Sweet Molecules Research Articles

Human sweet taste receptor: Complete structure prediction and evaluation

Human sweet taste receptor structure is predicted and was further evaluate using experimental result. This would enable receptor-based docking and pharmacophore studies since no structure, experimental or predicted model, for complete subunits of human sweet taste receptor (hSTR) is available till date. Different homology modelling as well as threading-based tools were employed for structure prediction of individual domains (amino terminal domain (ATD), cysteine rich domain (CRD) and transmembrane domain (TMD)) and complete subunit structure. Finally, complete subunits structure was built from combinations of individual domain models. These predicted models were validated and further evaluated using docking and interaction analysis of the experimentally studied sweet molecules. hSTR Modelling through threading based tools was of poor quality with the exception of ITASSER software that predicted models with greater than 90% residues in energetically favourable environment. Among homology based software, CPH Model, SWISS Model and Prime predicted hSTR model of acceptable quality with more than 95% residues in energetically favourable environment. This model can be used for receptor-based pharmacophore modelling or searching newer sweet molecules.

Multimodal function of the sweet taste receptor expressed in pancreatic β-cells: generation of diverse patterns of intracellular signals by sweet agonists

The sweet taste receptor is expressed in the taste bud and is activated by numerous sweet molecules with diverse chemical structures. It is, however, not known whether these sweet agonists induce a similar cellular response in target cells. Using MIN6 cells, a pancreatic β-cell line expressing endogenous sweet taste receptor, we addressed this question by monitoring changes in cytoplasmic Ca2+ ([Ca2+]i) and cAMP ([cAMP]i) induced by four sweet taste receptor agonists. Glycyrrhizin evoked sustained elevation of [Ca2+]i but [cAMP]i was not affected. Conversely, an artificial sweetener saccharin induced sustained elevation of [cAMP]i but did not increase [Ca2+]i. In contrast, sucralose and acesulfame K induced rapid and sustained increases in both [Ca2+]i and [cAMP]i. Although the latter two sweeteners increased [Ca2+]i and [cAMP]i, their actions were not identical: [Ca2+]i response to sucralose but not acesulfame K was inhibited by gurmarin, an antagonist of the sweet taste receptor which blocks the gustducin-dependent pathway. In addition, [Ca2+]i response to acesulfame K but not to sucralose was resistant to a Gq inhibitor. These results indicate that four types of sweeteners activate the sweet taste receptor differently and generate distinct patterns of intracellular signals. The sweet taste receptor has amazing multimodal functions producing multiple patterns of intracellular signals.

The good taste of peptides

The taste of peptides is seldom one of the most relevant issues when one considers the many important biological functions of this class of molecules. However, peptides generally do have a taste, covering essentially the entire range of established taste modalities: sweet, bitter, umami, sour and salty. The last two modalities cannot be attributed to peptides as such because they are due to the presence of charged terminals and/or charged side chains, thus reflecting only the zwitterionic nature of these compounds and/or the nature of some side chains but not the electronic and/or conformational features of a specific peptide. The other three tastes, that is, sweet, umami and bitter, are represented by different families of peptides. This review describes the main peptides with a sweet, umami or bitter taste and their relationship with food acceptance or rejection. Particular emphasis will be given to the sweet taste modality, owing to the practical and scientific relevance of aspartame, the well-known sweetener, and to the theoretical importance of sweet proteins, the most potent peptide sweet molecules.

Hydration properties and proton exchange in aqueous sugar solutions studied by time domain nuclear magnetic resonance

Proton NMR relaxation rates (R1 and R2) were measured in aqueous solutions of sucrose, d-glucose and d-fructose with increasing concentration. The measurements were carried out using Bruker PC 20 NMR Process Analyzer. Inversion recovery and CPMG pulse sequences are used for the measurement of relaxation rates. Results show that the values of relaxation rate increase as the concentration of the sugar is increased. The relaxation rate appears to be higher for sucrose solutions as compared to glucose or d-fructose solutions. These results were discussed on the basis of molecular association between sugar and water molecules through hydrogen bonding. The water self diffusion coefficient was measured in these sugar solutions by using pulse field gradient NMR method. As expected, the water self-diffusion coefficient was reduced with increased sugar concentrations. The results of translational mobility exhibited a higher mobility for fructose than glucose or sucrose in aqueous solutions.The dependence of R2 on the inter-pulse delay of the CPMG sequence gives information on the proton exchange mechanism involved. The mechanism of exchange was studied using R2 with increasing inter pulse delay of 0.05–2.0ms in aqueous solutions of 10%, 20% and 35% (w/v) of the above sugar solutions. From the plots of relaxation rates (R2) versus the 90°–180° pulse spacing it was possible to calculate the proton exchange rate (kb) of the different sugar solutions. Relaxation rates show characteristic variations with CPMG pulse spacing which can be interpreted on the basis of chemical exchange between solute and solvent molecules.The experimental results namely relaxation rates and CPMG pulse spacing data show the importance of water interactions with sweet molecules and this can lead to a better understanding of the effect of hydration water in taste chemoreception.

SuperSweet--a resource on natural and artificial sweetening agents

A vast number of sweet tasting molecules are known, encompassing small compounds, carbohydrates, d-amino acids and large proteins. Carbohydrates play a particularly big role in human diet. The replacement of sugars in food with artificial sweeteners is common and is a general approach to prevent cavities, obesity and associated diseases such as diabetes and hyperlipidemia. Knowledge about the molecular basis of taste may reveal new strategies to overcome diet-induced diseases. In this context, the design of safe, low-calorie sweeteners is particularly important. Here, we provide a comprehensive collection of carbohydrates, artificial sweeteners and other sweet tasting agents like proteins and peptides. Additionally, structural information and properties such as number of calories, therapeutic annotations and a sweetness-index are stored in SuperSweet. Currently, the database consists of more than 8000 sweet molecules. Moreover, the database provides a modeled 3D structure of the sweet taste receptor and binding poses of the small sweet molecules. These binding poses provide hints for the design of new sweeteners. A user-friendly graphical interface allows similarity searching, visualization of docked sweeteners into the receptor etc. A sweetener classification tree and browsing features allow quick requests to be made to the database. The database is freely available at: http://bioinformatics.charite.de/sweet/.

Sweet Taste in Man: A Review

A greater understanding of the molecular mechanisms of sweet taste has profound significance for the food industry as well as for consumers. Understanding the mechanism by which sweet taste is elicited by saccharides, peptides, and proteins will assist science and industry in their search for sweet substances with fewer negative health effects. The original AH-B theories have been supplanted by detailed structural models. Recent identification of the human sweet receptor as a dimeric G-protein coupled receptor comprising T1R2 and T1R3 subunits has greatly increased the understanding of the mechanisms involved in sweet molecule binding and sweet taste transduction. This review discusses early theories of the sweet receptor, recent research of sweetener chemoreception of nonprotein and protein ligands, homology modeling, the transduction pathway, the possibility of the sweet receptor functioning allosterically, as well as the implications of allelic variation.

Is rat LRRP Ba1-651 a Delta-1-pyrroline-5-carboxylate dehydrogenase activated by changes in the concentration of sweet molecules?

The liver is one of the most complex organs in the body, which responds to hepatocellular damage with inflammatory, regenerative and repair processes designed to restore functional liver tissue mass. Rat LRRP Ba1-651, a liver regeneration related protein induced during partial hepatectomy, is classified as a member of the aldehyde dehydrogenase (ALDh) 4A1 superfamily. During a BLAST protein search, this protein basically showed three structural and functional domains: an intermediate filament-like protein, a Delta-1-pyrroline-5-carboxylate dehydrogenase (P5CDh) and an atrial natriuretic factor (ANF) receptor. We suggest that all amniotic mammals possess a Ba1-651 ortholog to that of rats. The ANF receptor domain of rat LRRP Ba1-651, which domain is part of the receptor family ligand binding region, shows a very high sequence homology (almost identity) to the extracellular amino-terminal domains of the mammalian sweet taste receptor T1R2. This receptor belongs to the type C family of G protein coupled receptors (GPCRs) and is characterized by the presence of large extracellular amino-terminal domains, a nine cysteine domain of family 3 GPCR and a 7tm_3 transmembrane type domain. We suggest that rat LRRP Ba1-651 protein is a liver P5CDh-ANF that is activated by changes in the concentration of sweet molecules. If the sugar concentration in the organ increases due to liver damage or the intake of carbohydrate-rich or protein-rich foods, the P5CDh-ANF enzyme is activated to help in P5C catabolism. The hormone insulin probably plays a key role in the regulation of this enzyme. In the model that we propose, the P5CDh-ANF enzyme is activated by a conformational change in protein structure in the P5C docking site due to sugars binding in the AFN receptor region of the LRRP Ba1-651 protein. Our research could be a further understanding of the biological significance of this P5CDh-ANF enzyme, with important potential applications in the treatment of HPII and liver diseases and in liver transplantation. Further studies of our P5CDh-ANF enzyme are needed to clarify its features and functions, and which substances are involved in its induction. These might use liver cell lines or purified LRRP Ba1-651 protein with sweet molecules in vitro. Other experiments may help to localize LRRP Bal-651 in the organ and to link its abnormal presence or absence to certain tumors like hepatocellular carcinoma.

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.

Micro and Macro Models of the Sweet Receptor

The study of the structure–activity relationship (SAR) of sweet molecules has been traditionally based on indirect analyses that have led to several models of the receptor active site, consistent with the shape of many conformationally rigid sweeteners—see, for example, Temussi et al. (1991) and the references therein. Most sweeteners are low mol. wt compounds but a few sweet proteins are also known. Only recently it has been demonstrated that small mol. wt sweeteners and sweet macromolecules interact with the same T1R2–T1R3 receptor (Li et al., 2002), inferring that there is just one sweet taste receptor. However, it is not easy to understand (and show) how low mol. wt sweet compounds and sweet proteins can activate the same receptor. The three-dimensional structure of the receptor has not yet been elucidated, but it is possible to build homology models to shed light on the SAR of sweet molecules. The sweet taste receptor (SR) is a G protein coupled receptor similar to the dimeric metabotropic glutamate m1-LBR receptor (Margolskee, 2002). The similarity between the sequences of the two chains of the T1R2–T1R3 receptor and that of the single chain of the homodimer of the m1-LBR mGlu receptor is sufficient to allow homology modelling and to assume that it has the same general features. Existing three-dimensional SR models (Margolskee, 2002; Temussi, 2002) have been built using, as template, the N-terminal domain of mGluR1 whose crystal structure is available (Kunishima et al., 2000).

The Vomeronasal Receptor V2R2 Does Not Require Escort Molecules for Expression in Heterologous Systems

In rodents, many behavioural responses are triggered by pheromones. These molecules are believed to bind and activate two families of G-protein coupled receptors, namely V1Rs and V2Rs, which are specifically expressed in the chemosensory neurons of the vomeronasal organ. V2Rs are homologous with Group 3 of G-protein-coupled receptors, which includes metabotropic glutamate receptors, calcium-sensing receptors, fish olfactory receptors, and taste receptors for sweet molecules and amino acids. The large extracellular region of these receptors is folded as a dimer and, in this form, binds agonists that in many cases are amino acids. It has recently been reported that V2Rs must be physically associated with specific major histocompatibility complex class Ib molecules (MHC) for their expression in both mouse vomeronasal neurons and heterologous cell lines. Here, we show that in contrast to the other V2Rs, V2R2, an atypical member of this receptor family, can be successfully and abundantly expressed by insect cells without the requirement of escort molecules like MHC. Moreover, the extracellular binding domain of V2R2, secreted as a soluble product, forms dimers via cysteine-mediated sulphur bridges. Overall, the data presented in this paper confirm that V2R2 diverges from the other members of the V2R family and suggest a different role for this receptor in pheromonal communication.

Effect of sucrose on the properties of caffeine adsorption layers at the air/solution interface.

Sweet and bitter tastes are known to be mediated by G-protein-coupled receptors. The relationship between the chemical structure of gustable molecules and their molecular organization in saliva (aqueous solution) near the surface of the tongue provides a useful tool for elucidating the mechanism of chemoreception. The interactions between stimulus and membrane receptors occur in an anisotropic system. They might be influenced by the molecular packing of gustable molecules within an aqueous solvent (saliva) close to the receptor protein. To investigate the molecular organization of a sweet molecule (sucrose), a bitter molecule (caffeine), and their mixture in an aqueous phase near a "wall", a hydrophobic phase, we modeled this using an air/liquid interface as an anisotropic system. The experimental (tensiometry and ellipsometry) data unambiguously show that caffeine molecules form an adsorption layer, whereas sucrose induces a desorption layer at the air/water interface. The adsorption of caffeine molecules at the air/water interface gradually increases with the volume concentration and is delayed when sucrose is added to the solution. Spectroscopic ellipsometry data show that caffeine in the adsorption layer has optical properties practically identical to those of the molecule in solution. The results are interpreted in terms of molecular association of caffeine with itself at the interface with and without sucrose in the subphase, using the theory of ideal gases.

Carbohydrates: The Sweet Molecules of Life (by Robert V. Stick)

Carbohydrates: The Sweet Molecules of Life (by Robert V. Stick)

Role of water in sweet taste chemoreception

Abstract The mechanistic understanding of sweet taste chemoreception has been advanced by the microscopic and macroscopic studies of sweetener­water interactions. This approach has led to the concept of water mobility as a key to interpreting sweetness. The apparent specific volume of a solution is a determinant of its taste quality, as sweetness is known to be confined to the range 0.51­0.71 cm3 g-1. Thus, the "ideal" quality of the sugars is presumed to be due to their occupancy of the center of this range (i.e., 0.618 cm3 g-1). Most sweeteners elicit off-tastes and flavors and exhibit different apparent specific volumes. This leads to the conclusion that taste quality is broadly determined by the packing characteristics of sweet molecules among water molecules and the compactness of their hydration shells, expressed as their apparent specific isentropic compressibilities. The role of water can, therefore, be applied in modern attempts to optimize sweet taste quality, and different food salts can be explored as useful taste modifiers. Salts interact more strongly with water structure than do any other taste solutes, and it emerges that the ionic charge density is an important criterion. Such studies show how sweetener formulations are likely to improve within the next decade.

Some taste molecules and their solution properties.

The solution properties of a variety of different sapid substances from all four basic taste modalities, namely, sweet (n = 24), salty (n = 7), sour (n = 11) and bitter (n = 2), have been investigated. Some multisapophoric molecules, i.e. molecules exhibiting more than one taste, have also been included in the study in an attempt to define their properties in relation to the tastes they exhibit; eight sweet-bitter and three salty-bitter molecules were used. The density and sound velocity of their solutions in water have been measured and their apparent volumes, apparent compressibilities and compressibility hydration numbers calculated and compared. Apparent molar volumes (phi(v)) and apparent specific volumes (ASV) reflect the state of hydration of the molecules, and thus their extent of interaction with water structure. The range of ASVs reported are 0.13-0.49 cm3/g for salty molecules, 0.55-0.68 cm3/g for sweet molecules, 0.53-0.88 cm3/g for sweet-bitter molecules and a much wider range (0.16-0.85 cm3/g) for sour molecules. Isentropic apparent specific compressibilities range from -2.33 x 10(-5) to -8.06 x 10(-5) cm3/g x bar for salty molecules, -3.38 x 10(-7) to -2.34 x 10(-5) cm3/g x bar for sweet molecules, +6.35 x 10(-6) to -2.22 x 10(-5) cm3/g x bar for sweet-bitter molecules and +6.131 x 10(-6) to -2.99 x 10(-5) cm3/g x bar for sour molecules. Compressibility hydration numbers are also determinable from the measurements of isentropic compressibilities and these reflect the number of water molecules that are disturbed by the presence of the solutes in solution. This study also shows that it is possible to group isentropic apparent molar compressibility values by the taste quality exhibited by the molecules in the same order as for ASV.

Conformational analysis of potent sweet taste ligands by NMR and computer simulations

We report the conformational analysis by 1H-nmr and computer simulations of five potent sweet molecules, N-(3,3-dimethylbutyl)-L-aspartyl-S-(α-methyl)phenylalanine methylester (1; 5000 times more potent than sucrose), L-aspartyl-D-valine (S)-α-methoxycarbonylmethylbenzylamide (2; 1400 times more potent than sucrose), L-aspartyl-D-valine α-phenylcyclopentylamide (3; 1200 times more potent than sucrose), L-aspartyl-D-α-aminobutyric acid (S)-α-cyclohexylpropylamide (4; 2300 times more potent than sucrose), and L-aspartyl-D-valine (R)-α-methylthiomethylbenzylamide (5; 3000 times more potent than sucrose). The “L-shaped” structure, which we believe to be responsible for sweet taste, is accessible to all five sweet compounds in solution. This structure is characterized by a zwitterionic ring formed by the A-H and B containing moieties located in the +y axis and by the hydrophobic group X pointing into the +x axis. Other accessible conformations of these flexible molecules are extended conformations with the A-H and B containing moieties in the +y axis and the hydrophobic group X pointing in the −y axis and reversed L-shaped structures with the hydrophobic group X projecting along the −x axis. The remarkable potency of the N-alkylated compound 1 supports our recent hypothesis that a second hydrophobic binding domain in addition to interactions arising from the L-shaped structure leads to an enhancement of sweetness potency. © 1999 John Wiley & Sons, Inc. Biopoly 49: 525–539, 1999

Importance of molar volumes and related parameters in sweet taste chemoreception

Abstract

Specific volumes and sweet taste

Partial molar and specific volumes are now well-known parameters in assessing drug potency in general and have recently been explored in sweet taste chemoreception. At natural tasting concentrations the apparent specific volume seems to be the most useful index of taste with 0.51–0.71 cm 3 g −1 defining sweet taste quality. Most sugars are in the range 0.60–0.63 cm 3 g −1. Apparent specific volumes of sweet molecules correlate well with other volume parameters such as intrinsic viscosity, partial molar compressibility and theoretical molecular volume calculations. Fragments of sugar molecules contribute differently to overall volume. Lowering of molecular weight by removal of an oxygen atom or larger fragments from a sugar molecule may actually elevate specific volume by diminishing hydrogen bonding. The positional contributions of substituents around sugar rings to overall volume allow orientational comparisons to be made, and examples of these effects in multisapophoric molecules and L-sugars are illustrated. The interaction of a tastant molecule with water causes physical changes which may or may not give rise to a change in gustatory quality over the course of time. Detailed studies of specific volumes will therefore contribute to the understanding of the role of water in sweet taste chemoreception. However, differences in taste (if any) between enantiomers cannot be explained by differences in hydration.

Discovery of Highly Sweet Compounds from Natural Sources

Sucrose, the most widely used sweetener globally, is of plant origin. In addition, a number of other plant constituents are employed as dietary sucrose substitutes in one or more countries, including the diterpenoid, stevioside, the triterpenoid, glycyrrhizin, and the protein, thaumatin. Accordingly, there has been much interest in discovering further examples of potently sweet compounds of natural origin, for potential use in foods, beverages, and medicines. Approximately 75 plant-derived compounds are presently known, mainly representative of the flavonoid, proanthocyandin, protein, steroidal saponin, and terpenoid chemotypes.In our program directed towards the elucidation of further highly sweet molecules from plants, candidate sweet-tasting plants for laboratory investigation are obtained from ethnobotanical observations in the field or in the existing literature. Examples of novel sweet-tasting compounds obtained so far are the sesquiterpenoids, hernandulcin and 4beta-hydroxyhemandulcin; the triterpe...

Experimental study of the internal signal of alliesthesia induced by sweet molecules in rats

To identify the preabsorptive signal that arouses alliesthesia, we compared the effects of five sweet molecules: glucose (3 g · 5 ml−1), cyclamate (0.280 g · 5 ml−1), saccharin (0.016 g · 5 ml−1), aspartame (0.020 g · 5 ml−1), and mannitol (3 g · 5 ml−1) on the ingestiveaversive responses of rats. In Experiment 1, the sweet stimuli were adjusted to taste similarly sweet, and they were administered orally; they aroused similar ingestive responses. In Experiment 2, an isovolumetric load of each of the five molecules was administered in the stomach and its influence on ingestive/aversive response aroused by oral sucrose was recorded. Negative alliesthesia was obtained after gastric loads of glucose and mannitol, but not after gastric loads of cyclamate, saccharin, and aspartame.

The chemical basis of sweetness perception in beverages

For the chemist, sweet taste perception must begin with the simple principles of chemoreception. Molecules endowed with an appropriate ‘Glucophore’ are able to interact with, and elicit a response in, a putative receptor. The challenge is thus to explore the structures of sweet molecules, their modification in the environments in which they are presented and their access to, and activation of receptors. A recent approach to structure-activity relationships in sweeteners has centred on the role of water. This has led to a clearer picture of the real hydrated state of sweet molecules and differences between them based on their solution properties. The role of water is of particular relevance in beverages and offers the tantalising prospect of sensory control of formulations based on objective solution measurements. Parameters such as 1H NMR pulse relaxation times, intrinsic viscosities and apparent specific volumes can be compared to evaluate the solution behaviour of sweet molecules. Apparent specific volumes offer direct experimental verification of computed volumes and, more importantly, are measures of the effective volumes of sweet solutes in the vicinity of receptor sites. They have already been shown to be broad determinants of taste quality. Sweet molecules belong to vastly different chemical classes and they elicit different qualities, intensities and persistences of response. Future progress in the optimisation of sweet taste perception may lie in enhancement or inhibition of the response and control of solution interactions