeteners (saccharine). Taste buds on our tongue contain receptors that are responsive to sweet-tasting compounds. The sweet taste receptor gene family consists of only 3 members. Two of the 3 members are responsible for the production of a separate sweet receptor protein and these two proteins, T1R2 and T1R3, function as a combined complex called a protein dimer. Scientists conclude that the sensitivity and selectivity of the receptor to a wide variety of sweet-tasting compounds must reside in the nature of the interaction between a particular sweet-tasting compound and the different extracellular sites and transmembrane domains within the receptor molecule.

Advances: Scientists have recently found that the molecular basis for sensitivity to the sweet protein, brazzein, depends on a site within a region of the human T1R3 taste receptor but T1R2 is not involved in the binding. Moreover, this site differs from the site needed for the sensitivity to other smaller sweet-tasting molecules.

In another study, scientists have discovered that a mutation in the gene that encodes for T1R2 renders the receptors in cats non-functional; therefore, felines are unable to taste sweet compounds. This is an unusual example of the loss of function of a single gene leading to a specific change in a relatively complex behavior, namely the loss of preference for sweet-tasting compounds. In this case, the loss is selective for sweet taste but other elements of food intake remain normal.

Implications: Taste preference shapes diet, nutrition, and health. The underlying genetic bases for food preferences have significant health implications. This is most apparent in the realm of sweet consumption and the origins of obesity and diabetes. Since preference to sweets appear to have substantial genetic underpinnings, genetic variations, deletions or over-expression become potential behavioral modifiers with important long-term health consequences. It is important to understand sweet taste receptor function at the molecular level in order to design more effective low- and non-caloric sweeteners and sugar substitutes.

Citations: Jiang P, Ji Q, Liu Z, Snyder LA, Benard LM, Margolskee RF, Max M. The Cysteine-Rich Region of T1R3 Determines Responses to Intensely Sweet Proteins. J Bio Chem 279: 45068-45075, 2004.

Li X, Li W, Wang H, Cao J, Maehashi K, Huang L, Bachmanov AA, Reed DR, Legrand-Defretin V, Beauchamp GK, Brand JG. Pseudogenization of a Sweet-Receptor Gene Accounts for Cats’ Indifference Toward Sugar. PLoS Genetics 1: 27-35, 2005.

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