Monellin sweetness

Some proteins display rather exotic functions that do not quite fit the previous classifications. Monellin, a protein found in an African plant, has a very sweet taste and is being considered as an artificial sweetener for human consumption. Resilin, a protein having exceptional elastic properties, is found in... 

That the initial event of taste stimulation takes place on the cell surface of the taste receptor is now universally accepted. In addition, accumulated evidence strongly suggests that taste-bud stimulation is extracellular in nature. For example, (1) the sweet-taste response is both rapid and reversible, (2) the intensely sweet proteins monellin" and thaumatin could not possibly penetrate the cell, because of their size, and (3) miraculin, the taste-modifying glycoprotein, having a molecular weight of 44,000 would also be too large to penetrate the taste cell. ... 

In a study of the three-dimensional structure of thaumatin, it was reported that, not only do antibodies raised against thaumatin cross-react with monellin,but antibodies raised against monellin also cross-react with thaumatin, suggesting that there is some structural similarity between portions of the two sweet-protein molecules. Earlier studies " had shown that there is a limited homology in the amino acid sequence in the two proteins. Five tripeptides in monellin have their counterparts in thaumatin. 

The design of safe sweeteners is very important for people who are afiected by diabetes, hyperlipemia, caries and other diseases that are linked to sugar consumption. Sweet proteins, which are found in several tropical plants, are many times (100-100,000) sweeter than sucrose on a molar basis. Only a few sweet proteins are known miraculin, monellin, thaumatin, curculin, mabinlin. 

Native monellin consists of two polypeptide chains, a 45-residue A-chain and a 50-residue B-chain, linked by non-covalent interactions. At neutral pH, it is fairly resistant to heat denaturation with a higher than 80 °C. The crystal structure of native monellin shows a tertiary structure comprising an anti-parallel /1-sheet with five strands and an a-helix. H NMR spectroscopy and hydrogen exchange methods have been used to characterize the alcohol-denaturated state of monellin in order to understand how its secondary structure depends on environmental conditions. " Structural and dynamic studies by NMR have been carried out in order to compare native monellin and a non-sweet analogue in which Asp was replaced by Abu . The three-dimensional structures of the two proteins are found to be very... 

Fig. 8. Ribbon diagram of the restraint energy-minimized average structure of singlechain monellin displaying ordered secondary structure elements and relative orientation of secondary structures. The side chain atoms for residues thought to be responsible for sweetness and receptor binding are also displayed. Reprinted with permission from Biochemistry, Vol. 38, S. Y. Lee, J. H. Lee, H. J. Chang, J. M. Cho, J. W. Jung and W. Lee, 1999, p. 2340. Copyright (1999) American Chemical Society.

Brazzein is another small sweet-tasting protein whose solution structure has been recently solved by NMR. Brazzein tastes 2000 times sweeter than sucrose on a weight basis and is exceptionally thermostable. As indicated by NMR, the structure of this 54 residue, single-chain polypeptide does not change between 32 and 82 °C and retains its sweetness after incubation at 98 °C for two hours.Brazzein contains one a-helix and three strands of antiparallel jd-sheet stabilized by four intramolecular disulphide bonds. It has been proposed that the disulphide bonds could be responsible for the thermostability of brazzein by forming a compact structure at the tertiary level.The structure of brazzein does not resemble that of the other two sweet proteins with known structures, monellin and thaumatin, whereas sequence alignment and structural prediction indicate that brazzein shares the fold of a newly identified family of serine proteinase inhibitors. 

There are seven known sweet and taste-modifying proteins, namely (1) monellin and (2) thaumatin (3) mabinlin. and (4) curculin (5) pentadin, (6) brazzein and (7) miraculin.The properties and characteristics of these proteins are illustrated in Table 2. There are several recent reviews relating to sweet proteins. Apart from curculin and... 

Monellin can be purified from the fruit of Dioscoreophyllum cumminsii grown in West Africa and is approximately 100 000 times sweeter than sugar on a molar basis and several thousand times sweeter on a weight basis.Single-chain monellin (SCM), which is an engineered 94-residue polypeptide, has proven to be as the sweet as native two-chain monellin. It is more stable than the native monellin at high temperature and in acidic... 

The intensely sweet taste of monellin can be explained in a second way. The intense sweetness of monellin can be envisioned as being dependent upon the molecular size of monellin. The size of monellin is thereby large enough to span two or more individual receptors. In this way, amino acid residues 4-11 would enhance the sweet taste produced by another residue(s) further along the monellin molecule. These other residues that might impart a sweet taste could include amino acid residue 16-19 (asp-lys-leu-phe) or 33-37 (lys-leu-leu-arg-phe) on subunit 1 of monellin. Experiments examining the taste of monellin both with and without residue 4-11 should confirm or deny the above hypothesis. 

Figure 30-31 Structures of some very sweet compounds. The backbone structure of the protein thaumatin I is included. The main body of this structure consists of two (3 sheets forming a flattened (3 barrel. (3 Strands in the top sheet are shaded light, and those in the bottom sheet are darker. Open bars represent disulfide bonds, and the regions with sequences homologous to monellin are indicated by the hatched marks. From de Vos et al.9i0...

The occurrence of sweet-tasting proteins, such as thaumatin, monellin, mabinlin and pentadin, in the pulp of fruits of various rain forest species has been reported. The sweet-tasting proteins have different molecular lengths (from 54 residues of brazzein to 207 residues of thaumatin), virtually no sequence homology and very little structural homology. Thaumatin, the most characterised sweet protein, is 100,000 times sweeter than sugar on a molar... 

Monellin is a sweet protein that was isolated from the fruit of Dioscoreophyllum cumminsii (Staff Diels, which is known as the serendipity berry and is native to West Africa. It is a basic protein with an isoelectric point of approximately 9.3 and is 3000 times sweeter than sucrose.65 66 Perception lasts for more than 1 h and leaves an aftertaste. Heat denatures monellin proteins they lose their sweetness when heated over 50 °C at low pH. Monellin has a molecular mass of 10.7 kDa. Monellin has two noncovalently associated polypeptide chains chain A contains 44 amino acid residues and chain B has 50 residues. In 1976, the primary structure of monellin was proposed independently by three groups but their results all differed somewhat.67-69... 

Brazzein is a sweet protein that was isolated from the fruit of the West African climbing plant Oubli (Pentadiplandra brazzeana Baillon). Along with pentadin, which was discovered in 1989, brazzein is the second sweet protein that was discovered in this fruit. Like other natural sweet proteins such as monellin and thaumatin, it is highly sweet. On a weight basis, brazzein is 500 times sweeter than sucrose when compared to 10% sucrose solution and 2000 times sweeter when compared to 2% sucrose solution. Its sweet perception is more similar to that of sucrose than that of thaumatin, and it presents a clean sweet taste with a lingering aftertaste. Brazzein is stable over a broad pH range from 2.5 to 8 and is heat stable at 80 °C for 4h.84... 

Monellin Dioscoreophyllum cumminsii (serendipity Sweet protein... 

Single-chain Monellin (= MNEI Monellin B-Gly-Phe-Monellin A)] (94 aa, lOkDa protein) Synthetic Sweet ( = Monellin)... 

Additional evidence that the exterior surface of the taste receptor cell plasma membrane is the location of the sweet receptor is provided by the action of the chemostimulatory proteins, monellin and thaumatin. Monellin occurs in the fruit of the African serendipity berry (Dioscoreophyllum cumminsii), and thaumatin is found in the fruit of Thaumatococcus daniellii, also... 

The effects of miraculin, monellin and thaumatin, taken together, provide evidence that the sweetness receptor site is located at the surface of the taste receptor cell, in or in close proximity to, the plasma membrane. 

Sodium dodecyl sulfate, the gymnemic acids and the ziziphins have all been termed "surface active" taste modifiers because they all possess detergent-like properties. These molecules all have a polar and a non-polar end and they are capable of penetrating the phospholipid membranes that are believed to be components of sweetness receptors. Any speculation about the mechanism of action of these substances must take into account the experimental observations concerning miraculin, monellin, and thaumatin, which were presented at the beginning of this article. Those observations suggested that transport of the modifier to the cell s interior was not occurring and the inhibition effect is manifested at the surface of the cell. 

Thaumatin (10) and monellin (IJ,) are sweet and are proteins isolated from a plant native to Western Africa. Thaumatin and monellin are respectively 1600 and 3000 times sweeter than sucrose. As both proteins are basic, they are assumed to bind easily to the negatively charged taste cell. 

There have been no reports regarding the detection of sweet peptides in naturally occurring foodstuffs other than thaumatin and monellin. 

Interestingly, the human TAS1R2/TAS1R3, but not its mouse counterpart, are sensitive to the sweet proteins monellin, thaumatin, and brazzein, and to the artificial sweeteners neo-tame, cyclamate, and aspartame (9-11). This difference provides a molecular explanation for the previous observation that these compounds are sweet for humans but not attractive to rodents (9). The species difference also applies to the inhibitor lactisole that blocks the sweet taste in humans but not in rats, and only inhibits the response of human TAS1R2/TAS1R3 to sweet stimuli (9). 

Several plant-derived proteins have been reported previously as sweeteners, inclusive of curculin [103], mabinlin [104,105], monellin [28,106], pentadin [107], and thaumatin, with the latter compound already mentioned as having commercial use as a sweetener and flavor enhancer [22]. Recently, a sixth sweet protein of plant origin, brazzein, was isolated from the fruits of an African climbing vine, Pentadiplandra... 

Do these diverse compounds give rise to a common perception of sweetness or to qualitatively different sensations Sweetness does indeed appear to be a unitary percept (Breslin et al. 1994,1996). However, some sweeteners may be discriminable on the basis of their activation of other sensory transduction mechanisms or differences in the temporal properties of their sensory action. For example, the sweetener sodium saccharin activates bitter receptors in some people (Kuhn et al. 2004 Pronin et al. 2007), and also inhibits sweet taste at high concentrations (Galindo-Cuspinera et al. 2006). Sweet proteins such as thaumatin and monellin can have a slow onset or evoke a prolonged sweetness compared with sugars (Faus 2000), likely owing to a relatively high affinity for the sweet taste receptor. 

---