Sweet proteins have the advantages of high sweetness, low calories, non-toxicity and safety, and are expected to become a new type of health-care sweetener.
There are currently 8 types of plant sweet proteins discovered, thaumatin, monellin, mabinlin, brazzein, pentadin, miraculin, curculin and neoculin. To be precise, 5 of them are sweet proteins (thaumatin, monellin, mabinlin, brazzein, pentadin), which have sweetness themselves; two are sweetness-inducing proteins (miraculin, neoculi), which have taste regulation functions and can turn other tastes such as sourness into sweetness. Curculin has the sweetness characteristics of the first two types of proteins.
Thaumatin is extracted from Thaumatin Daniellii Benth, a perennial shrub of the Bambusaceae family in the tropical rainforests of West Africa. Thaumatin is 3,000 times sweeter than sucrose of the same mass and is considered to be the most promising sucrose substitute. Thaumatin is a light yellow-brown to gray-brown powder, odorless, extremely sweet, and has a sweetness threshold of 1.1 mg/kg. When it is below the sweetness threshold of 1.1 mg/kg, it can enhance the flavor of food. For example, adding 0.5 mg/kg of thaumatin can reduce the mint flavor threshold by 1/10-1/3.
Naturally occurring thaumatin is encoded by a multi-gene family, with at least 6-7 closely related proteins, mainly thaumatin I and thaumatin II. thaumatin I accounts for about 96% of the extract, and they are all composed of a straight polypeptide chain, containing 207 amino acid residues and a molecular weight of 22 kD. Thaumatin is an alkaline protein with an isoelectric point of 11.5-12.5. It can denature and lose its sweetness when heated. When heated at 80-100°C, the sweetness can decrease by more than 50%. Short-term heating above 100°C has little effect on its sweetness, but it can combine with tannins and lose its sweetness.
Crystal structure studies have shown that thaumatin consists of three domains. The core domain is a smooth β-folded barrel containing 11 β-folded sheets. Except for the N-terminal and C-terminal sheets, which are parallel to each other, the other sheets are anti-parallel. These known thaumatin homologues have only 5 or fewer amino acid differences, and the N-terminus is alanine.
Thaumatin is sweet and refreshing, has no bad aftertaste or bitterness, and has the function of enhancing product flavor and masking bad taste. It is a very promising natural food sweetener. Thaumatin has been approved for use in food, medicine and cosmetics in the United States, the United Kingdom, Japan, Germany and other countries.
Monellin is isolated from the West African plant Dioscoreophy Ilum cummitasii. The monellin molecule consists of two chains, A and B, with a total of 94 amino acid residues. The two chains are bound together by non-covalent interactions, of which the A chain contains 44 amino acids and the B chain contains 50 amino acids. After the A and B peptide chains are separated, its sweetness disappears. Its relative molecular weight is 10.7 kD, its isoelectric point is 9.0-9.3, and its sweetness is 3000 times that of sucrose of the same mass. The sweetness comes slowly and goes slowly, lasts for a long time, and the taste is lingering. Temperature and pH have a great influence on it. When its aqueous solution is heated to 55-60°C, the sweetness will be lost. When the pH is less than 2 or greater than 9 at room temperature, the sweetness will also be lost.
The factors that affect the stability of monellin include covalent bond forces and non-covalent bond forces. The charge and amino acid residues on the surface of the protein will affect its sweetness, especially Ca2+, which may be related to the Ca2+-mediated cation channels in taste cells.
Mabinlin is a seed storage protein of Capparis Srnasaikai Levi, a plant of the Cappafidaceae family in high altitude areas of China. Mabinlin can cause lasting sweetness, with a relative molecular mass of 11700, an isoelectric point of 11.8, and a maximum absorption peak at a wavelength of 280 nm. The lowest concentration that causes sweetness is 0.1%, and its sweetness is 100 times that of sucrose of the same mass. Mabinlin consists of an A chain of 3 amino acids and a B chain of 72 amino acids. Its thermal stability is related to the amino acid at position 47 of the B chain. If it is arginine, it is thermally stable, and if it is glutamic acid, it is thermally unstable.
After further separation and purification, it was found that mabinlin has five isoproteins, namely mabinlin I, I-1, II, III and IV. Among them, mabinlin II has the highest thermal stability, which is also the highest among all known sweet proteins. Its sweetness can be maintained at 80°C for 48 h, while the other four lose their sweetness after being maintained at 80°C for 0.5 h. Mabinlin II has a high development value.
Brazzein is isolated and purified from the fruit of Pentadiplandra Brazzeana Baillon, a wild plant in western Africa. Brazzein is a single-chain polypeptide composed of 54 amino acid residues, containing 8 cysteines, forming 4 pairs of intramolecular disulfide bonds. The relative molecular mass of brazzein is 6500, the isoelectric point is 5, and the sweetness is 2000 times that of sucrose of the same mass. Compared with other sweet proteins, brazzein has the smallest molecular weight and the best water solubility. Its aqueous solution still maintains sweetness after heat treatment at 80°C for 4 h, and has good thermal stability and pH stability.
Pentadin is also isolated from the fruit of Pentadiplandra Brazzeana Baillon. Pentadin is extracted from the fruit after heat drying, and Brazzein is extracted from fresh fruit. Pentadin has a molecular weight of about 12 kDa, and its sweetness is 500 times that of sucrose. The entire molecule is connected by substructures through disulfide bonds. Pentadin is a non-natural cross-linked brazzein molecule with a molecular weight of about twice that of brazzein and significantly lower sweetness than brazzein.
Miraculin can be extracted from the red berries of the West African plant Richadelladulcifa Baehni. Miraculin has a relative molecular weight of about 24.6 kDa and is a single polypeptide chain composed of 191 amino acids and N-linked oligosaccharides. Its isoelectric point is about 9. Miraculin is relatively stable below 100 °C and in the pH range of 3-12. It has no sweetness itself, but it can change people's taste when it encounters acidic substances.
Miraculin, as a natural non-sugar sweetener, is very likely to become a substitute for sugar compounds and a new type of health food sweetener. Based on the special property of miraculin that can change sourness to sweetness, it may be an ideal sweetener in the food of diabetic and obese patients.
Curculin exists in the Malaysian plant Curculingo latifolia. It is 550 times sweeter than sucrose. Its molecular weight is 24.9 kDa and it is composed of 114 amino acids. Curculin is a homodimer composed of two basic subunits. It is unstable and its activity decreases at temperatures above 50°C. Curculin can also make sour substances sweet. The sweetness can last for a few minutes in the mouth. If you drink water after the sweetness disappears in the mouth, it will recover and last for about 5 minutes. In addition to pure water and lemon, 0.5 mol/L sodium chloride aqueous solution can also induce its sweetness. It can be seen that curculin is also a sweet protein that changes taste. Ca2+ and Mg2+ may have an inhibitory effect on its sweetness, but whether in neutral solution or in acidic solution, monovalent ions such as Na+ and Cl+ will not affect its sweetness.
Neoculin is a sweet protein homologous to curculin and has a taste-modifying effect. Its sweetness is about 500 times that of sucrose of the same mass. Neoculin is a heterodimeric glycoprotein composed of an N-terminal glycosylated acidic subunit NAS and a basic unit NBS. The one-dimensional and three-dimensional structures of neoculin are similar to the mannose-binding lectins in monocots.
Sweet protein has many advantages and a wide range of applications, mainly including:
Although sweet protein has many advantages, its high cost, inconsistent quality standards, and chaotic product market order have greatly restricted the in-depth development and wide application of sweet protein. With the continuous development of biotechnology, the technical difficulties in its industrial production and application will no longer be the main problem, and the quality and safety of the product and the psychological barriers to consumption will also be solved with the upgrading of the product. Therefore, sweet protein has broad prospects in the future competition in the food additive market.
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