Curculin or neoculin is a sweet

protein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chains. Protein complexes are distinct from multidomain enzymes, in which multiple active site, catalytic domains are found in a single polypeptide chain. ...

that was discovered and isolated in 1990 from the fruit of '' Curculigo latifolia'' ( Hypoxidaceae). Like

miraculin Miraculin is a taste modifier, a glycoprotein extracted from the fruit of '' Synsepalum dulcificum''. The berry, also known as the miracle fruit, was documented by explorer Chevalier des Marchais, who searched for many different fruits during ...

, 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. mRNAs for a related protein complex is found in '' Curculigo capitulata'' fruits, though at a much lower level of expression – so low that the product is undetectable by immunoblotting.

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 Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the Proteinogenic amino acid, 22 α-amino acids incorporated into p ...

, 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. To avoid confusion, the heterodimeric form is sometimes referred to as "neoculin".

Amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

sequence of sweet

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

s curculin-1 and curculin-2 adapted from Swiss-Prot biological database of protein sequences. Intra-chain disulfide bonds in bold italics, inter-chain disulfide bonds underlined.

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. A sweet taste, equivalent to a 6.8% or 12%

sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refined ...

solution, was observed after holding curculin in the mouth in combination with clear water or acidified water (

citric acid Citric acid is an organic compound with the formula . It is a Transparency and translucency, colorless Weak acid, weak organic acid. It occurs naturally in Citrus, citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, ...

), respectively. The sweet taste lasts for 5 minutes with water and 10 minutes with an acidic solution. 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. Although the "sweet-inducing" mechanism is unknown, it is believed that one active site of curculin strongly binds to the

taste receptor A taste receptor or tastant is a type of cellular receptor that facilitates the sensation of taste. When food or other substances enter the mouth, molecules interact with saliva and are bound to taste receptors in the oral cavity and other locat ...

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. Curculin appears to use a unique binding site at the amino terminal of TAS1R3.

As a sweetener

Like most proteins, curculin is susceptible to heat. At a temperature of 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, 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 Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom (biology), kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are est ...

, but the recombinant protein did not exhibit "sweet-tasting" or "taste-modifying" activity. However, a 2004 study obtained a recombinant curculin, expressed in ''E. coli'', exhibiting "taste-modifying" and "sweet-tasting" properties: the authors discovered a new curculin gene named curculin-2, which was previously unknown. Only the heterodimer formed by curculin-1 and curculin-2 exhibit these activities. 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 The European Union (EU) is a supranational union, supranational political union, political and economic union of Member state of the European Union, member states that are Geography of the European Union, located primarily in Europe. The u ...

and

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

. However it is approved in

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

as a harmless

additive Additive may refer to: Mathematics * Additive function, a function in number theory * Additive map, a function that preserves the addition operation * Additive set-function see Sigma additivity * Additive category, a preadditive category with fin ...

, according to the List of Existing Food Additives established by the Ministry of Health and Welfare (English publication by JETRO).

References

External links

*{{Commons category-inline Plant proteins Taste modifiers Sugar substitutes Curculigo

Protein structure

Sweetness properties

As a sweetener

See also

References

External links