Curculin
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'Curculin'
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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.
Contents |
[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).
[edit] References
- ^ a b c d YYamashita, H., S. Theerasilp, T. Aiuchi, K. Nakaya, Y. Nakamura, and Y. Kurihara. 1990. Purification and complete amino acid sequence of a new type of sweet protein taste-modifying activity, curculin. J. Biol. Chem. 265:15770-15775.
- ^ UniProtKB/Swiss-Prot database entry #P19667
- ^ Kurihara, Y. 1992. Characteristics of antisweet substances, sweet proteins, and sweetness-inducing proteins. Crit. Rev. Food Sci. Nutr. 32:231-252.
- ^ a b c d Yamashita, H., T. Akabane, and Y. Kurihara. 1995. Activity and stability of a new sweet protein with taste-modifying action, curculin. Chem. Senses 20:239-243.
- ^ Y Kurihara, H Kohno, M Kato, K Ikeda and M Miyake. Protein curuculin and application of the same. US5242693 A. 1993.
- ^ Kurihara, Y. and Nirasawa, S. Structures and activities of sweetness-inducing substances (miraculin, curculin, strogin) and the heat-stable sweet protein, mabinlin. Foods and Food Ingredients Journal of Japan 1997, 67-74.
- ^ M Suzuki, E Kurimoto, et. al. Recombinant curculin heterodimer, exhibits taste-modifying and sweet-tasting activities. FEBS Letters 2004, 573, 135-138.