Curculin

Curculin is a sweet protein which has been discovered and isolated in 1990 from the fruit of Curculigo latifolia (Hypoxidaceae), a plant from Malaysia. Like miraculin, curculin exhibits taste-modifying activity; however unlike miraculin, it also exhibits a "sweet-tasting" by itself. After consumption of curculin, water and sour solution taste sweet.

Protein structure
Curculin is a dimer of two monomeric protein connected through 2 disulphide bridges. Each monomer weights 12.5 kda and contains a sequence of 114 amino acids.

SIGNAL (22): MAAKFLLTIL VTFAAVASLG MA

1-50: DNVLLSGQTL HADHSLQAGA YTLTIQNKCN LVKYQNGRQI WASNTDRRGS

51-100: GCRLTLLSDG NLVIYDHNNN DVWGSACWGD NGKYALVLQK DGRFVIYGPV

101-114: LWSLGPNGCR RVNG

PROPEP (22): GITVAKDSTE PQHEDIKMVI NN

''Amino acid sequence of sweet protein curculin adapted from Swiss-Prot biological database of protein sequences. ''

Sweetness properties
Curculin can be considered as an high intense sweetener with a reported relative sweetness 430-2070 times sweeter than sucrose on a weight basis.

A sweet taste, equivalent to 6.8% and 12% of sucrose solution, was observed after holding curculin in the mouth in combination with clear water and acidified water (citric acid) respectively. Sweet taste lasts 5 minutes with water and 10 minutes with acidic solution.

Sweetness was also observed with other acids such as ascorbic acid (vitamin C) and acetic acid.

The taste-modifying activity of curculin is reduced in the presence of ions with 2 positive charges (Ca2+ and Mg2+) in neutral solution while they have no effect in acidic solutions. In the same way monovalent ions (Na+ and Cl-) have no effect in both neutral and acidic pH.

The “sweet-inducing” mechanism is unknown, however it is believed that one site of curculin strongly bound to the taste receptor membranes while a second active site fits in the sweet receptor site. The later site is responsible for induction of the sweetness. Presence of Ca2+ and/or Mg2+, water and acids tune the binding of the active site of curculin to the receptor site and therefore modify perceived sweetness.

As a sweetener
Curculin is a protein, so not very stable to heat, over 50°C the proteins start to degrade, and therefore to lose its “sweet-tasting” and “taste-modifying” property. However below this temperature, both poperties of curculin were unaffected in basic and acidic solutions. Therefore curculin will be impossible to use in hot food or processed food, however it’s the perfect candidate for use in fresh food as a table-top sweetener.

As it is not widely spread in nature, scientists are looking produce thanks to recombinant technology. In 1997, curculin was expressed in E.coli and yeast, but the recombinant curculin didn’t exhibits the “sweet-tasting” nor "taste-modifying” activity, however a more recent study (2004) has obtained a recombinant curculin expressed in E.coli exhibiting “taste-modifying” and “sweet-tasting” property.

The main issue to use it as sweetener is the legal side as Curculin has no legal status in European Union and United States. However it is approved in Japan as a harmless additive, according to the List of Existing Food Additives established by the Ministry of Health and Welfare (English publication by JETRO).

Links

 * Photo of Curculigo latifolia