Miraculin is a taste modifier, a glycoprotein extracted from the fruit of Synsepalum dulcificum. The berry, also known as the miracle fruit, was first documented by explorer Chevalier des Marchais, who searched for many different fruits during a 1725 excursion to its native West Africa.
Miraculin itself is not sweet. After the taste buds are exposed to miraculin (which binds to sour receptors on the tongue), acidic foods that are ordinarily sour (such as citrus) are perceived as sweet. This effect lasts up to an hour. The active substance, isolated by Prof. Kenzo Kurihara (栗原 堅三 Kurihara Kenzō), a Japanese scientist, was named miraculin after the miracle fruit when Kurihara published his work in Science in 1968.
SIGNAL (29) MKELTMLSLS FFFVSALLAA AANPLLSAA 1-50 DSAPNPVLDI DGEKLRTGTN YYIVPVLRDH GGGLTVSATT PNGTFVCPPR 51-100 VVQTRKEVDH DRPLAFFPEN PKEDVVRVST DLNINFSAFM PNPGPETISS 101-150 WCRWTSSTVW RLDKYDESTG QYFVTIGGVK FKIEEFCGSG FYKLVFCPTV 151-191 CGSCKVKCGD VGIYIDQKGR GRRLALSDKP FAFEFNKTVY F Amino acids sequence of glycoprotein miraculin unit adapted from Swiss-Prot biological database of protein sequences.
The molecular weight of the glycoprotein is 24.6 kDa including 3.4 kDa (13.9% of the weight) of sugar constituted (on molar ratio) of glucosamine (31%), mannose (30%), fucose (22%), xylose (10%) and galactose (7%).
The taste-modifying protein, miraculin, has seven cystein residues in a molecule composed of 191 amino acid residues. Both tetramer miraculin and native dimer miraculin in its crude state have the taste-modifying activity of turning sour tastes into sweet tastes.
Miraculin, like curculin (another taste-modifying agent), is not sweet by itself, but it can change the perception of sourness to sweetness, even for a long period after consumption. The duration and intensity of the sweetness-modifying effect depends on various factors, such as miraculin concentration, duration of contact of the miraculin with the tongue, and acid concentration. Maximum sweet-induced response has been shown to be equivalent to the sweetness of 17% sucrose solution. Glycoprotein is sensitive to heat: when heated over 100 °C, miraculin loses its taste-modifying property. Miraculin activity is inactivated at pH below 3 and pH above 12 at room temperature.
Although the detailed mechanism of the taste-inducing behavior is unknown, it appears the sweet receptors are activated by acids which are related to sourness, an effect remaining until the taste buds perceive a neutral pH. Sweeteners are perceived by the human sweet taste receptor, hT1R2-hT1R3, which belongs to G protein-coupled receptors, modified by the two histidine residues (i.e. His30 and His60) which participate in the taste-modifying behavior. One site maintains the attachment of the protein to the membranes while the other (with attached xylose or arabinose) activates the sweet receptor membrane in acid solutions.
As a sweetener
As miraculin is a readily soluble protein and relatively heat stable, it is a potential sweetener in acidic food (e.g. soft drinks). While attempts to express it in yeast and tobacco plants have failed, researchers have succeeded in preparing genetically modified E. coli bacteria, lettuce and tomatoes that express miraculin. The scientists' crops resulted in 40 micrograms of miraculin per gram of lettuce leaves, with two grams of lettuce leaves producing roughly the same amount of miraculin as in one miracle fruit berry.
The use of miraculin as a food additive was forbidden in 1974 by the United States Food and Drug Administration, in circumstances that have been interpreted as suggesting influence by competing commercial interests. Since 2011, The FDA has imposed a ban on importing Synsepalum dulcificum (specifying 'miraculin') from its origin in Taiwan, declaring it as an "illegal undeclared sweetener". Although this ban does not apply to fresh and freeze-dried miracle fruit, the fresh or normally-frozen berry deteriorates rapidly. The ban also does not apply to sale as a dietary supplement. There is informed opinion that the FDA ban could be overturned given sufficient funding for the required safety studies. Miraculin has a novel food status in the European Union. It is approved in Japan as a safe food additive, according to the List of Existing Food Additives published by the Ministry of Health and Welfare (published by JETRO).
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