Three-dimensional mapping of the sweet taste receptor site

The Taste-Masking Mechanism of Chitosan at the Molecular Level on Bitter Drugs of Alkaloids and Flavonoid Glycosides from Traditional Chinese Medicine

Taste masking of traditional Chinese medicines (TCMs) containing multiple bitter components remains an important challenge. In this study, berberine (BER) in alkaloids and phillyrin (PHI) in flavonoid glycosides, which are common bitter components in traditional Chinese medicines, were selected as model drugs. Chitosan (CS) was used to mask their unfriendly taste. Firstly, from the molecular level, we explained the taste-masking mechanism of CS on those two bitter components in detail. Based on those taste-masking mechanisms, the bitter taste of a mixture of BER and PHI was easily masked by CS in this work. The physicochemical characterization results showed the taste-masking compounds formed by CS with BER (named as BER/CS) and PHI (named as PHI/CS) were uneven in appearance. The drug binding efficiency of BER/CS and PHI/CS was 50.15 ± 2.63% and 67.10 ± 2.52%, respectively. The results of DSC, XRD, FTIR and molecular simulation further indicated that CS mainly masks the bitter taste by disturbing the binding site of bitter drugs and bitter receptors in the oral cavity via forming hydrogen bonds between its hydroxyl or amine groups and the nucleophilic groups of BER and PHI. The taste-masking evaluation results by the electronic tongue test confirmed the excellent taste-masking effects on alkaloids, flavonoid glycosides or a mixture of the two kinds of bitter components. The in vitro release as well as in vivo pharmacokinetic results suggested that the taste-masked compounds in this work could achieve rapid drug release in the gastric acid environment and did not influence the in vivo pharmacokinetic results of the drug. The taste-masking method in this work may have potential for the taste masking of traditional Chinese medicine compounds containing multiple bitter components.

The Flexible Loop is a New Sweetness Determinant Site of the Sweet-Tasting Protein: Characterization of Novel Sweeter Mutants of the Single-Chain Monellin (MNEI)

The single-chain monellin (MNEI) displays same sweet potency as the natural monellin protein. To identify critical residues determining its sweetness, residues located at the loops region were selected for mutagenesis analysis. Mutations of positive-charge residues R31, R53, and R82 consistently led to obvious decrease of sweetness, whereas mutations of negative-charge residues resulted in variable sweet potency. Of note, the E50N mutant in the loop region linking the 2 natural chains showed significantly increased sweetness. Mutations of this residue to M or K led to similar effects, in accordance with the so-called wedge model for explanation of the sweet protein–receptor interaction. Homology modeling was carried out with the firstly reported crystal structure of sweet taste receptor (from medaka fish) as the template, and molecular docking and dynamics simulations suggested that flexible conformations of specific residues located in the loops region play essential roles for the interaction with the receptor and the sweetness of the protein. Moreover, obvious additive effects were found for the sweetness as 2 double-site mutants (E50N/Y65R and E2N/E50N) displayed increased sweetness than their single-site mutants. Our results revealed the flexible loop L23 linking the 2 natural chains as a novel sweetness determinant site of the sweet protein monellin and raised a series of new sweeter mutants, which could provide helpful guidance for molecular designing the sweet-tasting proteins.

The taste of D- and L-amino acids: In vitro binding assays with cloned human bitter (TAS2Rs) and sweet (TAS1R2/TAS1R3) receptors

The taste of different enantiomeric forms of amino acids has been deeply investigated because it represents the most impressive case of correlation between stereochemistry and flavour. Herein, we aimed to elucidate the molecular activity of d- and l-amino acids using an in vitro system based on a cellular model overexpressing sweet and bitter receptors, and to analyse the correlation between in vitro and sensory studies. With our work we demonstrated specific enantiomeric activities for several amino acids on TAS1R2-TAS1R3 sweet receptor. Moreover, we proved interaction of tryptophan and phenylalanine with a specific group of TAS2Rs bitter receptors, confirming and improving the results recently obtained in the tasting of amino acids. In addition, we provide the first systematic analysis of l- and d-amino acid actions on the sweet heterodimeric receptor.

New insights into the characteristics of sweet and bitter taste receptors

Understanding the molecular bases of taste is of primary importance for the field of human senses as well as for translational medical science. This chapter describes the complexity of the mechanism of action of sweet, bitter, and umami receptors. Most molecular weight sweeteners interact with orthosteric sites of the sweet receptor. The mechanism of action of sweet proteins is more difficult to interpret. In the only general mechanism proposed for the action of sweet proteins, the "wedge model," it is hypothesized that proteins bind to an external active site of the active conformation of the sweet receptor. This model can be updated by building topologically correct complexes of proteins with the receptor. Among the recent advances that will be described here are the discovery of taste modulators and the possibility that certain bitter compounds are recognized by the umami receptor.

The sweet taste receptor: a single receptor with multiple sites and modes of interaction

Elucidation of the molecular bases of sweet taste is very important not only for its intrinsic biological significance but also for the design of new artificial sweeteners. Up to few years ago design was complicated by the common belief that different classes of sweet compounds, notably sweet proteins, might interact with different receptors altogether. The recent identification and functional expression of the receptor for sweet taste have shown that there is but one receptor, drastically changing our approach to the development of new sweeteners. The explanation of how the sweet receptor can bind several different classes of molecules is that rather than multiple receptors there are, apparently, multiple sites on the single sweet taste receptor. In this chapter, the mechanisms of interaction of small and macromolecular sweet molecules will be examined, with particular emphasis on sweet proteins. Systematic homology modeling yields reliable models of all possible heterodimers of the human T1R2 and T1R3 sequences with the closed (A) and open (B) conformations of one of the metabotropic glutamate receptors (mGluR1), used as template. The most important result of these studies is the "wedge model," the first explanation of the taste of sweet proteins. In addition, it was shown that simultaneous binding to the A and B sites is not possible with two large sweeteners but is possible with a small molecule in site A and a large one in site B. This observation accounted for the first time for the peculiar phenomenon of synergy between some sweeteners.

The history of sweet taste: not exactly a piece of cake

Understanding the molecular bases of sweet taste is of crucial importance not only in biotechnology but also for its medical implications, since an increasing number of people is affected by food-related diseases like, diabetes, hyperlipemia, caries, that are more or less directly linked to the secondary effects of sugar intake. Despite the interest paid to the field, it is only through the recent identification and functional expression of the receptor for sweet taste that new perspectives have been opened, drastically changing our approach to the development of new sweeteners. We shall give an overview of the field starting from the early days up to discussing the newest developments. After a review of early models of the active site, the mechanisms of interaction of small and macromolecular sweet molecules will be examined in the light of accurate modeling of the sweet taste receptor. The analysis of the homology models of all possible dimers allowed by combinations of the human T1R2 and T1R3 sequences of the sweet receptor and the closed (A) and open (B) conformations of the mGluR1 glutamate receptor shows that only 'type B' sites, either T1R2(B) and T1R3(B), can host the majority of small molecular weight sweeteners. Simultaneous binding to the A and B sites is not possible with two large sweeteners but is possible with a small molecule in site A and a large one in site B. This observation accounted for the first time for the peculiar phenomenon of synergy between some sweeteners. In addition to these two sites, the models showed an external binding site that can host sweet proteins.

From small sweeteners to sweet proteins: anatomy of the binding sites of the human T1R2_T1R3 receptor

The sweet taste receptor, a heterodimeric G protein coupled receptor (GPCR) protein, formed by the T1R2 and T1R3 subunits, recognizes several sweet compounds including carbohydrates, amino acids, peptides, proteins, and synthetic sweeteners. Its similarity with the metabotropic glutamate mGluR1 receptor allowed us to build homology models. All possible dimers formed by combinations of the human T1R2 and T1R3 subunits, modeled on the A (closed) or B (open) chains of the extracellular ligand binding domain of the mGluR1 template, yield four ligand binding sites for low-molecular-weight sweeteners. These sites were probed by docking a set of molecules representative of all classes of sweet compounds and calculating the free energy of ligand binding. These sites are not easily accessible to sweet proteins, but docking experiments in silico showed that sweet proteins can bind to a secondary site without entering the deep cleft. Our models account for many experimental observations on the tastes of sweeteners, including sweetness synergy, and can help to design new sweeteners.

Structure–Sweetness Relationship in Egg White Lysozyme: Role of Lysine and Arginine Residues on the Elicitation of Lysozyme Sweetness

Lysozyme is one of the sweet-tasting proteins. To clarify the structure-sweetness relationship and the basicity-sweetness relationship in lysozyme, we have generated lysozyme mutants with Pichia systems. Alanine substitution of lysine residues demonstrated that two out of six lysine residues, Lys13 and Lys96, are required for lysozyme sweetness, while the remaining four lysine residues do not play a significant role in the perception of sweetness. Arginine substitution of lysine residues revealed that the basicity, but not the shape, of the side chain plays a significant role in sweetness. Single alanine substitutions of arginine residues showed that three arginine residues, Arg14, Arg21, and Arg73, play significant roles in lysozyme sweetness, whereas Arg45, Arg68, Arg125 and chemical modification by 1,2-cyclohexanedione did not affect sweetness. From investigation of the charge-specific mutations, we found that the basicity of a broad surface region formed by five positively charged residues, Lys13, Lys96, Arg14, Arg21, and Arg73, is required for lysozyme sweetness. Differences in the threshold values among sweet-tasting proteins might be caused by the broadness and/or the density of charged residues on the protein surface.

Interaction of sweet proteins with their receptor

The mechanism of interaction of sweet proteins with the T1R2-T1R3 sweet taste receptor has not yet been elucidated. Low molecular mass sweeteners and sweet proteins interact with the same receptor, the human T1R2-T1R3 receptor. The presence on the surface of the proteins of "sweet fingers", i.e. protruding features with chemical groups similar to those of low molecular mass sweeteners that can probe the active site of the receptor, would be consistent with a single mechanism for the two classes of compounds. We have synthesized three cyclic peptides corresponding to the best potential "sweet fingers" of brazzein, monellin and thaumatin, the sweet proteins whose structures are well characterized. NMR data show that all three peptides have a clear tendency, in aqueous solution, to assume hairpin conformations consistent with the conformation of the same sequences in the parent proteins. The peptide corresponding to the only possible loop of brazzein, c[CFYDEKRNLQC(37-47)], exists in solution in a well ordered hairpin conformation very similar to that of the same sequence in the parent protein. However, none of the peptides has a sweet taste. This finding strongly suggests that sweet proteins recognize a binding site different from the one that binds small molecular mass sweeteners. The data of the present work support an alternative mechanism of interaction, the "wedge model", recently proposed for sweet proteins [Temussi, P. A. (2002) FEBS Lett.526, 1-3.].

The mechanism of interaction of sweet proteins with the T1R2-T1R3 receptor: evidence from the solution structure of G16A-MNEI

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.

Why are sweet proteins sweet? Interaction of brazzein, monellin and thaumatin with the T1R2-T1R3 receptor

Sweet tasting proteins interact with the same receptor that binds small molecular weight sweeteners, the T1R2-T1R3 G-protein coupled receptor, but the key groups on the protein surface responsible for the biological activity have not yet been identified. I propose that sweet proteins, contrary to small ligands, do not bind to the 'glutamate-like' pocket but stabilize the free form II of the T1R2-T1R3 receptor by attachment to a secondary binding site. Docking of brazzein, monellin and thaumatin with a model of the T1R2-T1R3 sweet taste receptor shows that the most likely complexes can indeed stabilize the active form of the receptor.

Probing the surface of a sweet protein: NMR study of MNEI with a paramagnetic probe

The design of safe sweeteners is very important for people who are affected by diabetes, hyperlipemia, and caries and other diseases that are linked to the consumption of sugars. Sweet proteins, which are found in several tropical plants, are many times sweeter than sucrose on a molar basis. A good understanding of their structure-function relationship can complement traditional SAR studies on small molecular weight sweeteners and thus help in the design of safe sweeteners. However, there is virtually no sequence homology and very little structural similarity among known sweet proteins. Studies on mutants of monellin, the best characterized of sweet proteins, proved not decisive in the localization of the main interaction points of monellin with its receptor. Accordingly, we resorted to an unbiased approach to restrict the search of likely areas of interaction on the surface of a typical sweet protein. It has been recently shown that an accurate survey of the surface of proteins by appropriate paramagnetic probes may locate interaction points on protein surface. Here we report the survey of the surface of MNEI, a single chain monellin, by means of a paramagnetic probe, and a direct assessment of bound water based on an application of ePHOGSY, an NMR experiment that is ideally suited to detect interactions of small ligands to a protein. Detailed surface mapping reveals the presence, on the surface of MNEI, of interaction points that include residues previously predicted by ELISA tests and by mutagenesis.

Solution structure of a sweet protein: NMR study of MNEI, a single chain monellin

The sweet protein MNEI is a construct of 96 amino acid residues engineered by linking, with a Gly-Phe dipeptide, chains B and A of monellin, a sweet protein isolated from Discoreophyllum cuminsii. Here, the solution structure of MNEI was determined on the basis of 1169 nuclear Overhauser enhancement derived distance restraints and 184 dihedral angle restraints obtained from direct measurement of three-bond spin coupling constants. The identification of hydrogen bonded NH groups was obtained by a combination of H/(2)H exchange data and NH resonance temperature coefficients derived from a series of HSQC spectra in the temperature range 278-328 K. The good resolution of the structure is reflected by the Z-score of the quality checking program in WHAT IF (-0.61). The topology of MNEI, like that of natural monellin and of SCM, another single-chain monellin, is typical of the cystatin superfamily: an alpha-helix cradled into the concave side of a five-strand anti-parallel beta-sheet. The high resolution (14 restraints/residue) 3D structure of MNEI shows close similarity to the crystal structures of natural monellin and of SCM but differs from the solution structure of SCM. The structures of SCM in the crystal and in solution differ in some of the secondary structure elements, but most of all in the relative arrangement of the elements: the four main beta-strands that surround the helix in the crystal structure of SCM, are displaced far from the helix in the solution structure of SCM. These differences were attributed to the fact that SCM is a monomer in solution and a dimer in the crystal. This result is at variance with the observation that our solution structure, like that of SCM, corresponds to a monomeric state of the protein, as demonstrated by the insensitivity of HSQC spectra to extreme dilution (down to 20 microM). On the basis of the solution structure of MNEI it is possible to propose that the main glucophores are hosted on loop L34, whereas the N-terminal and C-terminal regions host two other important interaction regions, centered around segments 6-9 and 94-96.

Conformation and sweet tastes of L-aspartyl dipeptide methyl esters

In order to investigate the conformational preferences to elicit tastes, conformational free energy calculations using an empirical potential (ECEPP/2) and the hydration shell model were carried out on the L-aspartyl dipeptide methyl esters, L-(+)HAsp(-)-L-Xaa-OMe, in the hydrated state, where Xaa includes sweet (Phe, Tyr, Met, and Gly), bitter (Ala, Trp, Val, Leu, and Ile), and tasteless (Ser, Thr, and Abu) residues. The refined preferred conformation of the Phe dipeptide (aspartame) with side chain chi 1/2 conformation g- is g-Fg- in the hydrated state, which is consistent with the structure deduced from 1H-nmr experiments. Irrespective of the Xaa and taste, all the dipeptides have the same conformation for the Asp residue, which is attributable to the hydrogen bond between protonated amino hydrogen and carboxylate oxygen and the favored hydration of the carboxylate group. This implies that the L-aspartyl residue is a necessary factor for the dipeptides to be sweet not a sufficient factor. The computed conformational preferences for sweet, bitter, and tasteless dipeptides in the hydrated state indicate to us that the conformation about the N--C alpha bond of the Xaa residue, i.e., the orientation of the hydrophobic moiety with respect to the AH/B functionalities in the aspartyl moiety, seems to be crucial to elicit the tastes. In addition, the hydrophobicity and the size of the Xaa residue are found to play a major role in determining the tastes. These well accord with the related works reported previously.

Sweeteners: state of knowledge review

Sweeteners are widely used in the food and pharmaceutical industry. The purpose of this paper is to review our current knowledge of sweet taste from chemical, biochemical, electrophysiological, psychophysical, and psychological points of view. The most common sweetners likely to be used in food and pharmaceuticals will be examined in detail. First, the chemical structures of sweet compounds including saccharides, diterpene glycosides, polyols, amino acids, dipeptides, and other nonsugars will be discussed. Second, biochemical approaches to understanding sweetner receptors will be reviewed. Third, electrophysiological and behavioral approaches to understanding sweetner receptors will be discussed. Fourth, psychophysical studies in humans will be shown to be consistent with biochemical and neurophysiological data. In addition, the basic mechanisms of sweet taste revealed by psychophysical studies will be given, including the role of multiple receptor sites, hydrogen bonding, and sodium transport. Finally, the factors that affect preference for sweet taste including the psychological and physiological variables associated with sweet preference will be explored.

Structural determination of the active site of a sweet protein. A 1H NMR investigation of pMNEI

pMNEI, a single chain sweet protein related to monellin, has been studied by means of 1H NMR at 500 MHz. A partial sequential assignment performed by means of the MCD method allowed the determination of the secondary structure of a large portion of the beta-sheet of pMNEI that contains a likely 'sweet finger': the loop connecting the beta-strands from residue 59 to residue 78, corresponding to segment 16-35 of the A chain of monellin. The detailed three-dimensional structure of the loop (Tyr66-Ala67-Ser68-Asp69), determined from several interresidue and intraresidue NOEs and subsequent energy minimization, shows that the side chains of Tyr66 and Asp69 fit our model of the sweet receptor in a manner very similar to that of the side chains of Phe and Asp of aspartame.

A review of sweet taste potentiation brought about by divalent oxygen and sulfur incorporation

The plethora of high-potency sweetener research has allowed the construction of important structure-taste relationships. In light of new structure-taste relationships, it is instructive to review sweet taste potentiation brought about by divalent oxygen and sulfur incorporation. The taste of sulfur-containing organic compounds was reviewed in Japanese by Yasuo Ariyoshi in 1977. Several new representative examples of sweet taste potentiation and taste dichotomy (sweet and bitter) found within similar classes of oxygen- and sulfur-containing organic compound: amides, dipeptides, ureas, sulfamates, sulfonamides, oximes, sugars, dihydroisocoumarins, and others are reviewed. Special attention is given to the thioethers and thioureas in sulfamates, dipeptides, aryl ureas, and hybrid dipeptide ureas. The most notable contributions have arisen from the work of Nofre and Tinti at Université Claude Bernard in Lyons, France. A common trend emerges with certain sweeteners when a carbon atom is strategically replaced by sulfur or oxygen atoms. The net result is an increase in the sweetness two- to tenfold. With saccharins, the usual bitter, metallic taste is removed. Sweet taste receptor models that have been published are mainly based on the original Shallenberger and Acree model of the glucophores AH-B with contributions from Kier (AH-B-X). AH is a proton donor group, B is a proton acceptor group, and X is some hydrophobic group. All of the models have overlooked the contributions of divalent sulfur (often in place of oxygen) in bringing about sweetness potentiation. There is no precedence for localizing the energy-minimized structures of sulfur-containing sweeteners in a binding mode that includes sulfur. These sulfur potentiation loci are analyzed and illustrated in a computer-generated sweetener model to show the specific region in which sulfur is being "recognized" as a potentiating feature.

Conformation and hydration of aspartame

Conformational free energy calculations using an empirical potential (ECEPP/2) and the hydration shell model were carried out on the neutral, acidic, zwitterionic, and basic forms of aspartame in the hydrated state. The results indicate that as the molecule becomes more charged, the number of low energy conformations becomes smaller and the molecule becomes less flexible. The calculated free energies of hydration of charged aspartames show that hydration has a significant effect on the conformation in solution. Only two feasible conformations were found for the zwitterionic form, and these are consistent with the conformations deduced from NMR and X-ray diffraction experiments. The calculated free energy difference between these two conformations was 1.25 kcal/mol. The less favored of the two solvated conformations can be expected to be stabilized by hydrophobic interaction of the phenyl groups in the crystal.

Molecular structure influencing either a sweet or bitter taste among aldoximes

A series of cyclohexylaldoximes was examined for their sweet or bitter taste using discriminant analysis. The structures of the molecules were described using molecular connectivity. A two-variable linear discriminant function and critical value were computed that correctly assigned 17 of the 20 molecules to their observed sweet or bitter taste categories. The same discriminant function can predict correctly the taste categories of seven of eight additional molecules.