{{Use dmy dates|date=October 2019}} {{distinguish|Curcumin}} {{short description|Sweet protein with taste-modifying activity}} {{Infobox nonhuman protein | Name = Curculin-1 | caption = A curculin homodimer. From {{PDB|2DPF}}. | image = Curculin 2DPF.png | width = | HGNCid = | Symbol = CURC_CURLA | AltSymbols = | EntrezGene = | OMIM = | RefSeq = | UniProt = P19667 | PDB = 2DPF | PDB_supplemental = | ECnumber = | Chromosome = | Arm = | Band = | LocusSupplementaryData = | Organism = ''Molineria latifolia'' }} {{Infobox nonhuman protein | Name = Curculin-2 | caption = | image = | width = | Symbol = CURC2_CURLA | AltSymbols = | EntrezGene = | OMIM = | RefSeq = | UniProt = Q6F495 | PDB = 2D04 | PDB_supplemental = | ECnumber = | Chromosome = | Arm = | Band = | LocusSupplementaryData = | Organism = ''Molineria latifolia'' }} '''Curculin''' or '''neoculin''' is a sweet protein complex that was discovered and isolated in 1990 from the fruit of ''Curculigo latifolia'' (Hypoxidaceae).<ref name = jbc-265-15770>{{cite journal | vauthors = Yamashita H, Theerasilp S, Aiuchi T, Nakaya K, Nakamura Y, Kurihara Y |display-authors=3 | title = Purification and complete amino acid sequence of a new type of sweet protein taste-modifying activity, curculin | journal = The Journal of Biological Chemistry | volume = 265 | issue = 26 | pages = 15770–5 | date = September 1990 |doi=10.1016/S0021-9258(18)55464-8 | pmid = 2394746 |doi-access=free }}</ref> 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.
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.<ref>{{cite journal |last1=Okubo |first1=S |last2=Terauchi |first2=K |last3=Okada |first3=S |last4=Saito |first4=Y |last5=Yamaura |first5=T |last6=Misaka |first6=T |last7=Nakajima |first7=KI |last8=Abe |first8=K |last9=Asakura |first9=T |title=De novo transcriptome analysis and comparative expression profiling of genes associated with the taste-modifying protein neoculin in Curculigo latifolia and Curculigo capitulata fruits. |journal=BMC Genomics |date=13 May 2021 |volume=22 |issue=1 |pages=347 |doi=10.1186/s12864-021-07674-3 |doi-access=free |pmid=33985426|pmc=8120819 }} – ''Background'' section cites some more recent work on elucidating the mechanism of sweetness and taste modification.</ref>
== 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. The structure is typical for a leptin.<ref name="Suzuki2004">{{cite journal | vauthors = Suzuki M, Kurimoto E, Nirasawa S, Masuda Y, Hori K, Kurihara Y, Shimba N, Kawai M, Suzuki E, Kato K |display-authors=3 | title = Recombinant curculin heterodimer exhibits taste-modifying and sweet-tasting activities | journal = FEBS Letters | volume = 573 | issue = 1–3 | pages = 135–8 | date = August 2004 | pmid = 15327988 | doi = 10.1016/j.febslet.2004.07.073 | doi-access = |bibcode=2004FEBSL.573..135S }}</ref> To avoid confusion, the heterodimeric form is sometimes referred to as "neoculin".
{|class=wikitable |- |1, 1-50 || {{mono|DNVLLSGQTL HADHSLQAGA YTLTIQNK'''''C'''''N LVKYQNGRQI WASNTDRRGS}} |- |1, 51-100 || {{mono|G'''''C'''''RLTLLSDG NLVIYDHNNN DVWGSA<u>C</u>WGD NGKYALVLQK DGRFVIYGPV}} |- |1, 101-114 || {{mono|LWSLGPNG<u>C</u>R RVNG}} |- |2, 1-50 || {{mono|DSVLLSGQTL YAGHSLTSGS YTLTIQNN'''''C'''''N LVKYQHGRQI WASDTDGQGS}} |- |2, 51-100 || {{mono|Q'''''C'''''RLTLRSDG NLIIYDDNNM VVWGSD<u>C</u>WGN NGTYALVLQQ DGLFVIYGPV}} |- |2, 101-113 || {{mono|LWPLGLNG<u>C</u>R SLN}} |} 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 italics, inter-chain disulfide bonds underlined.<ref name="swissprot">{{UniProt Full|P19667|Curculin-1}} {{UniProt Full|Q6F495|Curculin-2}}</ref>
==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.<ref name="jbc-265-15770" /><ref>{{cite journal | vauthors = Kurihara Y | title = Characteristics of antisweet substances, sweet proteins, and sweetness-inducing proteins | journal = Critical Reviews in Food Science and Nutrition | volume = 32 | issue = 3 | pages = 231–52 | year = 1992 | pmid = 1418601 | doi = 10.1080/10408399209527598 }}</ref><ref name = cc-20-239>{{cite journal | vauthors = Yamashita H, Akabane T, Kurihara Y | title = Activity and stability of a new sweet protein with taste-modifying action, curculin | journal = Chemical Senses | volume = 20 | issue = 2 | pages = 239–43 | date = April 1995 | pmid = 7583017 | doi = 10.1093/chemse/20.2.239 }}</ref>
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.<ref name="jbc-265-15770" />
The taste-modifying activity of curculin is reduced in the presence of ions with two positive charges (such as Ca<sup>2+</sup> and Mg<sup>2+</sup>) in neutral pH solutions, although these ions have no effect in acidic solutions. In the same way, monovalent ions (such as Na<sup>+</sup> and Cl<sup>−</sup>) have no effect in solutions with either neutral or acidic pH.<ref name="jbc-265-15770" /><ref name="cc-20-239" />
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 Ca<sup>2+</sup> and/or Mg<sup>2+</sup>, water and acids tune the binding of the active site of curculin to the receptor site and therefore modify perceived sweetness.<ref name="cc-20-239" /> Curculin appears to use a unique binding site at the amino terminal of TAS1R3.<ref>{{cite journal |name-list-style=vanc |last1=Koizumi |first1=A |last2=Nakajima |first2=K |last3=Asakura |first3=T |last4=Morita |first4=Y |last5=Ito |first5=K |last6=Shmizu-Ibuka |first6=A |last7=Misaka |first7=T |last8=Abe |first8=K |display-authors=3 |title=Taste-modifying sweet protein, neoculin, is received at human T1R3 amino terminal domain. |journal=Biochemical and Biophysical Research Communications |date=29 June 2007 |volume=358 |issue=2 |pages=585–9 |doi=10.1016/j.bbrc.2007.04.171 |pmid=17499612}}</ref>
==As a sweetener== Like most proteins, curculin is susceptible to heat. At a temperature of {{convert|50|C|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,<ref name="cc-20-239" /> 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.<ref name="ffijoj-67">{{cite journal | vauthors = Kurihara Y, Nirasawa S | title = Structures and activities of sweetness-inducing substances (miraculin, curculin, strogin) and the heat-stable sweet protein, mabinlin. | journal = Foods and Food Ingredients Journal of Japan | year = 1997 | pages = 67–74 }}</ref> 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.<ref name="Suzuki2004" />
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 == {{Reflist}}
== External links == *{{Commons category-inline}}
Category:Plant proteins Category:Taste modifiers Category:Sugar substitutes Category:Curculigo