Curculin

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'Curculin'
Identifiers
Symbol	CURC_CURLA
UniProt	P19667
Other data

Curculin is a sweet protein that was discovered and isolated in 1990 from the fruit of Curculigo latifolia (Hypoxidaceae),^[1] a plant from Malaysia. Like miraculin, curculin exhibits taste-modifying activity; however unlike miraculin, it also exhibits a "sweet-taste" by itself. After consumption of curculin, water and sour solutions taste sweet.

1 Protein structure
2 Sweetness properties
3 As a sweetener
4 References
5 See also
6 Links

[edit] Protein structure

Curculin is a homodimer consisting of two monomeric units connected through two disulfide bridges. Each mature monomer weighs 12.5 kDa and consists of 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.^[2]

[edit] Sweetness properties

Curculin is considered to be a high-intensity sweetener, with a reported relative sweetness of 430-2070 times sweeter than sucrose on a weight basis.^[1]^[3]^[4]

A sweet taste, equivalent to a 6.8% or 12% sucrose solution, was observed after holding curculin in the mouth in combination with clear water or acidified water (citric acid), respectively. The sweet taste lasts for 5 minutes with water and 10 minutes with an acidic solution.^[1]
Sweetness was also observed with other acids such as ascorbic acid (vitamin C)^[5] and acetic acid.

The taste-modifying activity of curculin is reduced in the presence of ions with two positive charges (such as Ca²⁺ and Mg²⁺) in neutral pH solutions, although these ions have no effect in acidic solutions. In the same way, monovalent ions (such as Na⁺ and Cl^-) have no effect in solutions with either neutral or acidic pH.^[1]^[4]

Although the "sweet-inducing" mechanism is unknown, it is believed that one active site of curculin strongly binds to the taste receptor membranes while a second active site fits into the sweet receptor site. The latter site is thought to be responsible for the induction of 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.^[4]

[edit] As a sweetener

Like most proteins, curculin is susceptible to heat. At a temperature of 50°C the protein starts to degrade and lose its "sweet-tasting" and "taste-modifying" properties, so it is not a good candidate for use in hot or processed foods. However, below this temperature both properties of curculin are unaffected in basic and acidic solutions.^[4], so it has potential for use in fresh foods and as a table-top sweetener.

Because curculin is not widely found in nature, efforts are underway to produce a recombinant form of the protein. In 1997, curculin was expressed in E.coli and yeast, but the recombinant protein did not exhibit "sweet-tasting" or "taste-modifying" activity.^[6] However, a 2004 study obtained a recombinant curculin, expressed in E.coli, exhibiting "taste-modifying" and "sweet-tasting" properties.^[7]

In addition to challenges related to commercial production of the protein, there are many regulatory and legal issues remaining to be resolved before it can be marketed as a sweetener. Curculin currently 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).