Amino monellin

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. 

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. 

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, the smallest of sweet proteins, was discovered only in 1994 (Ming and Hellekant, 1994) in Pentadiplandra brazzeana B. This protein, whose sequence contains 54-amino acid residues, is 2000 times sweeter than sucrose when compared to a 2% sucrose aqueous solution. Its taste was described as more similar to sucrose than that of thaumatin (Ming and Hellekant, 1994). As can be seen in Figure 5C, the 3D structure of brazzein, determined by NMR spectroscopy in solution at pH 5.2 (Caldwell et al., 1998), is very simple. It contains one a-helix and three strands of antiparallel )3-sheet. The structure is stabilized by four disulfide bonds, three connecting the helix to the jS-sheet. It does not resemble either that of monellin or that of thaumatin instead, it resembles those of plant y-thionins and defensins and arthropod toxins. According to the SCOP classification (Murzin et al., 1995), brazzein belongs to the Scorpion toxin-like superfamily. 

Monellin an intensely sweet tasting dimeric protein the A-chain contains 44, the B-chain SO amino acid residues. M. tastes 3,000 times (weight basis) or 90,000 times (molar basis) sweeter than sucrose. It was discovered in the tesh fruits of an African berry, DioscoreophyUum cumminsU ( wild red berry ) of the Menispermaceae. Another sweet protein is Thaumatin (see). 

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. 

Table 8.4. Amino acid sequences of the A and B chains of monellin. The sequence YASD shown in bold type, which is localized in a p-turn, is regarded as a part of the structure responsible for the cross reaction of monellin with antibodies against thaumatin as well as for making contact with the sweetness receptor (cf. Table 8,5 and Fig. 8.7 and 8.8)...

The fruit of Thaumatococcus daniellii contains two sweet proteins thaumatin I and II, with /sac,g 2000. There are also low amounts of three other sweet proteins (thaumatin a, b and c). The complete amino acid sequence and the conformation (Fig. 8.7 and 8.8) of thaumatin I, a peptide chain with 207 amino acid residues, has been established (Table 8.5). As a result of cross reactions with an anti-serum against monellin (cf. 8.8.4), sequence Y(57)FD in a j3-tum is regarded as the site of contact with the sweetness receptor. It corresponds to sequence Y(A13)ASD of monellin. 

Monellin is extracted from the fruit Dioscoreophyllum volkensii (syn. D. cumminsii, Menispermaceae) native to tropical African rainforests. Monellin is a sweet protein with liquorice-like flavour, which consists of two peptide chains, A and B, composed of a sequence of 45 and 50 amino acids, respectively (11.5 kDa). Under food processing conditions it is unstable and has no practical significance as a sweet substance. 

Taste also registers differences between enantiomers. L-Glutamic acid imparts a meaty flavor and has been sold as taste intensifier for meats. The D-isomer, however, is almost tasteless. The proteins thaumalin (MW about 21,000) and monellin (MW about 10,700) have been observed to exhibit intense sweetness. An even more potent taste modifier is miraculin (MW about 44,000), found, like the other two, in African berries, which causes acids to taste sweet. Monellin contains 92 amino acid residues. Its intact tertiary (three-dimensional) structure is necessary to produce sweetness. The protein is in fact composed of two noncovalently bound chains of 50 and 42 residues. When separated, neither shows sweetness. This represents a case of molecular recognition at a conformational level. 

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