Monellin amino acid sequence

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 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. 

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 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. 

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