Sweetness receptor, glucose

Fig. 8.4. Model of a sweetness receptor according to Nofre and Tinti (1996). a) Possible interactions of a sweet substance with the receptor. Interactions of the receptor with b) glucose, c) sucrose and d) lugduname...

Sucrose is rapidly dissociated into glucose and fructose by the enzymes in your mouth and in your stomach, and your taste receptors sense sweetness. A problem (for Coca Cola) is that fructose tastes five times as sweet as glucose so 40% of the sucrose they purchase is wasted compared to pure fructose. A problem for you is that both sugars have the same calories, and the soft drink companies want to advertise lower calories for an aceeptable sweetness. 

Elicitation of sweetness is explained by the AH—B—X Theory, or the Sweetness Triangle (1,23,24), wherein AH and B represent a hydrogen donor and acceptor, respectively, of sucrose. These interact with complementary regions of taste-receptor proteins. An extended hydrophobic region (X) of sucrose docks in a hydrophobic cleft of the receptor, facilitating optimal electrostatic interaction and sensory stimulation. The C2- and C3-hydroxyls of glucose (AH and B, respectively) and the back side of the fmctose ring (X) are critical to this interaction (1). 

Figure 3. Positioning of /3-d-glucose over the proposed taste receptor site to initiate sweet taste response...

Sugar is added to chocolate to counteract the bitter taste. The arrangement of atoms in sugar molecules allows them to fit into the receptor sites of sweet taste cells. When a sugar molecule such as glucose or sucrose attaches to a receptor of a sweet taste cell, the sweet signal is sent to the brain. 

For a molecule to taste sweet, it must bind to a receptor on a taste bud cell of the tongue. The binding of this molecule causes a nerve impulse to pass from the taste bud to the brain, where the molecule is interpreted as being sweet. Sugars differ in their degree of sweetness. The relative sweetness of glucose is 1.00, that of sucrose is 1.45, and that of fmctose, the sweetest of all sugars, is 1.65. Developers of synthetic sweeteners must consider several factors—such as toxicity, stability, and cost—in addition to taste. 

Often, the biological difference between enantiomers is one of activity or effectiveness, with no difference in toxicity. Aspartame (NutraSweet), widely used as an artificial sweetener, has two enantiomers. However, one enantiomer has a sweet taste, while the other enantiomer is bitter. This indicates that the receptor sites on our taste buds must be chiral, since they respond differently to the handedness of aspartame enantiomers. This becomes clearer when looking at the properties of the simple sugars. D-Glucose is sweet and nutritious, whereas L-glucose is tasteless and cannot be metabolized by the body. 

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