Crystallization and preliminary X-ray diffraction studies of curculin. A new type of sweet protein having taste-modifying action

Bioprospecting and biotechnological insights into sweet-tasting proteins by microbial hosts-a review

Owing to various undesirable health effects of sugar overconsumption, joint efforts are being made by industrial sectors and regulatory authorities to reduce sugar consumption practices, worldwide. Artificial sweeteners are considered potential substitutes in several products, e.g., sugar alcohols (polyols), high-fructose corn syrup, powdered drink mixes, and other beverages. Nevertheless, their long-standing health effects continue to be debatable. Consequently, growing interest has been shifted in producing non-caloric sweetenersfrom renewable resources to meet consumers' dietary requirements. Except for the lysozyme protein, various sweet proteins including thaumatin, mabinlin, brazzein, monellin, miraculin, pentadin, and curculin have been identified in tropical plants. Given the high cost and challenging extortion of natural resources, producing these sweet proteins using engineered microbial hosts, such as Yarrowia lipolytica, Pichia pastoris, Hansenula polymorpha, Candida boidinii, Arxula adeninivorans, Pichia methanolica, Saccharomyces cerevisiae, and Kluyveromyces lactis represents an appealing choice. Engineering techniques can be applied for large-scale biosynthesis of proteins, which can be used in biopharmaceutical, food, diagnostic, and medicine industries. Nevertheless, extensive work needs to be undertaken to address technical challenges in microbial production of sweet-tasting proteins in bulk. This review spotlights historical aspects, physicochemical properties (taste, safety, stability, solubility, and cost), and recombinant biosynthesis of sweet proteins. Moreover, future opportunities for process improvement based on metabolic engineering strategies are also discussed.

De novo transcriptome analysis and comparative expression profiling of genes associated with the taste-modifying protein neoculin in Curculigo latifolia and Curculigo capitulata fruits

Background

Curculigo latifolia is a perennial plant endogenous to Southeast Asia whose fruits contain the taste-modifying protein neoculin, which binds to sweet receptors and makes sour fruits taste sweet. Although similar to snowdrop (Galanthus nivalis) agglutinin (GNA), which contains mannose-binding sites in its sequence and 3D structure, neoculin lacks such sites and has no lectin activity. Whether the fruits of C. latifolia and other Curculigo plants contain neoculin and/or GNA family members was unclear.

Results

Through de novo RNA-seq assembly of the fruits of C. latifolia and the related C. capitulata and detailed analysis of the expression patterns of neoculin and neoculin-like genes in both species, we assembled 85,697 transcripts from C. latifolia and 76,775 from C. capitulata using Trinity and annotated them using public databases. We identified 70,371 unigenes in C. latifolia and 63,704 in C. capitulata. In total, 38.6% of unigenes from C. latifolia and 42.6% from C. capitulata shared high similarity between the two species. We identified ten neoculin-related transcripts in C. latifolia and 15 in C. capitulata, encoding both the basic and acidic subunits of neoculin in both plants. We aligned these 25 transcripts and generated a phylogenetic tree. Many orthologs in the two species shared high similarity, despite the low number of common genes, suggesting that these genes likely existed before the two species diverged. The relative expression levels of these genes differed considerably between the two species: the transcripts per million (TPM) values of neoculin genes were 60 times higher in C. latifolia than in C. capitulata, whereas those of GNA family members were 15,000 times lower in C. latifolia than in C. capitulata.

Conclusions

The genetic diversity of neoculin-related genes strongly suggests that neoculin genes underwent duplication during evolution. The marked differences in their expression profiles between C. latifolia and C. capitulata may be due to mutations in regions involved in transcriptional regulation. Comprehensive analysis of the genes expressed in the fruits of these two Curculigo species helped elucidate the origin of neoculin at the molecular level.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07674-3.

3M-Brazzein as a Natural Sugar Substitute Attenuates Obesity, Metabolic Disorder, and Inflammation

Obesity is a global chronic disease linked to various diseases. Increased consumption of added sugars, especially in beverages, is a key contributor to the obesity epidemic. It is essential to reduce or replace sugar intake with low-calorie sweeteners. Here, a natural sweet protein, 3M-brazzein, was investigated as a possible sugar substitute. Mice were exposed to 3M-brazzein or 10% sucrose of equivalent sweetness, in drinking water to mimic human obesity development over 15 weeks. Consumption of 3M-brazzein in liquid form did not cause adiposity hypertrophy, resulting in 33.1 ± 0.4 g body weight and 0.90 ± 0.2 mm fat accumulation, which were 35.9 ± 0.7 g (p = 0.0094) and 1.53 ± 0.067 mm (p = 0.0031), respectively, for sucrose supplement. Additionally, 3M-brazzein did not disrupt glucose homeostasis or affect insulin resistance and inflammation. Due to its naturally low-calorie content, 3M-brazzein could also be a potential sugar substitute that reduces adiposity.

Bioproduction of the Recombinant Sweet Protein Thaumatin: Current State of the Art and Perspectives

There is currently a worldwide trend to reduce sugar consumption. This trend is mostly met by the use of artificial non-nutritive sweeteners. However, these sweeteners have also been proven to have adverse health effects such as dizziness, headaches, gastrointestinal issues, and mood changes for aspartame. One of the solutions lies in the commercialization of sweet proteins, which are not associated with adverse health effects. Of these proteins, thaumatin is one of the most studied and most promising alternatives for sugars and artificial sweeteners. Since the natural production of these proteins is often too expensive, biochemical production methods are currently under investigation. With these methods, recombinant DNA technology is used for the production of sweet proteins in a host organism. The most promising host known today is the methylotrophic yeast, Pichia pastoris. This yeast has a tightly regulated methanol-induced promotor, allowing a good control over the recombinant protein production. Great efforts have been undertaken for improving the yields and purities of thaumatin productions, but a further optimization is still desired. This review focuses on (i) the motivation for using and producing sweet proteins, (ii) the properties and history of thaumatin, (iii) the production of recombinant sweet proteins, and (iv) future possibilities for process optimization based on a systems biology approach.

A novel family of lectins evolutionarily related to class V chitinases: an example of neofunctionalization in legumes

A lectin has been identified in black locust (Robinia pseudoacacia) bark that shares approximately 50% sequence identity with plant class V chitinases but is essentially devoid of chitinase activity. Specificity studies indicated that the black locust chitinase-related agglutinin (RobpsCRA) preferentially binds to high-mannose N-glycans comprising the proximal pentasaccharide core structure. Closely related orthologs of RobpsCRA could be identified in the legumes Glycine max, Medicago truncatula, and Lotus japonicus but in no other plant species, suggesting that this novel lectin family most probably evolved in an ancient legume species or possibly an earlier ancestor. This identification of RobpsCRA not only illustrates neofunctionalization in plants, but also provides firm evidence that plants are capable of developing a sugar-binding domain from an existing structural scaffold with a different activity and accordingly sheds new light on the molecular evolution of plant lectins.

Developments in biotechnological production of sweet proteins

Most proteins are tasteless and flavorless, while some proteins elicit a sweet-taste response on the human palate. Six proteins, thaumatin, monellin, mabinlin, brazzein, egg lysozyme, and neoculin (previously considered as curculin) have been identified as sweet-tasting proteins. However, no common features among them have been observed. Herein, recent advances in the research of sweet-tasting proteins and the production of such proteins by biotechnological approaches are reviewed. Information on the structure-sweetness relationship for these proteins would help not only in the clarification of the mechanism of interaction of sweet-tasting proteins with their receptors, but also in the design of more effective low-calorie 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.

Crystal Structure of Neoculin: Insights into its Sweetness and Taste-modifying Activity

Although the majority of sweet compounds are of low molecular mass, several proteins are known to elicit sweet taste responses in humans. The fruit of Curculigo latifolia contains a heterodimeric protein, neoculin, which has both sweetness and a taste-modifying activity that converts sourness to sweetness. Here, we report the crystal structure of neoculin at 2.76A resolution. This is the first well-defined tertiary structure of a taste-modifying protein of this kind. The overall structure is quite similar to those of monocot mannose-binding lectins. However, crucial topological differences are observed in the C-terminal regions of both subunits. In both subunits of neoculin, the C-terminal tails turn up to form loops fixed by inter-subunit disulfide bonds that are not observed in the lectins. Indeed, the corresponding regions of the lectins stretch straight over the surface of another subunit. Such a C-terminal structural feature as is observed in neoculin results in a decrease in subunit-subunit interactions. Moreover, distribution of electrostatic potential on the surface of neoculin is unique and significantly different from those of the lectins, particularly in the basic subunit (NBS). We have found that there is a large cluster composed of six basic residues on the surface of NBS, and speculate that it might be involved in the elicitation of sweetness and/or taste-modifying activity of neoculin. Molecular dynamics simulation based on the crystallography results suggests that neoculin may adopt a widely "open" conformation at acidic pH, while unprotonated neoculin at neutral pH is in a "closed" conformation. Based on these simulations and the generation of a docking model between neoculin and the sweet-taste receptor, T1R2-T1R3, we propose the hypothesis that neoculin is in dynamic equilibrium between open and closed states, and that the addition of an acid shifts the equilibrium to the open state, allowing ligand-receptor interaction.

Sweet proteins--potential replacement for artificial low calorie sweeteners

Exponential growth in the number of patients suffering from diseases caused by the consumption of sugar has become a threat to mankind's health. Artificial low calorie sweeteners available in the market may have severe side effects. It takes time to figure out the long term side effects and by the time these are established, they are replaced by a new low calorie sweetener. Saccharine has been used for centuries to sweeten foods and beverages without calories or carbohydrate. It was also used on a large scale during the sugar shortage of the two world wars but was abandoned as soon as it was linked with development of bladder cancer. Naturally occurring sweet and taste modifying proteins are being seen as potential replacements for the currently available artificial low calorie sweeteners. Interaction aspects of sweet proteins and the human sweet taste receptor are being investigated.

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.].

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.

Brazzein, a new high-potency thermostable sweet protein from Pentadiplandra brazzeana B

We have discovered a new high-potency thermostable sweet protein, which we name brazzein, in a wild African plant Pentadiplandra brazzeana Baillon. Brazzein is 2,000 times sweeter than sucrose in comparison to 2% sucrose aqueous solution and 500 times in comparison to 10% of the sugar. Its taste is more similar to sucrose than that of thaumatin. Its sweetness is not destroyed by 80 degrees C for 4 h. Brazzein is comprised of 54 amino acid residues, corresponding to a molecular mass of 6,473 Da.