Curculin

Curculin
A curculin homodimer. From PDB 2DPF.
Identifiers
Symbol	CURC_CURLA
PDB	2PDF 2D04
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. The plant is referred to locally as 'Lumbah'.

Protein structure

The active form of curculin is a heterodimer consisting of two monomeric units connected through two disulfide bridges. The mature monomers each consist of a sequence of 114 amino acids, weighing 12.5 kDa (curculin 1) and 12.7 kDa (curculin 2), respectively. While each of the two isoforms is capable of forming a homodimer, these do not possess the sweet taste nor the taste-modifying activity of the heterodimeric form. ^[2] To avoid confusion, the heterodimeric form is sometimes referred to as "neoculin".

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.^[3]

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][4][5]

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)^[6] 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][5]

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.^[5]

As a sweetener

Like most proteins, curculin is susceptible to heat. At a temperature of 50 °C (122 °F) 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,^[5] 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.^[7] However, a 2004 study obtained a recombinant curculin, expressed in E. coli, exhibiting "taste-modifying" and "sweet-tasting" properties. ^[2]

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).

See also

• Brazzein
• Miraculin
• Monellin
• Thaumatin

References

1. ^ ^{a b c d} Yamashita H, Theerasilp S, Aiuchi T, Nakaya K, Nakamura Y, Kurihara Y (September 1990). "Purification and complete amino acid sequence of a new type of sweet protein taste-modifying activity, curculin". J. Biol. Chem. 265 (26): 15770–5. PMID 2394746. http://www.jbc.org/cgi/pmidlookup?view=long&pmid=2394746. 
2. ^ ^{a b} Suzuki, M; Kurimoto, E; Nirasawa, S; Masuda, Y; Hori, K; Kurihara, Y; Shimba, N; Kawai, M; Suzuki, E; Kato, K (August 2004). "Recombinant curculin heterodimer exhibits taste-modifying and sweet-tasting activities". FEBS letters 573 (1–3): 135–8. doi:10.1016/j.febslet.2004.07.073. PMID 15327988. 
3. ^ UniProtKB/Swiss-Prot database entry #P19667
4. ^ Kurihara Y (1992). "Characteristics of antisweet substances, sweet proteins, and sweetness-inducing proteins". Crit Rev Food Sci Nutr 32 (3): 231–52. doi:10.1080/10408399209527598. PMID 1418601. 
5. ^ ^{a b c d} Yamashita H, Akabane T, Kurihara Y (April 1995). "Activity and stability of a new sweet protein with taste-modifying action, curculin". Chem. Senses 20 (2): 239–43. doi:10.1093/chemse/20.2.239. PMID 7583017. http://chemse.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=7583017. 
6. ^ Y Kurihara, H Kohno, M Kato, K Ikeda and M Miyake. Protein curuculin and application of the same. US5242693 A. 1993.
7. ^ 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.