The 2.5th generation enzymatic sensors based on the construction of quasi-direct electron transfer type NAD(P)-Dependent dehydrogenases

We introduce a versatile method to convert NAD+ or NADP+ -dependent dehydrogenases into quasi-direct electron transfer (quasi-DET)-type dehydrogenases, by modifying with a mediator on the enzyme surface toward the development of 2.5th generation enzymatic sensors. In this study, we use β-hydroxybutyrate (BHB) dehydrogenase (BHBDh) from Alcaligenes faecalis (AfBHBDh) as a representative NAD+ or NADP+ -dependent dehydrogenase. BHBDhs are important in ketone monitoring, especially for the diagnosis of diabetic ketoacidosis. We modified AfBHBDh with a thiol-reactive phenazine ethosulfate (trPES). We designed, constructed, and modified mutant BHBDhs harboring cysteine residues within 20 Å from the C4 nicotinamide in NAD+/NADH. Mutants Ser65Cys, Thr96Cys, and Lys106Cys showed indistinguishable catalytic activities from the wild-type enzyme, even after trPES modification. These trPES-modified mutants were immobilized on gold disk electrodes via amine coupling with succinimide-groups of dithiobis (succinimidyl hexanoate) self-assembled monolayers for electrochemical measurements. Considering there is a wide range of BHB concentrations, we exploited the linear regression in log scales. The linear range for the sensors with trPES-modified BHBDh mutants Ser65Cys, Thr96Cys, and Lys106Cys were 0.1-4.0 mM in both buffer solution and artificial interstitial fluid (ISF). They have limits of detection of 0.047 mM for Ser65Cys, 0.15 mM for Thr96Cys, and 0.060 mM for Lys106Cys in buffer solution, and 0.12 mM, 0.089 mM, and 0.044 mM in artificial ISF, respectively. These results indicate that redox mediator modification of NAD(P)-dependent dehydrogenases converts them into quasi-DET-type dehydrogenases, thereby enabling their utilization in 2.5th generation enzymatic sensors, which will facilitate the construction of enzymatic sensors suitable for continuous monitoring systems.

Development of a glycated albumin sensor employing dual aptamer-based extended gate field effect transistors

Glycated albumin (GA), defined as the percentage of serum albumin glycation, is a mid-term glycemic control marker for diabetes. The concentrations of both glycated human serum albumin (GHSA) and total human serum albumin (HSA) are required to calculate GA. Here, we report the development of a GA sensor employing two albumin aptamers: anti-GHSA aptamer which is specific to GHSA and anti-HSA aptamer which recognizes both glycated and non-glycated HSA. We combine these aptamers with extended gate field effect transistors (EGFETs) to realize GA monitoring without the need to pretreat serum samples, and therefore suitable for point of care and home-testing applications. Using anti-GHSA aptamer-immobilized electrodes and EGFETs, we measured GHSA concentrations between 0.1-10 μM within 20 min. The sensor was able to measure GHSA concentration in the presence of BSA for a range of known GA levels (5-29%). With anti-HSA aptamer-immobilized electrodes and EGFETs, we measured total HSA concentrations from 1-17 μM. Furthermore, GHSA and total HSA concentrations of both healthy and diabetic-level samples were determined with GHSA and HSA sensors. The measured GHSA and total HSA concentrations in three samples were used to determine respective GA percentages, and our calculations agreed with GA levels determined by reference methods. Thus, we developed simple and rapid dual aptamer-based EGFET sensors to monitor GA through measuring GHSA and total HSA concentration, without the need for sample pretreatment, a mandatory step in the current standard of enzymatic GA monitoring.

Development of an electrochemical impedance spectroscopy immunosensor for insulin monitoring employing pyrroloquinoline quinone as an ingestible redox probe

Contemporary electrochemical impedance spectroscopy (EIS)-based biosensors face limitations in their applicability for in vivo measurements, primarily due to the necessity of using a redox probe capable of undergoing oxidation and reduction reactions in solution. Although previous investigations have demonstrated the effectiveness of EIS-based biosensors in detecting various target analytes using potassium ferricyanide as a redox probe, its unsuitability for blood or serum measurements, attributed to its inherent toxicity, poses a significant challenge. In response to this challenge, our study adopted a unique approach, focusing on the use of ingestible materials, by exploring naturally occurring substances within the body, with a specific emphasis on pyrroloquinoline quinone (PQQ). Following an assessment of PQQ's electrochemical attributes, we conducted a comprehensive series of EIS measurements. This involved the thorough characterization of the sensor's evolution, starting from the bare electrode and progressing to the immobilization of antibodies. The sensor's performance was then evaluated through the quantification of insulin concentrations ranging from 1 pM to 100 nM. A single frequency was identified for insulin measurements, offering a pathway for potential in vivo applications by combining PQQ as a redox probe with EIS measurements. This innovative approach holds promise for advancing the field of in vivo biosensing based on the EIS method.

Development of Direct Electron Transfer-Type Extended Gate Field Effect Transistor Enzymatic Sensors for Metabolite Detection

In this work, direct electron transfer (DET)-type extended gate field effect transistor (EGFET) enzymatic sensors were developed by employing DET-type or quasi-DET-type enzymes to detect glucose or lactate in both 100 mM potassium phosphate buffer and artificial sweat. The system employed either a DET-type glucose dehydrogenase or a quasi-DET-type lactate oxidase, the latter of which was a mutant enzyme with suppressed oxidase activity and modified with amine-reactive phenazine ethosulfate. These enzymes were immobilized on the extended gate electrodes. Changes in the measured transistor drain current (ID) resulting from changes to the working electrode junction potential (φ) were observed as glucose and lactate concentrations were varied. Calibration curves were generated for both absolute measured ID and ΔID (normalized to a blank solution containing no substrate) to account for variations in enzyme immobilization and conjugation to the mediator and variations in reference electrode potential. This work resulted in a limit of detection of 53.9 μM (based on ID) for glucose and 2.12 mM (based on ID) for lactate, respectively. The DET-type and Quasi-DET-type EGFET enzymatic sensor was then modeled using the case of the lactate sensor as an equivalent circuit to validate the principle of sensor operation being driven through OCP changes caused by the substrate-enzyme interaction. The model showed slight deviation from collected empirical data with 7.3% error for the slope and 8.6% error for the y-intercept.

Affinity sensor for haemoglobin A1c based on single-walled carbon nanotube field-effect transistor and fructosyl amino acid binding protein

Haemoglobin A1c (HbA1c) is a significant glycaemic marker for diabetes mellitus. The level of HbA1c reflects the mean blood glucose level over the prior 2-3 months and it is useful for the assessment of therapeutic effectiveness and for diagnosis. In this study, we report the label-free affinity sensor for HbA1c based on the chemiresistor-type field-effect transistor, which has a simple sensor configuration. Single-walled carbon nanotubes (SWNTs) were used as the transducing element. The fructosyl amino acid binding protein from Rhizobium radiobacter (SocA), which binds to α-fructosyl amino acid specifically, was used as the biorecognition element for fructosyl valine (FV), the product of the proteolytic hydrolysis of HbA1c. The developed sensor shows the ability to measure as low as 1.2 nM FV, which is 14-fold more sensitive compared to the previously reported fluorescence-based sensor using SocA. This sensor also exhibits high specificity where no significant response is observed from either fructosyl lysine (FK) or glucose, which are potential interferents. FK is the ε-fructosyl amino acid from glycated albumin, another glycated protein, whereas glucose is naturally present at very high concentration in the blood. We propose that the modulation of the surface charges on the SWNTs caused by the conformational change in SocA upon ligand binding leads to the proportionate changes in the number of carriers in the SWNT channel.

Crystallization at low salt concentration and alkaline pH and preliminary crystallographic data for a monoclinic form of yeast enolase

Yeast enolase (2-phospho-D-glycerate hydrolyase, E.C. 4.2.1.11) has been crystallized by vapor diffusion from a solution containing 22% PEG 4000, 100 mM Tris buffer pH = 9.3, 200 mM Li(2)SO(4). The crystals are monoclinic with a = 122.5, b = 111.8, c = 63.7 A, beta = 95.6 degrees, space group P2(1) and two dimeric molecules are present in an asymmetric part of the unit cell. Crystals have been successfully transferred to an artificial mother liquor, pH = 7.8, 20 mM in Mg(2+) and 5 mM in 2-phospho-D-glycerate. We believe that under these lower salt concentration and more alkaline conditions we should be able to localize the two metal ions that participate in catalysis as well as examine binding of high-affinity inhibitors.

A novel phosphotyrosine mimetic 4'-carboxymethyloxy-3'-phosphonophenylalanine (Cpp): exploitation in the design of nonpeptide inhibitors of pp60(Src) SH2 domain

The novel phosphotyrosine (pTyr) mimetic 4'-carboxymethyloxy-3'-phosphonophenylalanine (Cpp) has been designed and incorporated into a series of nonpeptide inhibitors of the SH2 domain of pp60(c-Src) (Src) tyrosine kinase. A 2.2 A X-ray crystal structure of 1a bound to a mutant form of Lck SH2 domain provides insight regarding the structure-activity relationships and supports the design concept of this new pTyr mimetic.

Structure-based design of an osteoclast-selective, nonpeptide src homology 2 inhibitor with in vivo antiresorptive activity

Targeted disruption of the pp60(src) (Src) gene has implicated this tyrosine kinase in osteoclast-mediated bone resorption and as a therapeutic target for the treatment of osteoporosis and other bone-related diseases. Herein we describe the discovery of a nonpeptide inhibitor (AP22408) of Src that demonstrates in vivo antiresorptive activity. Based on a cocrystal structure of the noncatalytic Src homology 2 (SH2) domain of Src complexed with citrate [in the phosphotyrosine (pTyr) binding pocket], we designed 3',4'-diphosphonophenylalanine (Dpp) as a pTyr mimic. In addition to its design to bind Src SH2, the Dpp moiety exhibits bone-targeting properties that confer osteoclast selectivity, hence minimizing possible undesired effects on other cells that have Src-dependent activities. The chemical structure AP22408 also illustrates a bicyclic template to replace the post-pTyr sequence of cognate Src SH2 phosphopeptides such as Ac-pTyr-Glu-Glu-Ile (1). An x-ray structure of AP22408 complexed with Lck (S164C) SH2 confirmed molecular interactions of both the Dpp and bicyclic template of AP22408 as predicted from molecular modeling. Relative to the cognate phosphopeptide, AP22408 exhibits significantly increased Src SH2 binding affinity (IC(50) = 0.30 microM for AP22408 and 5.5 microM for 1). Furthermore, AP22408 inhibits rabbit osteoclast-mediated resorption of dentine in a cellular assay, exhibits bone-targeting properties based on a hydroxyapatite adsorption assay, and demonstrates in vivo antiresorptive activity in a parathyroid hormone-induced rat model.

A ligand-reversible dimerization system for controlling protein-protein interactions

Chemically induced dimerization provides a general way to gain control over intracellular processes. Typically, FK506-binding protein (FKBP) domains are fused to a signaling domain of interest, allowing crosslinking to be initiated by addition of a bivalent FKBP ligand. In the course of protein engineering studies on human FKBP, we discovered that a single point mutation in the ligand-binding site (Phe-36 --> Met) converts the normally monomeric protein into a ligand-reversible dimer. Two-hybrid, gel filtration, analytical ultracentrifugation, and x-ray crystallographic studies show that the mutant (F(M)) forms discrete homodimers with micromolar affinity that can be completely dissociated within minutes by addition of monomeric synthetic ligands. These unexpected properties form the basis for a "reverse dimerization" regulatory system involving F(M) fusion proteins, in which association is the ground state and addition of ligand abolishes interactions. We have used this strategy to rapidly and reversibly aggregate fusion proteins in different cellular compartments, and to provide an off switch for transcription. Reiterated F(M) domains should be generally useful as conditional aggregation domains (CADs) to control intracellular events where rapid, reversible dissolution of interactions is required. Our results also suggest that dimerization is a latent property of the FKBP fold: the crystal structure reveals a remarkably complementary interaction between the monomer binding sites, with only subtle changes in side-chain disposition accounting for the dramatic change in quaternary structure.

Selective epimerization of rapamycin via a retroaldol/aldol mechanism mediated by titanium tetraisopropoxide

We describe the efficient and selective epimerization of the immunosuppressant rapamycin to 28-epirapamycin under mild conditions. The mechanism of epimerization involves an equilibrium of the four C28/C29 diastereomers through a two-step retroaldol/aldol (macrocycle ring-opening/ring-closing) sequence. This retroaldol/aldol equilibration is not restricted to rapamycin but is also applicable to acyclic beta-hydroxyketones. A potentially useful extension of the method--the use of beta-hydroxyketones as enolate synthons for effecting inter- or intramolecular aldol reactions under neutral conditions--is demonstrated.

Structure-based design and synthesis of a novel class of Src SH2 inhibitors

The structure-based design and synthesis of a novel class of 2,4-disubstituted thiazoles as Src SH2 inhibitors is described. Initial results are presented, including the X-ray and NMR analysis of one thiazole inhibitor bound to Lck and Src SH2.

Redesigning an FKBP-ligand interface to generate chemical dimerizers with novel specificity

FKBP ligand homodimers can be used to activate signaling events inside cells and animals that have been engineered to express fusions between appropriate signaling domains and FKBP. However, use of these dimerizers in vivo is potentially limited by ligand binding to endogenous FKBP. We have designed ligands that bind specifically to a mutated FKBP over the wild-type protein by remodeling an FKBP-ligand interface to introduce a specificity binding pocket. A compound bearing an ethyl substituent in place of a carbonyl group exhibited sub-nanomolar affinity and 1,000-fold selectivity for a mutant FKBP with a compensating truncation of a phenylalanine residue. Structural and functional analysis of the new pocket showed that recognition is surprisingly relaxed, with the modified ligand only partially filling the engineered cavity. We incorporated the specificity pocket into a fusion protein containing FKBP and the intracellular domain of the Fas receptor. Cells expressing this modified chimeric protein potently underwent apoptosis in response to AP1903, a homodimer of the modified ligand, both in culture and when implanted into mice. Remodeled dimerizers such as AP1903 are ideal reagents for controlling the activities of cells that have been modified by gene therapy procedures, without interference from endogenous FKBP.

A newly identified horseshoe crab lectin with binding specificity to O-antigen of bacterial lipopolysaccharides

We identified a novel horseshoe crab hemocyte-derived lectin, which we named tachylectin-4. It has more potent hemagglutinating activity against human A-type erythrocytes than a previously identified hemocyte lectin with an affinity to N-acetylglucosamine, tachylectin-2. The purified tachylectin-4 is an oligomeric glycoprotein of 470 kDa, composed of subunits of 30 and 31.5 kDa. Ca2+ at 10 mM enhanced the hemagglutinating activity 4-fold, and the activity was inhibited by EDTA and o-phenanthroline. L-Fucose and N-acetylneuraminic acid at 100 mM completely inhibited the activity of tachylectin-4. The activity was also inhibited more strongly by bacterial S-type lipopolysaccharides (LPS) but not by R-type LPS lacking O-antigen. The most effective S-type LPS was from Escherichia coli O111:B4, and the minimum concentration required for inhibiting agglutination against human A-type erythrocytes (0.1 microg/ml) was 160-fold lower than those of S-type LPS from Salmonella minnesota. Therefore, colitose (3-deoxy-L-fucose), a unique sugar present in the O-antigen of E. coli O111:B4 with structural similarity to L-fucose, is the most probable candidate for a specific ligand of tachylectin-4. A cDNA coding for tachylectin-4 was isolated from a hemocyte cDNA library. The open reading frame of the 1344-base pair cDNA coded for the mature protein with 232 amino acids. There is no significant sequence similarity to any other known LPS-binding lectins, whereas tachylectin-4 is homologous to the NH2-terminal domain with unknown functions of Xenopus laevis pentraxin 1.

Solution structure of the C-terminal SH2 domain of the human tyrosine kinase Syk complexed with a phosphotyrosine pentapeptide

BACKGROUND

Recruitment of the intracellular tyrosine kinase Syk to activated immune-response receptors is a critical early step in intracellular signaling. In mast cells, Syk specifically associates with doubly phosphorylated immunoreceptor tyrosine-based activation motifs (ITAMs) that are found within the IgE receptor. The mechanism by which Syk recognizes these motifs is not fully understood. Both Syk SH2 (Src homology 2) domains are required for high-affinity binding to these motifs, but the C-terminal SH2 domain (Syk-C) can function independently and can bind, in isolation, to the tyrosine-phosphorylated IgE receptor in vitro. In order to improve understanding of the cellular function of Syk, we have determined the solution structure of Syk-C complexed with a phosphotyrosine peptide derived from the gamma subunit of the IgE receptor.

RESULTS

The Syk-C:peptide structure is compared with liganded structures of both the SH2 domain of Src and the C-terminal SH2 domain of ZAP-70 (the 70 kDa zeta-associated protein). The topologies of these domains are similar, although significant differences occur in the loop regions. In the Syk-C structure, the phosphotyrosine and leucine residues of the peptide ligand interact with pockets on the protein, and the intervening residues are extended.

CONCLUSIONS

Syk-C resembles other SH2 domains in its peptide-binding interactions and overall topology, a result that is consistent with its ability to function as an independent SH2 domain in vitro. This result suggests that Syk-C plays a unique role in the intact Syk protein. The determinants of the binding affinity and selectivity of Syk-C may reside in the least-conserved structural elements that comprise the phosphotyrosine- and leucine-binding sites. These structural features can be exploited for the design of Syk-selective SH2 antagonists for the treatment of allergic disorders and asthma.

Expression, characterization and crystallographic analysis of telluromethionyl dihydrofolate reductase

Selenomethionine-containing proteins analyzed by multi-wavelength anomalous diffraction provide a facile means of addressing the phase problem, whose solution is necessary to determine protein structures by X-ray crystallography [Hendrickson (1991). Science, 254, 51-58]. Since this method requires synchrotron radiation, we sought to incorporate a true heavy atom into protein, allowing the solution of the phase problem by more traditional methods of data collection. Media containing TeMet alone or TeMet with low levels of Met failed to sustain growth of a methione auxotroph of Escherichia coli carrying the dihydrofolate reductase expression vector. Growth of the organism to stationary phase and incorporation of TeMet was observed when the culture was initiated in media containing minimal Met levels and TeMet was added after induction with isopropyl-1-thio-beta-D-galactopyranoside. The purified enzyme exhibited properties similar to those of the native enzyme. Atomic absorption spectroscopy and amino-acid analysis indicated that 40% of the methionines were replaced with TeMet. Sequence analysis did not indicate significant levels of replacement in the first three sites (1, 16 and 20), suggesting that TeMet was present only in the last two sites (42 and 92). Crystals of this enzyme were grown in the presence of methotrexate and were isomorphous with crystals of wild-type dihydrofolate reductase. Difference Fourier maps and restrained least-squares refinement showed no substitution at the first three methionines, while incorporation was seen at positions 42 and 92.

Catalytic metal ion binding in enolase: the crystal structure of an enolase-Mn2+-phosphonoacetohydroxamate complex at 2.4-A resolution

Enolase, a glycolytic enzyme that catalyzes the dehydration of 2-phospho-D-glycerate (PGA) to form phosphoenolpyruvate (PEP), requires two divalent metal ions per active site for activity. The first metal ion, traditionally referred to as "conformational", binds in a high-affinity site I. The second metal ion, "catalytic", binds in site II only in the presence of a substrate or substrate analogue and with much lower affinity for the physiological cofactor Mg2+. While the high-affinity site has been well characterized, the position of the lower affinity site has not been established so far. Here, we report the structure of the quaternary complex between enolase, the transition-state analogue phosphonoacetohydroxamate (PhAH), and two Mn2+ ions. The structure has been refined by using 16 561 reflections with F/sigma (F) > or = 3 to an R = 0.165 with average deviations of bond lengths and bond angles from ideal values of 0.013 A and 3.1 degrees, respectively. The "catalytic" metal ion is coordinated to two oxygen atoms of the phosphono moiety of PhAH and to the carbonyl oxygen of Gly37. Most likely, disordered water molecules complement its coordination sphere. The interaction with the site II metal ion must stabilize negative charge on the phosphate group and produce electron withdrawal from carbon 2 of the substrate, facilitating proton abstraction from carbon 2, the rate-limiting step in the catalytic process. The Gly37 residue is located in the flexible loop Ser36-His43, which assumes an "open" conformation in the absence of substrate and a "closed" conformation in the presence of a substrate.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural comparisons among the short-chain helical cytokines

BACKGROUND

Cytokines and growth factors are soluble proteins that regulate the development and activities of many cell types. One group of these proteins have structures based on a four-helix bundle, though this similarity is not apparent from amino acid sequence comparisons. An understanding of how diverse sequences can adopt the same fold would be useful for recognizing and aligning distant homologs and for applying structural information gained from one protein to other sequences.

RESULTS

We have approached this problem by comparing the five known structures which adopt a granulocyte-macrophage colony-stimulating factor (GM-CSF)-like, or short-chain fold: interleukin (IL)-4, GM-CSF, IL-2, IL-5, and macrophage colony-stimulating factor. The comparison reveals a common structural framework of five segments including 31 inner-core and 30 largely exposed residues. Buried polar interactions found in each protein illustrate how complementary substitutions maintain protein stability and may help specify unique core packing. A profile based on the known structures is not sufficient to guarantee accurate amino acid sequence alignments with other family members. Comparisons of the conserved short-chain framework with growth hormone define the optimal structural alignment.

CONCLUSIONS

Our results are useful for extrapolating functional results among the short-chain cytokines and growth hormone, and provide a foundation for similar characterization of other subfamilies. These results also show that the placement of polar residues at different buried positions in each protein complicates sequence comparisons, and they document a challenging test case for methods aimed at recognizing and aligning distant homologs.

Protein permeation through poly(vinyl alcohol) hydrogel membranes

Hydrogel membranes were prepared by radiation and chemical cross-linking of poly(vinyl alcohol) (PVA) in aqueous solutions. Effects of PVA concentration, PVA molecular weight and radiation dose, as well as concentration of cross-linking agent, in the case of the chemical cross-linking procedure, on the permeation of insulin, albumin and immunoglobulin (IgG) through the membranes were investigated. Glucose permeation was also studied. The cross-linking density affected the size of the macromolecular mesh of the hydrogel network and thus the water content of the membrane responsible for the diffusion of the solutes. The diffusion coefficient linearly increased for all the solutes with increasing water content in the PVA hydrogels, indicating that diffusion occurs primarily through the water hydrating the polymer network. The permeability study showed that the water content as well as the mesh size had an influence on the diffusion of low molecular weight glucose and insulin. Although the diffusion of higher molecular weight solutes, such as albumin and IgG, was not so much affected by the mesh size of elaborated PVA hydrogel membranes, the diffusion of these proteins was very low.

Identification of the discontinuous binding site in human interleukin 1 beta for the type I interleukin 1 receptor

Human interleukin 1 beta (IL-1 beta) exerts its diverse biological effects by binding to specific receptors on target cells. Two types of IL-1 receptor (IL-1R) have been identified: the type I IL-1R (p80) and the type II IL-1R (p68). Using site-specific mutagenesis, we have identified the binding site on IL-1 beta for the murine type I IL-1R. Analogs of the IL-1 beta protein containing defined amino acid substitutions were produced and tested for competitive binding to the two IL-1Rs. Substitutions of the amino acids at seven positions resulted in analogs that had greater than or equal to 100-fold reductions in competitive binding to the type I IL-1R, while maintaining substantial binding to the type II IL-1R. These seven amino acids (Arg-4, Leu-6, Phe-46, Ile-56, Lys-93, Lys-103, and Glu-105) are clustered in the IL-1 beta molecule, forming a discontinuous binding site. The side chains of all seven residues are exposed on the surface of IL-1 beta. The cumulative binding energies contributed by each of the residues predict a binding affinity that is consistent with the observed Kd of the wild-type protein for the type I IL-1R.

Crystal structure of the intensely sweet protein monellin

Two unusual proteins, discovered in African berries, possess the interesting property of having a very high specificity for the sweet receptors. These proteins, monellin and thaumatin, are approximately 100,000 times sweeter than sugar on a molar basis and several thousand times sweeter on a weight basis. Neither contains carbohydrates or modified amino acids. Several interesting observations have been made about the two proteins: native conformations are important for the sweet taste, although both proteins are intensely sweet, there are no statistically significant sequence similarities between them; and despite the absence of sequence similarity, antibodies against thaumatin compete for monellin (as well as many other sweet compounds, but not for chemically modified non-sweet monellin) and vice versa. To understand the structural basis of these observations we determined the crystal structure of thaumatin, and report here the structure of monellin at 3 A resolution. Monellin consists of two peptide chains, the A chain of 44 residues and the B chain of 50 residues. We find no similarity between the backbone structure of monellin and that of thaumatin.

Three-dimensional structure of thaumatin I, an intensely sweet protein

Thaumatin and monellin are the two sweetest compounds known to man--about 100,000 times sweeter than sugar on a molar basis and 3000 times on a weight basis. These proteins represent a unique class of proteins that are taste-active. We report the three-dimensional structure of thaumatin I at 3.1 A resolution.