Curculin or neoculin is a sweet

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

that was discovered and isolated in 1990 from the fruit of ''

Curculigo latifolia ''Curculigo'' is a flowering plant genus in the family Hypoxidaceae, first described in 1788. It is widespread across tropical regions of Asia, Africa, Australia, and the Americas. Curculin is a sweet protein that was discovered and isolated in ...

'' (

Hypoxidaceae Hypoxidaceae is a family of flowering plants, placed in the order Asparagales of the monocots. The APG IV system of 2016 (unchanged from the 1998, 2003, and 2009 versions) recognizes this family. The family consists of four genera totalling s ...

), a plant from

Malaysia Malaysia ( ; ) is a country in Southeast Asia. The federation, federal constitutional monarchy consists of States and federal territories of Malaysia, thirteen states and three federal territories, separated by the South China Sea into two r ...

. 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 a ...

, 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'.

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 hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

, 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". *1, 1-50: DNVLLSGQTL HADHSLQAGA YTLTIQNKCN LVKYQNGRQI WASNTDRRGS *1, 51-100: GCRLTLLSDG NLVIYDHNNN DVWGSACWGD NGKYALVLQK DGRFVIYGPV *1, 101-114: LWSLGPNGCR RVNG *2, 1-50: DSVLLSGQTL YAGHSLTSGS YTLTIQNNCN LVKYQHGRQI WASDTDGQGS *2, 51-100: QCRLTLRSDG NLIIYDDNNM VVWGSDCWGN NGTYALVLQQ DGLFVIYGPV *2, 101-113: LWPLGLNGCR SLN

Amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

sequence of sweet

s 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.

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 chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in t ...

), 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 which 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 loc ...

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 Taste receptor type 1 member 3 is a protein that in humans is encoded by the ''TAS1R3'' gene. The ''TAS1R3'' gene encodes the human homolog of mouse Sac taste receptor, a major determinant of differences between sweet-sensitive and -insensitive ...

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 {{Wiktionary, sweetener A sweetener is a substance added to food or drink to impart the flavor of sweetness, either because it contains a type of sugar, or because it contains a sweet-tasting sugar substitute. Many artificial sweeteners have been ...

. 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. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitut ...

, 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. 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 political and economic union of member states that are located primarily in Europe. The union has a total area of and an estimated total population of about 447million. The EU has often been des ...

and

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

. However it is approved in

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

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-functionn see Sigma additivity * Additive category, a preadditive category with f ...

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

JETRO is an Independent Administrative Institution established by Japan Export Trade Research Organization as a nonprofit corporation in Osaka in February 1952, reorganized under the Ministry of International Trade and Industry (MITI) in 1958 (later the ...

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