Miraculin encyclopedia topics

Glycoprotein structure

Miraculin was first sequenced in 1989 and was found to be a glycoprotein consisting of 191 amino acids and some carbohydrate chains.
Miraculin occurs as a tetramer (98.4 kDa), a combination of 4 monomers group by dimer. Within each dimer 2 miraculin glycoproteins are linked by a disulfide bridge.

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.

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%).

Sweetness properties

Miraculin, unlike curculin (another taste-modifying agent), is not sweet by itself, but it can change a sour beverage into a sweet beverage, even for a long period after consumption. The anti-sweet compound, Gymnemic acid suppresses the sweet taste of miraculin, like it does for sucrose. The duration and intensity of the taste-modifying phenomena depends on various factors — 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 100ºC, miraculin loses its taste-modifying property. Miraculin activity is inactivated at pH below 3 and pH above 12 at room temperature. The sweet modifying effect stays at pH 4 (in acetate buffer), for 6 months at 5ºC.

The detailed mechanism of the taste-inducing behaviour is still unknown. It has been suggested that the miraculin protein can change the structure of taste cells on the tongue. As a result, the sweet receptors are activated by acids, which are sour in general. This effect remains until the taste buds return to normal. The two histidine residues (i.e. His29 and His59) appear to be mainly responsible for the taste-modifiying 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. The presence of positively charged ions (Ca²⁺ and/or Mg²⁺) interferes with the binding of the active sugar of miraculin to the sweet receptor and therefore inactivates the effect.

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). Japanese researchers' more or less successful attempts to mass produce it are focused on recombinant technology. While attempts to express it in E. coli bacteria, yeast, and tobacco plants have failed, researchers have succeeded in preparing genetically modified lettuce that express miraculin. The scientists' crops resulted in 40 micrograms of miraculin per gram of lettuce leaves, which was considered a large amount. Two grams of lettuce leaves produced roughly the same amount of miraculin as in one miracle fruit berry.

Miraculin was denied approval for use as a sweetener by the United States Food and Drug Administration (FDA).

Miraculin has no legal status in the European Union. However it is approved in Japan as a harmless additive, according to the List of Existing Food Additives published by the Ministry of Health and Welfare (published by JETRO).