Monellin three-dimensional structure

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. 

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... 

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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