New insights into the characteristics of sweet and bitter taste receptors

A Super Stable Mutant of the Plant Protein Monellin Endowed with Enhanced Sweetness

Sweet proteins are a class of proteins with the ability to elicit a sweet sensation in humans upon interaction with sweet taste receptor T1R2/T1R3. Single-chain Monellin, MNEI, is among the sweetest proteins known and it could replace sugar in many food and beverage recipes. Nonetheless, its use is limited by low stability and high aggregation propensity at neutral pH. To solve this inconvenience, we designed a new construct of MNEI, dubbed Mut9, which led to gains in both sweetness and stability. Mut9 showed an extraordinary stability in acidic and neutral environments, where we observed a melting temperature over 20 °C higher than that of MNEI. In addition, Mut9 resulted twice as sweet than MNEI. Both proteins were extensively characterized by biophysical and sensory analyses. Notably, Mut9 preserved its structure and function even after 10 min boiling, with the greatest differences being observed at pH 6.8, where it remained folded and sweet, whereas MNEI lost its structure and function. Finally, we performed a 6-month shelf-life assessment, and the data confirmed the greater stability of the new construct in a wide range of conditions. These data prove that Mut9 has an even greater potential for food and beverage applications than MNEI.

The anatomy of mammalian sweet taste receptors

All sweet-tasting compounds are detected by a single G-protein coupled receptor (GPCR), the heterodimer T1R2-T1R3, for which no experimental structure is available. The sweet taste receptor is a class C GPCR, and the recently published crystallographic structures of metabotropic glutamate receptor (mGluR) 1 and 5 provide a significant step forward for understanding structure-function relationships within this family. In this article, we recapitulate more than 600 single point site-directed mutations and available structural data to obtain a critical alignment of the sweet taste receptor sequences with respect to other class C GPCRs. Using this alignment, a homology 3D-model of the human sweet taste receptor is built and analyzed to dissect out the role of key residues involved in ligand binding and those responsible for receptor activation. Proteins 2017; 85:332-341. © 2016 Wiley Periodicals, Inc.

Sweeter and stronger: enhancing sweetness and stability of the single chain monellin MNEI through molecular design

Sweet proteins are a family of proteins with no structure or sequence homology, able to elicit a sweet sensation in humans through their interaction with the dimeric T1R2-T1R3 sweet receptor. In particular, monellin and its single chain derivative (MNEI) are among the sweetest proteins known to men. Starting from a careful analysis of the surface electrostatic potentials, we have designed new mutants of MNEI with enhanced sweetness. Then, we have included in the most promising variant the stabilising mutation E23Q, obtaining a construct with enhanced performances, which combines extreme sweetness to high, pH-independent, thermal stability. The resulting mutant, with a sweetness threshold of only 0.28 mg/L (25 nM) is the strongest sweetener known to date. All the new proteins have been produced and purified and the structures of the most powerful mutants have been solved by X-ray crystallography. Docking studies have then confirmed the rationale of their interaction with the human sweet receptor, hinting at a previously unpredicted role of plasticity in said interaction.

Molecular Dynamics Driven Design of pH-Stabilized Mutants of MNEI, a Sweet Protein

MNEI is a single chain derivative of monellin, a plant protein that can interact with the human sweet taste receptor, being therefore perceived as sweet. This unusual physiological activity makes MNEI a potential template for the design of new sugar replacers for the food and beverage industry. Unfortunately, applications of MNEI have been so far limited by its intrinsic sensitivity to some pH and temperature conditions, which could occur in industrial processes. Changes in physical parameters can, in fact, lead to irreversible protein denaturation, as well as aggregation and precipitation. It has been previously shown that the correlation between pH and stability in MNEI derives from the presence of a single glutamic residue in a hydrophobic pocket of the protein. We have used molecular dynamics to study the consequences, at the atomic level, of the protonation state of such residue and have identified the network of intramolecular interactions responsible for MNEI stability at acidic pH. Based on this information, we have designed a pH-independent, stabilized mutant of MNEI and confirmed its increased stability by both molecular modeling and experimental techniques.

Glucose-Sensing Receptor T1R3: A New Signaling Receptor Activated by Glucose in Pancreatic β-Cells

Subunits of the sweet taste receptors T1R2 and T1R3 are expressed in pancreatic β-cells. Compared with T1R3, mRNA expression of T1R2 is considerably lower. At the protein level, expression of T1R2 is undetectable in β-cells. Accordingly, a major component of the sweet taste-sensing receptor in β-cells may be a homodimer of T1R3 rather than a heterodimer of T1R2/T1R3. Inhibition of this receptor by gurmarin or deletion of the T1R3 gene attenuates glucose-induced insulin secretion from β-cells. Hence the T1R3 homodimer functions as a glucose-sensing receptor (GSR) in pancreatic β-cells. When GSR is activated by the T1R3 agonist sucralose, elevation of intracellular ATP concentration ([ATP]i) is observed. Sucralose increases [ATP]i even in the absence of ambient glucose, indicating that sucralose increases [ATP]i not simply by activating glucokinase, a rate-limiting enzyme in the glycolytic pathway. In addition, sucralose augments elevation of [ATP]i induced by methylsuccinate, suggesting that sucralose activates mitochondrial metabolism. Nonmetabolizable 3-O-methylglucose also increases [ATP]i and knockdown of T1R3 attenuates elevation of [ATP]i induced by high concentration of glucose. Collectively, these results indicate that the T1R3 homodimer functions as a GSR; this receptor is involved in glucose-induced insulin secretion by activating glucose metabolism probably in mitochondria.

BitterX: a tool for understanding bitter taste in humans

BitterX is an open-access tool aimed at providing a platform for identifying human bitter taste receptors, TAS2Rs, for small molecules. It predicts TAS2Rs from the molecular structures of arbitrary chemicals by integrating two individual functionalities: bitterant verification and TAS2R recognition. Using BitterX, several novel bitterants and their receptors were predicted and experimentally validated in the study. Therefore, BitterX may be an effective method for deciphering bitter taste coding and could be a useful tool for both basic bitter research in academia and new bitterant discoveries in the industry.

Expanding the fragrance chemical space for virtual screening

The properties of fragrance molecules in the public databases SuperScent and Flavornet were analyzed to define a “fragrance-like” (FL) property range (Heavy Atom Count ≤ 21, only C, H, O, S, (O + S) ≤ 3, Hydrogen Bond Donor ≤ 1) and the corresponding chemical space including FL molecules from PubChem (NIH repository of molecules), ChEMBL (bioactive molecules), ZINC (drug-like molecules), and GDB-13 (all possible organic molecules up to 13 atoms of C, N, O, S, Cl). The FL subsets of these databases were classified by MQN (Molecular Quantum Numbers, a set of 42 integer value descriptors of molecular structure) and formatted for fast MQN-similarity searching and interactive exploration of color-coded principal component maps in form of the FL-mapplet and FL-browser applications freely available at http://www.gdb.unibe.ch. MQN-similarity is shown to efficiently recover 15 different fragrance molecule families from the different FL subsets, demonstrating the relevance of the MQN-based tool to explore the fragrance chemical space.

Peripheral coding of taste

Five canonical tastes, bitter, sweet, umami (amino acid), salty, and sour (acid), are detected by animals as diverse as fruit flies and humans, consistent with a near-universal drive to consume fundamental nutrients and to avoid toxins or other harmful compounds. Surprisingly, despite this strong conservation of basic taste qualities between vertebrates and invertebrates, the receptors and signaling mechanisms that mediate taste in each are highly divergent. The identification over the last two decades of receptors and other molecules that mediate taste has led to stunning advances in our understanding of the basic mechanisms of transduction and coding of information by the gustatory systems of vertebrates and invertebrates. In this Review, we discuss recent advances in taste research, mainly from the fly and mammalian systems, and we highlight principles that are common across species, despite stark differences in receptor types.

Sensomics analysis of taste compounds in balsamic vinegar and discovery of 5-acetoxymethyl-2-furaldehyde as a novel sweet taste modulator

Sensory-directed fractionation of traditional balsamic vinegar of Modena (TBV) led to the identification of the sweet-bitter tasting hexose acetates 6-O-acetyl-α/β-d-glucopyranose and 1-O-acetyl-β-d-fructopyranose as well as the previously unknown sweetness modulator 5-acetoxymethyl-2-furaldehyde. Taste re-engineering experiments and sensory time-intensity studies confirmed 5-acetoxymethyl-2-furaldehyde to contribute to the typical long-lasting sweet taste quality of TBV. Moreover, the response of the sweet taste receptor to this furaldehyde was verified by means of a functional hTAS1R2/hTAS1R3 receptor assay. Quantitative analysis of a total of 59 nonvolatile sensometabolites and taste modulators revealed higher concentrations of the sweet-modulating 5-acetoxymethyl-2-furaldehyde, nonvolatile organic acids and polyphenols such as wood-derived ellagitannins, and lower concentrations of acetic acid in the premium quality TBV when compared to balsamic vinegar of Modena (BV). Quantitative monitoring of sensometabolites throughout TBV manufacturing, followed by agglomerative hierarchical clustering and sensomics heatmapping, gave molecular insights into the taste alterations occurring during TBV maturation.