Curculin-1 | |||||||
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Identifiers | |||||||
Organism | |||||||
Symbol | CURC_CURLA | ||||||
PDB | 2DPF | ||||||
UniProt | P19667 | ||||||
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Curculin-2 | |||||||
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Identifiers | |||||||
Organism | |||||||
Symbol | CURC2_CURLA | ||||||
PDB | 2D04 | ||||||
UniProt | Q6F495 | ||||||
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Curculin or neoculin 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' or 'Lemba'.
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".
Amino acid sequence of sweet proteins curculin-1 and curculin-2 adapted from Swiss-Prot biological database of protein sequences. Intra-chain disulfide bonds in bold, inter-chain disulfide bonds underlined. [3]
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]
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] Curculin appears to use a unique binding site at the amino terminal of TAS1R3. [6]
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).