Monellin Chain B | |||||||
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X-ray crystal structure of an engineered single-chain monellin protein. [1] | |||||||
Identifiers | |||||||
Organism | |||||||
Symbol | MonB | ||||||
PDB | 2O9U | ||||||
UniProt | P02882 | ||||||
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Monellin Chain A | |||||||
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Identifiers | |||||||
Organism | |||||||
Symbol | MonA | ||||||
PDB | 1IV7 | ||||||
UniProt | P02881 | ||||||
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Monellin | |||||||||
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Identifiers | |||||||||
Symbol | Monellin | ||||||||
Pfam | PF09200 | ||||||||
InterPro | IPR015283 | ||||||||
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Monellin, a sweet protein, was discovered in 1969 in the fruit of the West African shrub known as serendipity berry ( Dioscoreophyllum cumminsii ); it was first reported as a carbohydrate. [2] The protein was named in 1972 after the Monell Chemical Senses Center in Philadelphia, U.S.A., where it was isolated and characterized. [3]
Monellin's molecular weight is 10.7 kDa. It has two noncovalently associated polypeptide chains: an A chain sequence with 44 amino acid residues, and a B chain with 50 residues. [3] [4]
Monellin chain A (44 AA):
REIKGYEYQL YVYASDKLFR ADISEDYKTR GRKLLRFNGP VPPP
Monellin chain B (50 AA):
GEWEIIDIGP FTQNLGKFAV DEENKIGQYG RLTFNKVIRP CMKKTIYEENAmino acid sequence of the sweet protein monellin adapted from Swiss-Prot biological database of protein. [5] [6]
Monellin has a secondary structure consisting of five beta-strands that form an antiparallel beta-sheet and a 17-residue alpha-helix. [1]
In its natural form, monellin is composed of the two chains shown above ( PDB: 3MON ), but the protein is unstable at high temperatures or at extremes of pH. [1] To enhance its stability, single-chain monellin proteins were created in which the two natural chains are joined via a Gly-Phe dipeptide linker. [1] This modified version of the protein (MNEI) has been studied using NMR and X-ray diffraction.
In addition to its secondary structure, four stably bound sulfate ions were located on the monellin protein, three on the concave face of the protein and one on the convex face of the protein. [1] The sulfate ion on the convex face of the protein is of particular interest because it lies adjacent to a patch of positive surface potential, which may be important in electrostatic interactions with the negative T1R2-T1R3 sweet taste protein receptor. [1]
Monellin is perceived as sweet by humans and some Old World primates, but is not preferred by other mammals. [1] The relative sweetness of monellin varies from 800 to 2000 times sweeter than sucrose, depending on the sweet reference against which it is assessed. It is reported to be 1500-2000 times sweeter than a 7% sucrose solution on a weight basis [7] [8] [9] and 800 times sweeter than sucrose when compared with a 5% sucrose solution on a weight basis. [10]
Monellin has a slow onset of sweetness and lingering aftertaste. Like miraculin, monellin's sweetness is pH-dependent; the protein is tasteless below pH 2 and above pH 9. Blending the sweet protein with bulk and/or intense sweeteners reduces the persistent sweetness and shows a synergistic sweet effect. [11]
Heat over 50 °C at low pH denatures monellin proteins, causing a loss of the sweetness. [11]
So far, five high-intensity sweet proteins have been reported: monellin (1969), thaumatin (1972), pentadin (1989), mabinlin (1983) and brazzein (1994). [12]
Monellin can be useful for sweetening some foods and drinks, as it is a protein readily soluble in water due to its hydrophilic properties. However, it may have limited application because it denatures under high temperature conditions, which makes it unsuitable for processed food. It may be relevant as noncarbohydrate tabletop sweetener, especially for individuals such as diabetics who must control their sugar intake. [1]
In addition, monellin is costly to extract from the fruit and the plant is difficult to grow. Alternative production such as chemical synthesis and expression in micro-organisms are being investigated. For instance, monellin has been expressed successfully in yeast ( Candida utilis ) [13] and synthesised by solid-phase method. [14] The synthetic monellin produce by yeast was found to be 4000 times sweeter than sucrose when compared to 0.6% sugar solution.
Legal issues are the main barrier in its widespread use as a sweetener, as monellin has no legal status in the European Union or the United States. However, it is approved in Japan as a harmless additive, according to the List of Existing Food Additives issued by the Ministry of Health and Welfare (published in English by JETRO).
A sugar substitute is a food additive that provides a sweet taste like that of sugar while containing significantly less food energy than sugar-based sweeteners, making it a zero-calorie (non-nutritive) or low-calorie sweetener. Artificial sweeteners may be derived through manufacturing of plant extracts or processed by chemical synthesis. Sugar alcohols such as erythritol, xylitol, and sorbitol are derived from sugars. In 2017, sucralose was the most common sugar substitute used in the manufacture of foods and beverages; it had 30% of the global market, which was projected to be valued at $2.8 billion by 2021.
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Acesulfame potassium, also known as acesulfame K or Ace K, is a calorie-free sugar substitute often marketed under the trade names Sunett and Sweet One. In the European Union, it is known under the E number E950. It was discovered accidentally in 1967 by German chemist Karl Clauss at Hoechst AG. In chemical structure, acesulfame potassium is the potassium salt of 6-methyl-1,2,3-oxathiazine-4(3H)-one 2,2-dioxide. It is a white crystalline powder with molecular formula C
4H
4KNO
4S and a molecular weight of 201.24 g/mol.
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Sugar alcohols are organic compounds, typically derived from sugars, containing one hydroxyl group (–OH) attached to each carbon atom. They are white, water-soluble solids that can occur naturally or be produced industrially by hydrogenation of sugars. Since they contain multiple –OH groups, they are classified as polyols.
UniProt is a freely accessible database of protein sequence and functional information, many entries being derived from genome sequencing projects. It contains a large amount of information about the biological function of proteins derived from the research literature. It is maintained by the UniProt consortium, which consists of several European bioinformatics organisations and a foundation from Washington, DC, United States.
Miraculin is a taste modifier, a glycoprotein extracted from the fruit of Synsepalum dulcificum. The berry, also known as the miracle fruit, was documented by explorer Chevalier des Marchais, who searched for many different fruits during a 1725 excursion to its native West Africa.
Lactisole is a carboxylic acid salt. Like gymnemic acid, it is a sweet inhibitor or taste modifier.
Sweetness is a basic taste most commonly perceived when eating foods rich in sugars. Sweet tastes are generally regarded as pleasurable, except when in excess. In addition to sugars like sucrose, many other chemical compounds are sweet, including aldehydes, ketones, and sugar alcohols. Some are sweet at very low concentrations, allowing their use as non-caloric sugar substitutes. Such non-sugar sweeteners include saccharin and aspartame. Other compounds, such as miraculin, may alter perception of sweetness itself.
Brazzein is a sweet-tasting protein extracted from the West African fruit of the climbing plant Oubli. It was first isolated by the University of Wisconsin–Madison in 1994.
Perillartine, also known as perillartin and perilla sugar, is a sweetener that is about 2000 times as sweet as sucrose. It is mainly used in Japan. Perillartine is the oxime of perillaldehyde, which is found in Perilla frutescens (Lamiaceae).
Maltisorb is a natural sweetening alcohol made by Roquette of France. It can be used in baking, especially in making fillings for cakes and cookies. It is available in a number of grades, labelled M59, M200 and M40. Maltisorb is a brand name for crystalline maltitol, the formula for which Roquette licenses from Towa Chemical Industry of Japan.
Curculin or neoculin 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' or 'Lemba'.
Gymnemic acids are a class of chemical compounds isolated from the leaves of Gymnema sylvestre (Asclepiadaceae). They are anti-sweet compounds, or sweetness inhibitors. After chewing the leaves, solutions sweetened with sugar taste like water.
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Mabinlins are sweet-tasting proteins extracted from the seed of mabinlang, a Chinese plant growing in Yunnan province. There are four homologues. Mabinlin-2 was first isolated in 1983 and characterised in 1993, and is the most extensively studied of the four. The other variants of mabinlin-1, -3 and -4 were discovered and characterised in 1994.
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