Thaumatin

Thaumatin family
Cartoon diagram of thaumatin I. From PDB 1RQW.
Identifiers
Symbol	Thaumatin
Pfam	PF00314
InterPro	IPR001938
SMART	SM00205
PROSITE	PDOC00286
SCOP	1thu
OPM family	189
OPM protein	1aun
CDD	cd09215
Available protein structures: Pfam structures PDB RCSB PDB; PDBe PDBsum structure summary

Thaumatin is a low-calorie sweetener and flavour modifier. The substance, a natural protein, is often used primarily for its flavour-modifying properties and not exclusively as a sweetener.^[1]

The thaumatins were first found as a mixture of proteins isolated from the katemfe fruit (Thaumatococcus daniellii Bennett) of west Africa. Some of the proteins in the thaumatin family are natural sweeteners roughly 2000 times more potent than sugar. Although very sweet, thaumatin's taste is markedly different from sugar's. The sweetness of thaumatin builds very slowly. Perception lasts a long time, leaving a liquorice-like aftertaste at high usage levels. Thaumatin is highly water soluble, stable to heating, and stable under acidic conditions.

Biological role

Thaumatin production is induced in katemfe in response to an attack upon the plant by viroid pathogens. Several members of the thaumatin protein family display significant in vitro inhibition of hyphal growth and sporulation by various fungi. The thaumatin protein is considered a prototype for a pathogen-response protein domain. This thaumatin domain has been found in species as diverse as rice and Caenorhabditis elegans. Thaumatins are pathogenesis-related (PR) proteins, which are induced by various agents ranging from ethylene to pathogens, and are structurally diverse and ubiquitous in plants:^[2] They include thaumatin, osmotin, tobacco major and minor PR proteins, alpha-amylase/trypsin inhibitor, and P21 and PWIR2 soybean and wheat leaf proteins. The proteins are involved in systematically acquired resistance and stress response in plants, although their precise role is unknown.^[2] Thaumatin is an intensely sweet-tasting protein (on a molar basis about 100,000 times as sweet as sucrose^[3]) found in the West African shrub Thaumatococcus daniellii: it is induced by attack by viroids, which are single-stranded unencapsulated RNA molecules that do not code for protein. The thaumatin protein I consists of a single polypeptide chain of 207 residues.

Like other PR proteins, thaumatin is predicted to have a mainly beta structure, with a high content of beta-turns and little helix.^[2] Tobacco cells exposed to gradually increased salt concentrations develop a greatly increased tolerance to salt, due to the expression of osmotin,^[4] a member of the PR protein family. Wheat plants attacked by barley powdery mildew express a PR protein (PWIR2), which results in resistance against that infection.^[5] The similarity between this PR protein and other PR proteins to the maize alpha-amylase/trypsin inhibitor has suggested that PR proteins may act as some form of inhibitor^[5]

Thaumatin-like protein has been found in olive fruit, and it can cause allergic reactions. A worker in an olive-oil mill in Jaen, Spain had respiratory symptoms at work. His blood serum was used to extract a 23-kilodalton protein from olive pulp, which was purified and confirmed to cause an allergic reaction in a skin prick test. The protein had amino acid sequence ATFXIVNQXTYTVXAAASP, which is similar (homologous) to that of thaumatin-like proteins.^[6]

Production

Within west Africa, the katemfe fruit has been locally cultivated and used to flavor foods and beverages for some time. The fruit's seeds are encased in a membranous sac, or aril, that is the source of thaumatin. In the 1970s, Tate and Lyle began extracting thaumatin from the fruit. In 1990, researchers at Unilever reported the isolation and sequencing of the two principal proteins found in thaumatin, which they dubbed thaumatin I and thaumatin II. These researchers were also able to express thaumatin in genetically engineered bacteria.

Thaumatin has been approved as a sweetener in the European Union (E957), Israel, and Japan. In the United States, it is a Generally Recognized as Safe flavoring agent (FEMA GRAS 3732).

See also

• Curculin
• Miraculin
• Monellin
• Stevia

References

Further reading