What is Thaumatin? Since the 1970s, significant progress has been made in the search for new sweeteners. To date, seven sweet-tasting proteins have been discovered: thaumatin, monellin, miraculin, pentadin, curcumin, mabinlin, and brazzein1. Thaumatin has gained widespread attention due to its high sweetness, low-calorie content, non-toxicity, and inability to cause tooth decay or obesity. Over the past thirty years, researchers have conducted in-depth studies on thaumatin’s biochemical properties, sweetness mechanisms, physiological functions, and genetic engineering, achieving remarkable results1. Thaumatin has also been developed into a commercial product, sold in markets in Europe, America, and Japan. In recent years, agricultural scientists have attempted to introduce sweet protein genes into crops such as potatoes, corn, cucumbers, and tomatoes to improve their taste1.
Discovery and Biochemical Characteristics of Thaumatin
Thaumatin is extracted from the fruit of a plant called Thaumatococcus daniellii Benth, which belongs to the Marantaceae family. In 1855, British doctor Daniell first described T. daniellii[1]. In 1972, van der Wel and colleagues isolated thaumatin I, II, and III from the aril of T. daniellii[2]. Subsequently, thaumatin b and c were also isolated from the fruit[3]. In 1988, Jar-how Lee and others isolated thaumatin A and B from T. daniellii leaves[4]. These proteins are encoded by a multigene family and share some similar properties, such as being a single polypeptide chain, having a molecular weight of approximately 22 kDa, and consisting of 207 amino acids. The proteins are alkaline, with an isoelectric point of 12.0. Among these proteins, thaumatin I and II are the most abundant, each accounting for more than 20% of the aril’s dry weight[1].
Thaumatin’s Physiological Functions
Thaumatin’s physiological functions are not yet fully understood, but some phenomena can be used to deduce its possible functions. Thaumatin is first translated in the body into a precursor protein (preprothaumatin) with extra peptides at the N-terminus and C-terminus. After post-translational processing, the extra peptides at the N-terminus and C-terminus are removed, forming mature thaumatin[1]. Thaumatin I contains eight disulfide bonds (9~204, 56~66, 71~77, 121~193, 126~177, 134~145, 149~158, 159~164). The presence of so many disulfide bonds in a relatively small protein is crucial for maintaining and restoring the protein’s higher-order structure, allowing thaumatin to retain its sweetness even after being boiled in water for an hour and then cooled.
Thaumatin as a Commercial Product
Thaumatin has been developed into a commercial product due to its high sweetness, low calorie content, non-toxicity, and inability to cause tooth decay or obesity. It is sold in markets in Europe, America, and Japan[1]. In recent years, agricultural scientists have attempted to introduce sweet protein genes into crops such as potatoes, corn, cucumbers, and tomatoes to improve their taste[1].
Mechanism of Thaumatin’s Sweetness
Thaumatin is an ultra-sweet substance, with a sweetness 2000-3000 times greater than sucrose by weight and 105 times greater by molar concentration[1]. The sweetness threshold of thaumatin is 10-8mol/L. When the concentration of thaumatin is below this threshold, it can selectively enhance other flavors. For example, adding 5×10-5%w/v of Talin can reduce the threshold of mint flavor by 90% and beef-extract taste by 50%.
Unique Sweetness Perception of Thaumatin
Thaumatin’s sweetness perception is different from other sweeteners, as it can produce a persistent sweet sensation on most parts of the tongue, lasting for about 30 minutes[1]. This property may not be suitable for some people’s taste preferences. To address this issue, Blair and colleagues[1] used site-directed mutagenesis on the thaumatin synthesis gene to obtain thaumatin variants with shorter aftertaste durations, making them more suitable for a wider range of tastes.
Role of Lysine Residues in Thaumatin’s Sweetness
The Lysine (Lys) residues in thaumatin play a crucial role in its sweetness. This is because Lys residues that affect thaumatin’s sweetness are not found in non-sweet thaumatin-like proteins (TLPs), and chemical modifications of Lys residues can alter thaumatin’s sweetness[1]. Van der Wel and Bel found that acetylating Lys residues with acetic anhydride could reduce thaumatin’s sweetness. When three of the 11 Lys residues in thaumatin were acetylated, its sweetness disappeared completely. Similarly, modifying Lys residues with succinic anhydride or phosphopyridoxylating them significantly reduced thaumatin’s sweetness.
Implications for Thaumatin’s Sweetness Mechanism
Given that thaumatin’s sweetness threshold is comparable to the binding threshold of hormones and receptors (10-8~10-11mol/L), it has been speculated that the sweetness of thaumatin may be caused by the binding of specific groups on thaumatin to receptors in the oral cavity[1]. The importance of Lys residues in thaumatin’s sweetness, as well as the effects of chemical modifications on these residues, further supports this hypothesis.
Possible Physiological Functions of Thaumatin
Currently, little is known about the physiological functions of thaumatin, and its possible functions can only be inferred from certain observations. Thaumatin is first translated in the body as a precursor protein (preprothaumatin) with extra peptides at the N-terminus and C-terminus. After post-translational processing, the extra peptides at both termini are removed to form mature thaumatin[1]. Thaumatin is stored in vesicle-like organelles in the false seed coat, and the extra peptide at the C-terminus may serve as a signal peptide for entering these organelles, where it is then removed[1]. Only a few proteins, such as vicilin precursors and tobacco alkaline chitinase, have been found to have extra peptides at the C-terminus, suggesting a possible relationship with protein compartmentalization within cells[1].
Thaumatin Accumulation and Plant Development
Thaumatin gradually accumulates in the false seed coat during fruit maturation, reaching up to 50% of the dry weight[1]. This suggests that thaumatin may play a role in plant development[1]. In addition to thaumatin, many homologous proteins, known as the PR-5 protein family or thaumatin-like proteins (TLPs), have been discovered in crops such as barley, potatoes, tomatoes, wheat, and soybeans[1]. Although induced by different factors, TLPs are expressed when plants are under stress, enhancing their resistance to adverse conditions[1][2].
Thaumatin-like Proteins and Stress Resistance
Thaumatin-like proteins (TLPs) are part of the pathogenesis-related (PR) protein family and are involved in various biotic and abiotic stresses[3]. TLPs are named due to their significant similarity to thaumatin, a sweet-tasting protein first isolated from the African shrub Thaumatococcus daniellii[3]. TLPs from several plant species have shown in vitro anti-fungal activity, making them good candidates for enhancing plant resistance to diseases[1]. These proteins are induced by various agents, ranging from ethylene to pathogens, and are structurally diverse, playing a role in plant defense mechanisms[1].