Monellin conformation

Tancredi, T., Pastore, A., Salvatori, S., Esposito, V., and Temussi, P.A. (2004). Interaction of sweet proteins with their receptor A conformational study of peptides corresponding to loops of brazzein, monellin and thaumatin. Eur. J. Biochem. 271, 2231-2240. 

The studies discussed above gives an idea of conformational stability of proteins in ILs as inferred from the t measurement and melting temperature determination followed by various spectroscopic methods. However, stability of a protein/enzyme has contributions from both thermodynamic and kinetic parameters [55]. While thermal unfolding studies help to extract important thermodynamic parameters, stability of the proteins could also arise from the kinetic barrier to the conformational changes in the protein scaffold [55]. However, except for monellin [48], the thermodynamic stability in other cases has not been examined. Thus, in context of stability of proteins/enzymes in ILs, it is important and imperative to study and probe thermal unfolding as well as the thermodynamics and kinetics of the conformational change of the proteins in ILs. 

Fig. 8.7. A two dimensional representation of the conformation of the peptide chains of monellin (a) and thaumatin (b). (p-structure a-helix d p-turn d ...

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. 

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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