Curculin
Encyclopedia
Curculin is a sweet protein
that was discovered and isolated in 1990 from the fruit of Curculigo latifolia (Hypoxidaceae
), 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'.
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.
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.
Sweetness was also observed with other acids such as ascorbic acid
(vitamin C
) 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.
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.
.
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. 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
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).
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
that was discovered and isolated in 1990 from the fruit of Curculigo latifolia (Hypoxidaceae
Hypoxidaceae
Hypoxidaceae is the botanical name of a family of flowering plants, placed in the order Asparagales of the monocots.The family has only recently been recognized by taxonomists. The APG III system of 2009 does recognize this family...
), a plant from Malaysia. Like miraculin
Miraculin
Miraculin is a natural sugar substitute, a glycoprotein extracted from the fruit of Synsepalum dulcificum. The berry, which contains active polyphenols, was first documented by explorer Chevalier des Marchais, who searched for many different fruits during a 1725 excursion to its native West...
, 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.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
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
curculin adapted from Swiss-Prot biological database of protein sequences.
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
Sucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...
solution, was observed after holding curculin in the mouth in combination with clear water or acidified water (citric acid
Citric acid
Citric acid is a weak organic acid. It is a natural preservative/conservative and is also used to add an acidic, or sour, taste to foods and soft drinks...
), respectively. The sweet taste lasts for 5 minutes with water and 10 minutes with an acidic solution.
Sweetness was also observed with other acids such as ascorbic acid
Ascorbic acid
Ascorbic acid is a naturally occurring organic compound with antioxidant properties. It is a white solid, but impure samples can appear yellowish. It dissolves well in water to give mildly acidic solutions. Ascorbic acid is one form of vitamin C. The name is derived from a- and scorbutus , the...
(vitamin C
Vitamin C
Vitamin C or L-ascorbic acid or L-ascorbate is an essential nutrient for humans and certain other animal species. In living organisms ascorbate acts as an antioxidant by protecting the body against oxidative stress...
) and acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
.
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
Taste receptor
A Taste receptor is a type of receptor which facilitates the sensation of taste.Examples include TAS2R16 and TAS2R38.They are divided into two families:* Type 1, sweet, first characterized in 2001: –...
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.
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, so it has potential for use in fresh foods and as a table-top sweetenerSugar substitute
A sugar substitute is a food additive that duplicates the effect of sugar in taste, usually with less food energy. Some sugar substitutes are natural and some are synthetic. Those that are not natural are, in general, called artificial sweeteners....
.
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
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
and yeast
Yeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...
, 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
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
, 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
Sugar substitute
A sugar substitute is a food additive that duplicates the effect of sugar in taste, usually with less food energy. Some sugar substitutes are natural and some are synthetic. Those that are not natural are, in general, called artificial sweeteners....
. Curculin currently has no legal status in European Union
European Union
The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...
and United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. However it is approved in Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
as a harmless additive
Food additive
Food additives are substances added to food to preserve flavor or enhance its taste and appearance.Some additives have been used for centuries; for example, preserving food by pickling , salting, as with bacon, preserving sweets or using sulfur dioxide as in some wines...
, according to the List of Existing Food Additives established by the Ministry of Health and Welfare (English publication by JETRO).