Passage
Umami is one of five basic tastes that humans recognize, and it is triggered by L-amino acids as well as some synthetic ingredients and organic acids. Three common umami tastant molecules are shown in Figure 1.
Figure 1 Three common tastant molecules known to trigger the umami taste sensationUmami taste signals are triggered upon binding of tastant molecules to the G protein-coupled receptor (GPCR) heterodimer T1R1-T1R3, which ultimately causes the breakdown of phosphatidylinositol 4,5-bisphosphate (PIP₂) into the messenger molecules inositol-1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG) . IP₃ triggers the release of Ca²⁺ cations and the subsequent gating of the taste-transduction channels (Figure 2) .
Figure 2 Signal pathway for transduction of umami taste sensationInosine monophosphate (IMP) has been suggested as enhancing (potentiating) the response of T1R1+T1R3 GPCRs to umami tastant molecules. Scientists tested this hypothesis by culturing receptors from human embryonic kidney (HEK) cells containing promiscuous G proteins. The HEK cells were then stimulated with umami tastants in the presence or absence of IMP. In each test, the cells were exposed to the same amounts of amino acid tastant and IMP. The number of cells that responded to each tastant was measured using a calcium indicator dye (FURA-2AM) in conjunction with fluorescence microscopy. By comparing the cellular response ratio of the stimulated to unstimulated HEK cells, the extent of potentiation can be assessed. The results are shown in Figure 3.
Figure 3 Quantification of amino-acid responses for T1R1+T1R3 with and without IMP
Adapted from Nelson G, Chandrashekar J, Hoon MA, et al. An amino-acid taste receptor. Nature. 2002;416(6877) :199-202.
-Which of the following best explains the results regarding D-amino acids in Figure 3?

Question

Passage
Umami is one of five basic tastes that humans recognize, and it is triggered by L-amino acids as well as some synthetic ingredients and organic acids. Three common umami tastant molecules are shown in Figure 1.

Figure 1 Three common tastant molecules known to trigger the umami taste sensationUmami taste signals are triggered upon binding of tastant molecules to the G protein-coupled receptor (GPCR) heterodimer T1R1-T1R3, which ultimately causes the breakdown of phosphatidylinositol 4,5-bisphosphate (PIP₂) into the messenger molecules inositol-1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG) . IP₃ triggers the release of Ca²⁺ cations and the subsequent gating of the taste-transduction channels (Figure 2) .

Figure 2 Signal pathway for transduction of umami taste sensationInosine monophosphate (IMP) has been suggested as enhancing (potentiating) the response of T1R1+T1R3 GPCRs to umami tastant molecules. Scientists tested this hypothesis by culturing receptors from human embryonic kidney (HEK) cells containing promiscuous G proteins. The HEK cells were then stimulated with umami tastants in the presence or absence of IMP. In each test, the cells were exposed to the same amounts of amino acid tastant and IMP. The number of cells that responded to each tastant was measured using a calcium indicator dye (FURA-2AM) in conjunction with fluorescence microscopy. By comparing the cellular response ratio of the stimulated to unstimulated HEK cells, the extent of potentiation can be assessed. The results are shown in Figure 3.

Figure 3 Quantification of amino-acid responses for T1R1+T1R3 with and without IMP
Adapted from Nelson G, Chandrashekar J, Hoon MA, et al. An amino-acid taste receptor. Nature. 2002;416(6877) :199-202.
-Which of the following best explains the results regarding D-amino acids in Figure 3?

Quizplus · Accepted Answer

11ecba2d_11a2_f69c_a3bf_a5eb28eea8b5_MD0008_00 The umami sensation is triggered by binding of tastant molecules to the GPCR T1R1+T1R3 heterodimer, ultimately leading to the release of Ca²⁺ ions. The number of cells that respond to a given tastant was determined using a calcium dye binding assay with fluorescence microscopy. As shown in Figure 3, the y-axis indicates cell response, and it is evident that D-amino acids elicit only a small response, if any, from cells; of the D-amino acids, only D-Ala caused a noticeable cell response. Because responses are caused by tastant binding, it can be concluded that a majority of D-amino acids do not bind effectively to T1R1+T1R3 GPCR cells. (Choice A) According to the experiment, each cell was exposed to an equal amount of both L-amino acids and D-amino acids. (Choice B) The FURA-2AM dye causes fluorescence when in the presence of calcium, not amino acids. (Choice C) Although D-amino acids do not trigger DAG formation, cell response is measured according to the amount of calcium release, which is triggered by the presence of IP₃, not DAG. Educational objective: Interpretation of graphic data requires an understanding of the underlying mechanisms. Cells may have altered responses to stimuli for a number of reasons. In this case, amino acids induce a response by binding to a receptor, so failure to induce a response indicates a lack of binding.

Passage Umami Is One of Five Basic Tastes That Humans Recognize