Figure 10. Assignment of glucophore unit in sweet compounds: (1) fructose, (2) saccharin, (3) aspartame. Source: Ref. 64.
subunit I and 3 from subunit II) where each strand contains an average of 10 residues (67). There is only one sulfhydryl group (in subunit II), and in contrast to thau-matin I, no disulfide bond. Despite the intense sweetness of both proteins, there is little similarity in their three-dimensional structures. However, antibodies raised against thaumatin also cross-react with monellin (68). Five homologous amino acid sequences are found in thaumatin I and monellin; two are topologically similar and one of these is located in a /? bend protruding from the surface of the molecule. This tripeptide sequence (-Glu-Tyr-Gly-) has been postulated to be the active site responsible for the sweet taste (69,70). The structure-function relationship of these proteins is not clear.
A similar AH, B concept has been employed to explain bitterness with little success. The structural requirements for bitter taste have been proposed, and it has been shown that, in contrast to the AH/B system for sweeteners, bitter compounds need only one polar group (electrophilic or nu-cleophilic with a negative charge) and a hydrophobic group, with the intensity of bitter taste depending on the size and shape of the hydrophobic part of the molecule (71,72). Although the monopolar-hydrophobic concept for bitter taste requires more research, the importance of hy-drophobicity to bitterness has been well established. Introduction of an additional hydrophobic group onto a sweet compound often results in modifying the taste from sweet to bitter.
Several theories have been presented over the years to correlate the molecular structure with the perceived odor quality of a chemical compound. The infrared theory has had some success in correlating odor quality with low-energy molecular vibrations (73). The site-fitting theory attempts to link the size, shape, and electronic status of a molecule to a complementary receptor site (74). It has been suggested that although the overall shape and size of an odorous compound is responsible for the sensory perception, the functional group determines the orientation and the affinity of the molecule to the receptor (75). Molecular connectivity terms have been derived to relate the quantitative description of certain aspects of molecular topology to several classes of odor compounds and found significant correlations between the two (76). Another study on the qualitative structure activity relationship has been con ducted using computerized pattern recognition techniques (77). Both chemical composition and the shape of the molecule seem to be important contributing factors. In addition to the properties mentioned, it is also evident that the chirality and hydrophobicity of the compound are important for odor intensity.
Current knowledge about olfactory reception suggests the initial interaction occurs in the olfactory epithelium between the odorant with receptor proteins that extend from the dendrites of the olfactory sensory neurons (78,79). In humans, there are around 1,000 genes encoding 1,000 different odor receptors, each expressed in thousands of olfactory sensory neurons. A particular odor would bind to a characteristic set of multiple receptors rather than individual receptor cells, depending on the chemical groups of the odor molecule. Binding of an odorant to receptors activates a G protein-adenylate cyclase cascade, which results in the generation of cAMP. The cAMP causes the opening of an ion channel either directly or by phosphorylation via a cAMP-dependent protein kinase (80,81). The signals are transmitted to the localized glomeruli in the olfactory bulb, but exactly how the cortex decodes and reconstructs the signals into sensory responses is not known. There is still lack of a clear relationship between chemical structure and odor quality, and it is impossible to predict odor quality of a compound with a known structure.
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