Sweet macromolecules

FIGURE 5 Ribbon representations of the three sweet proteins of known structure. 

FIGURE 6 Ribbon representations of lysozyme and neoculin. (A) Tetragonal form of lysozyme solved at 1.3-A resolution (pdb entry 193L). (B) One of the four crystallographically independent heterodimers of neoculin (pdb entry 2d04). 

The fruits of Synsepalum dulcificum have been known for more than a 

Interaction of sweet proteins with the sweet receptor 

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Most sweet compounds including the commercial sweeteners, are small molecular weight compounds but there are also sweet macromolecules both synthetic and natural. It was thought that compounds with molecular masses over 2 500 would generally be tasteless. It was assumed that macromolecules such as proteins could elicit a sweet taste similar to small molecules such as sucrose and stevioside until the discovery of miraculin. 

Although not very numerous, sweet macromolecules, both natural (Morris, 1976) and synthetic (Zaffaroni, 1975), are crucial for an understanding of the mechanism of the sweet receptor. The best known among proteins with a very strong sweet taste are brazzein (Ming and HeUekant, 1994), monellin, and thaumatin (Kurihara, 1992). Figure 5 shows molecular models of these three proteins. Other two known sweet proteins are mabinlin (Kurihara, 1992) and hen egg white (HEW) lysozyme (Maehashi and Udaka, 1998), whereas miraculin and curculin, which taste sweet when combined with sour substances, can be better described as taste-modifier proteins (Kurihara, 1992). 

Temussi, P.A. (2006). Natural sweet macromolecules How sweet proteins work. CMLS 63, 1876-1888. 

FIGURE 3.1 Crystals of a variety of macromolecules. In (a), clusters of needle crystals of bacterial a amylase in (b), crystals of a protease from pineapple, in (c), a crystal of satellite tobacco mosaic virus and in (d), a crystal of the sweet protein thaumatin. 

Fury, W., and Swaminathan, S. 1997. Phases-95 A program package for processing and analyzing diffraction data from macromolecules. In Methods in Enzymology, Macromolecular Crystallography (eds. C. W. Carter and R. W. Sweet), vol. 277, pp. 590-619. 

Ipomeanol. This pulmonary toxin is produced by a mould which grows on sweet potatoes. The toxin produces oedema, congestion and haemorrhage resulting from necrosis of the Clara cells (non-ciliated bronchiolar cells). Metabolic activation by a specific form of cytochrome P-450 in these cells produces a reactive metabolite which binds to macromolecules in these cells causing necrosis. Induction and inhibition of cytochrome P-450 may increase and decrease toxicity and depleting GSH increases the toxicity. 

Other large scale biopolymers are polysaccharides. Polysaccharides are relatively complex carbohydrates. They are polymers made up of many monosaccharides joined together by glycosidic linkages. They are therefore very large, often branched, macromolecules. Polysaccharides tend to be amorphous, insoluble in water, and have no sweet taste. When all the constituent monosaccharides are of the same type they are termed homopoly saccharides, when more than one type of monosaccharide is present they are termed heteropolysaccharides. Examples include storage polysaccharides such as starch and glycogen and structural polysaccharides such as cellulose and chitin. 

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