Monellin

The pulp of Dioscoreophyllum cumminsii fruit contains monellin, a sweet protein with a molecular weight of ll.Skdal. It consists of two peptide chains, A and B, which are not covalently bound. Their amino acid sequences are shown in Table 8.4. 

---

Monel alloy Monel alloy 400 Monel alloy K-500 Monell Center Monellin [9062-83-3] Monensin... 

Exotic proteins Monellin Resilin Glue proteins... 

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.

An engineered single-chain monellin (SCM) constructed by fusing the two chains, A and B, covalently has been proved to retain all of the sweetening power and has a greater thermal stability.SCM was studied by NMR both in solution (Fig. and in the solid-state. Several SCM mutants were... 

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... 

Table 2. Comparison of Thaumatin, Monellin, Mabinlin, Pentadin, Brazzein, Curculin and Miraculin... 

Thaumatin Monellin Mabinlin Pentadin Brazzein Curculin Miraculin... 

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... 

Of course, the goal of every synthetic organic chemist is to obtain crystalline products, and a few cases of crystalline synthetic proteins have been reported. These include the ribonuclease A synthesized in solution/35 which crystallized after chemical workup and affinity chromatography (see Section 5.1.6.2.2). Further examples include an HIV protease analogue/701 a ubiquitin analogue/59 and monellin/89 which were each prepared by solid-phase methods and purified by HPLC. 

Unusual proteins are also characteristic of the biome free-sulfydryl type for both tropical Americas and the East, and taste-modifying type, like miraculin, monellins, and thaumatin, for tropical Africa. 

Adapted from references 31. 37. and BMP amino acids 1-S from 3g. Monellin s amino acids 4-11 from. sour a>b>c savory a=b>c astringent a[Pg.92]

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...

---