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Abstract: Miraculin differs from other sweet-tasting proteins because it is a taste-modifier having the unusual property of modifying sourness into sweetness. Its dimer is covalently linked by an inter-chain disulphide bond, and shows its taste-modifying activity at acidic pH, with maximum at pH 3.0, while it is flat at neutral pH. Previous studies suggested the importance of two histidine residues for the taste-modifying activity of miraculin. In this work, we have conducted molecular dynamics simulations on wild type miraculin and on three mutated dimers (H29A, H59A and H29A/H59A) both at neutral and acidic pH to investigate the structural and functional role of these two His residues. Our results suggested that at acidic pH the presence of two charged His at the interface induced a structural rearrangement of the two monomers, thus leading to their relative opening and the following adaptation of their conformation to the receptor surface. On the other hand the simulations on three mutants showed that the mutated dimers had a closed form, and highlighted the important role of H29 in stabilizing/destabilizing the dimer arrangement and also a cooperative effect of the two histidines. [Copyright &y& Elsevier]

Abstract: Miraculin is a plant protein that displays the peculiar property of modifying taste by swiching sour into a sweet taste. Its monomer is flavourless at all pH as well as at high concentration; the dimer form elicits its taste-modifying activity at acidic pH; a tetrameric form is also reported as active. Two histidine residues, located in exposed regions, are the main responsible of miraculin activity, as demonstrated by mutagenesis studies. Since structural data of miraculin are not available, we have predicted its three-dimensional structure and simulated both its dimer and tetramer forms by comparative modelling and molecular docking techniques. Finally, molecular dynamics simulations at different pH conditions have indicated that at acidic pH the dimer assumes a widely open conformation, in agreement with the hypotheses coming from other studies. [Copyright &y& Elsevier]

The mechanism by which sweet proteins elicit a response on the T1R2-T1R3 sweet taste receptor is still mostly unknown but has been so far related to the presence of “sweet fingers” on the protein surface able to interact with the same mechanism as that of low molecular mass sweeteners. In the search for the identification of sweet fingers, we have solved the solution structure of G16A MNEI, a structural mutant that shows a reduction of one order of magnitude in sweetness with respect to its parent protein, MNEI, a single-chain monellin. Comparison of the structures of wild-type monellin and its G16A mutant shows that the mutation does not affect the structure of potential glucophores but produces a distortion of the surface owing to the partial relative displacement of elements of secondary structure. These results show conclusively that sweet proteins do not possess a sweet finger and strongly support the hypothesis that the mechanism of interaction of sweet-tasting proteins with the recently identified T1R2-T1R3 GPC receptor is different from that of low molecular mass sweeteners. [Copyright &y& Elsevier]