Sucrose, sweetness receptor

Flavouring substances that cause only taste impressions are defined as substances that are usually non-volatile at room temperature. Therefore, they are only perceived by the taste receptors [7]. Examples are sucrose (sweet) or caffeine (bitter). Flavouring substances causing odour impressions are volatiles that are perceived by the odour receptors [7]. Examples are ethyl butyrate or dimethyl sulfide. Some flavouring substances are perceived by taste and odour receptors (e.g. acetic acid, butyric acid). 

Much of the research on sweeteners involves probing the structure of sweetness receptor sites. One model proposed for a sweetness receptor incorporates eight binding interactions that involve hydrogen bonding as well as van der Waals forces. Sucronic acid is a synthetic compound designed on the basis of this model. Sucronic acid is reported to be 200,000 times as sweet as sucrose. 

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...

The newest frontier in sweetener research is the area of sweet enhancers. These substances, while tasteless themselves, cause sweetness receptors to respond more strongly to sweet substances. For example, the simple experimental molecule dubbed SE-2 enhances the sweetness of sucralose up to eightfold. It is thought that SE-2 and related structures bind to the Venus flytrap complex at a remote site from the sweetener itself, causing the trap to stay shut for longer periods of time. Sucrose and sucralose enhancers are beginning to be commercialized around the... 

A persistent idea is that there is a very small number of flavor quaUties or characteristics, called primaries, each detected by a different kind of receptor site in the sensory organ. It is thought that each of these primary sites can be excited independently but that some chemicals can react with more than one site producing the perception of several flavor quaUties simultaneously (12). Sweet, sour, salty, bitter, and umami quaUties are generally accepted as five of the primaries for taste sucrose, hydrochloric acid, sodium chloride, quinine, and glutamate, respectively, are compounds that have these primary tastes. Sucrose is only sweet, quinine is only bitter, etc saccharin, however, is slightly bitter as well as sweet and its Stevens law exponent is 0.8, between that for purely sweet (1.5) and purely bitter (0.6) compounds (34). There is evidence that all compounds with the same primary taste characteristic have the same psychophysical exponent even though they may have different threshold values (24). The flavor of a complex food can be described as a combination of a smaller number of flavor primaries, each with an associated intensity. A flavor may be described as a vector in which the primaries make up the coordinates of the flavor space. 

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. 

An interesting structure-taste study of sweet Ao-vanillyl derivatives has been published <1998JFA4002, 2001QSA3>. It was found that only one enantiomer of each pair proved to be sweet, the other being tasteless. The R-(+)-enantiomer of compound 129 was the sweetest molecule among the variety tested with a relative sweetness, RS, of 20000 (RS = [sucrose]/[compound]). (The 6 -(—(-enantiomer was also tasteless.) As in these Ao-vanillyl derivatives, the difference in the taste of two enantiomers seems to be general and helps in defining receptor-active sites. 

Aspartame, discovered by Mazur in 1969 (5), is 200 times sweeter than sucrose. Aspartame has a large commercial market as an artificial sweetening agent. It is apparent that the sweetness exhibited by aspartame requires amino (AH, electropositive) and carboxyl (B, electronegative) groups of aspartic acid moiety and the hydrophobic side chain (X) of the phenylalanine moiety (4). The sweetness of aspartame is exhibited by the trifunctional units AH, B, and X. It is thought that when the trifunctional units of aspartame, X, AH, and B, fit the corresponding receptor sites, a sweet taste is produced. 

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). 

Mixed acetyl-isobutanoyl sucrose esters (SAIB) are used as phase densifiers in beverages.426,427 Sucralose (trichlorogalactosucrose) is produced at the industrial scale and marketed in the UK and other countries as a sweetener, having a 650 times the sweetening power of sucrose.428 Being more stable to heat as compared to other synthetic sweeteners, it can be used in cooking. It has also been shown that the sweet taste of sucralose is based on interactions with both subunits of the sweet-taste receptor.429... 

Further examinations of the molecular features and of the model of receptor have suggested that several aspartyl tripeptide esters may also taste sweet. In confirmation of the idea, several tripeptide esters have been synthesized. In the first place, L-Asp-Gly-Gly-OMe (38) was synthesized as an arbitrarily-selected standard of tripeptides, because it was considered that this peptide ester had the simplest structure, and correlation of other peptides to (38) was easy. The tripeptide ester was predicted that it would be slightly sweet or tasteless because its projection formula was similar in size and shape to that of L-Asp-Gly-0Bum which is 13 times sweeter than sucrose (16) and because it is more hydrophilic than the dipeptide. The tripeptide (38) was devoid of sweetness and almost tasteless. 

It is generally accepted that a drug initiates a chain of events which eventually leads to a specific biological effect but which does not involve the drug after it triggers the mechanism through a drug-receptor interaction. For example, sucrose tastes sweet, but the role of sucrose molecules is to stimulate the taste buds, and they do not participate in the process of sensory conduction as such. 

Almost all amino acids elicit taste. Most hydrophobic L-amino acids have a bitter taste. However, hydrophobic D-amino acids, which are formed simultaneously by the synthesis of L-amino acids, bring out a strong sweet taste. D-Trp, Phe, His, Tyr and Leu are 35, 7, 7, 6 and A times as sweet as sucrose, respectively (2). Gly and L-Ala elicit a strong sweet taste. It is thought that the strong sweet taste elicited by these amino acids is due to the ability of these molecules to bind to the sweet substance receptors. 

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