Critical molecular regions for elicitation of the sweetness of the sweet-tasting protein, thaumatin I

Thaumatin is an intensely sweet-tasting protein. To identify the critical amino acid residue(s) responsible for elicitation of the sweetness of thaumatin, we prepared mutant thaumatin proteins, using Pichia pastoris, in which alanine residues were substituted for lysine or arginine residues, and the sweetness of each mutant protein was evaluated by sensory analysis in humans. Four lysine residues (K49, K67, K106 and K163) and three arginine residues (R76, R79 and R82) played significant roles in thaumatin sweetness. Of these residues, K67 and R82 were particularly important for eliciting the sweetness. We also prepared two further mutant thaumatin I proteins: one in which an arginine residue was substituted for a lysine residue, R82K, and one in which a lysine residue was substituted for an arginine residue, K67R. The threshold value for sweetness was higher for R82K than for thaumatin I, indicating that not only the positive charge but also the structure of the side chain of the arginine residue at position 82 influences the sweetness of thaumatin, whereas only the positive charge of the K67 side chain affects sweetness.     

Atomic structure of the sweet-tasting protein thaumatin I at pH 8.0 reveals the large disulfide-rich region in domain II to be sensitive to a pH change

Thaumatin, an intensely sweet-tasting plant protein, elicits a sweet taste at 50 nM. Although the sweetness remains when thaumatin is heated at 80 °C for 4 h under acid conditions, it rapidly declines when heating at a pH above 6.5. To clarify the structural difference at high pH, the atomic structure of a recombinant thaumatin I at pH 8.0 was determined at a resolution of 1.0 Å. Comparison to the crystal structure of thaumatin at pH 7.3 and 7.0 revealed the root-mean square deviation value of a Cα atom to be substantially greater in the large disulfide-rich region of domain II, especially residues 154–164, suggesting that a loop region in domain II to be affected by solvent conditions. Furthermore, B-factors of Lys137, Lys163, and Lys187 were significantly affected by pH change, suggesting that a striking increase in the mobility of these lysine residues, which could facilitate a reaction with a free sulfhydryl residue produced via the β-elimination of disulfide bonds by heating at a pH above 7.0. The increase in mobility of lysine residues as well as a loop region in domain II might play an important role in the heat-induced aggregation of thaumatin above pH 7.0.     

Cloning of the thaumatin I cDNA and characterization of recombinant thaumatin I secreted by Pichia pastoris

Thaumatin is a sweet-tasting protein comprising a mixture of some variants. The major variants are thaumatins I and II. Although the amino acid sequence of thaumatin I was known and the nucleotide sequence of cDNA of thaumatin II was elucidated, the nucleotide sequence of thaumatin I has been controversial. We have cloned two thaumatin cDNAs from the fruit of Thaumatococcus daniellii Benth. One is the same nucleotide sequence as that of thaumatin II already reported, and the other is a novel nucleotide sequence. The amino acid sequence deduced from the novel cDNA was the same amino acid sequence as that of thaumatin I, the only exception being the residue at position 113 (Asp instead of Asn), indicating that the novel thaumatin cDNA is that for thaumatin I. This thaumatin I cDNA was transformed into Pichia pastoris X-33, and the recombinant thaumatin I expressed was purified and characterized. The threshold value of sweetness of the recombinant thaumatin I was the same as that of the plant thaumatin I, although several unexpected amino acid residues were attached to the N-terminal of the recombinant thaumatin I. These indicate that the N-terminal portion of thaumatin is not critical for the elicitation of sweetness.     

EFFECT OF ACETYLATION AND METHYLATION ON THE SWEETNESS INTENSITY OF THAUMATIN I

The lysine residues in thaumatin I were chemically modifiedby acetylation with acetic anhydride and by reductive methylation,under various conditions. The acetylated and methylated thaumatinswere isolated by ion-exchange chromatography. The number ofremaining free amino groups was determined by trinitrophenylation. At least four acetylated thaumatins with either one, two, threeor four acetylated amino groups were obtained as well as onemethylated thaumatin with six dimethyl lysine residues and onemonomethyl lysine residue. The sweetness intensity of the acetylated thaumatins decreasedwith the increasing number of acetylated amino groups; the sweettaste had disappeared completely when four amino groups wereacetylated. The methylated thaumatin with seven modified lysineresidues had a sweetness intensity practically equal to thatof the original thaumatin. The total net change, i.e. the isoelectric point of thaumatin,might play a role in the physiological behaviour of thaumatincausing a sweet taste sensation.     

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.     

Cloning, expression and characterization of recombinant sweet-protein thaumatin II using the methylotrophic yeast Pichia pastoris

Thaumatin, an intensely sweet-tasting protein, was secreted by the methylotrophic yeast Pichia pastoris. The mature thaumatin II gene was directly cloned from Taq polymerase-amplified PCR products by using TA cloning methods and fused the pPIC9K expression vector that contains Saccharomyces cerevisiae prepro alpha-mating factor secretion signal. Several additional amino acid residues were introduced at both the N- and C-terminal ends by genetic modification to investigate the role of the terminal end region for elicitation of sweetness in the thaumatin molecule. The secondary and tertiary structures of purified recombinant thaumatin were almost identical to those of the plant thaumatin molecule. Recombinant thaumatin II elicited a sweet taste as native plant thaumatin II; its threshold value of sweetness to humans was around 50 nM, which is the same as that of plant thaumatin II. These results demonstrate that the functional expression of thaumatin II was attained by Pichia pastoris systems and that the N- and C-terminal regions of the thaumatin II molecule do not -play an important role in eliciting the sweet taste of thaumatin.     

Effects of chemical modification of lysine residues on the sweetness of lysozyme

Lysozyme is a sweet-tasting protein with a sweetness threshold value of around 7 microM. To clarify the effect of basicity at the side chain of lysine residues on the threshold values of sweetness, charge-specific chemical modifications such as guanidination, acetylation and phosphopyridoxylation of lysine residues were performed. Sensory analysis showed that the sweetness threshold value of lysozyme was not changed by guanidination, whereas it was increased markedly by acetylation and phosphopyridoxylation. To confirm the importance of the basicity in the lysine residues in detail, purification of acetylated (Ac-) and phosphopyridoxylated (PLP-) lysozymes using SP-ion exchange column chromatography was performed. The threshold values were not changed by modification with fewer than two residues (approximately 7 microM), whereas the threshold values significantly increased to 15 and 34 microM when tetra-Ac and tri-PLP, respectively. Furthermore, sweetness was not detected at 30 microM (hexa-, penta-Ac and tetra-PLP). It should be noted that removal of the negative charges of the phosphate groups in the tri-PLP lysozyme by acid phosphatase resulted in the recovery of sweetness (6.4 microM), indicating that basicity at the position of the lysine residues is responsible for lysozyme sweetness and that strict charge complementarities might be required for interaction to its putative receptor.     

Introduction of a negative charge at Arg82 in thaumatin abolished responses to human T1R2-T1R3 sweet receptors

Thaumatin, an intensely sweet-tasting protein, elicits a sweet-taste sensation at a level as low as 50 nM. Although previous sensory analyses have suggested that Lys67 and Arg82 are important to the sweetness of thaumatin, the exact effects of each residue on sweet receptors are still unknown. In the present study, various mutants of thaumatin altered at Arg82 as well as Lys67 were prepared and their sweetness levels were quantitatively evaluated by cell-based assays using HEK293 cells expressing human sweet receptors. Mutations at Arg82 had a more deteriorative effect on sweetness than mutations at Lys67. Particularly, a charge inversion at Arg82 (R82E) resulted in an abolishment of the response to sweet receptors even at a concentration as high as 1 mM. These results indicate that Arg82 plays a central role in determining the sweetness of thaumatin. A strict spatial charge location at residue 82 appears to be required for interaction with sweet receptors.     

Crystal structure of the sweet-tasting protein thaumatin II at 1.27Å

Thaumatin, an intensely sweet-tasting protein, elicits a sweet taste sensation at 50 nM. Here the X-ray crystallographic structure of one of its variants, thaumatin II, was determined at a resolution of 1.27 ?. Overall structure of thaumatin II is similar to thaumatin I, but a slight shift of the Cα atom of G96 in thaumatin II was observed. Furthermore, the side chain of residue 67 in thaumatin II is highly disordered. Since residue 67 is one of two residues critical to the sweetness of thaumatin, the present results suggested that the critical positive charges at positions 67 and 82 are disordered and the flexibility and fluctuation of these side chains would be suitable for interaction of thaumatin molecules with sweet receptors.     

Crystal structure of the sweet-tasting protein thaumatin II at 1.27 Å

Thaumatin, an intensely sweet-tasting protein, elicits a sweet taste sensation at 50 nM. Here the X-ray crystallographic structure of one of its variants, thaumatin II, was determined at a resolution of 1.27 Å. Overall structure of thaumatin II is similar to thaumatin I, but a slight shift of the Cα atom of G96 in thaumatin II was observed. Furthermore, the side chain of residue 67 in thaumatin II is highly disordered. Since residue 67 is one of two residues critical to the sweetness of thaumatin, the present results suggested that the critical positive charges at positions 67 and 82 are disordered and the flexibility and fluctuation of these side chains would be suitable for interaction of thaumatin molecules with sweet receptors.     

Identification of amino acid residues contributing to the ATP-binding site of a purinergic P2X receptor

P2X receptor subunits have intracellular N and C termini, two membrane-spanning domains, and an extracellular loop of about 280 amino acids. We expressed the rat P2X(2) receptor in human embryonic kidney cells, and used alanine-scanning mutagenesis on 30 residues with polar side chains conserved among the seven rat P2X receptor subunits. This identified a region proximal to the first transmembrane domain which contained 2 lysine residues that were critical for the action of ATP (Lys(69) and Lys(71)). We substituted cysteines in this region (Asp(57) to Asp(71)) and found that for S65C and I67C ATP-evoked currents were inhibited by methanethiosulfonates. At I67C, the inhibition by negatively charged ethylsulfonate and pentylsulfonate derivatives could be overcome by increasing the ATP concentration, consistent with a reduced affinity of ATP binding. The inhibitory action of the methanethiosulfonates was prevented by pre-exposure to ATP, suggesting occlusion of the binding site. Finally, introduction of negative charges into the receptor by mutagenesis at this position (I67E and I67D) also gave receptors in which the ATP concentration-response curve was right-shifted. The results suggest that residues close to Ile(67) contribute to the ATP-binding site.     

A structure-activity study of thaumatin using pyridoxal 5'-phosphate (PLP) as a probe

The structural features responsible for the sensory propertiesof the sweet protein, thaumatin, have been investigated by sidechain modification of amino acid residues using pyridoxal 5'-phosphate(PLP). PLP molecules bind covalently to proteins by reactingwith the -amino group and the -amino group of lysine residues.Spectral and sensory studies have been performed on thaumatin-PLPderivatives prepared at various molar ratios. The incorporationof one mole of PLP into thaumatin causes substantial modificationof the sensory properties which include generation of astringency,an unpleasant taste and the loss of sweetness intensity. Theintroduction of more than one mole of PLP has no further effecton the gustatory properties of thaumatin. Removal by alkalinephosphatase of the phosphate group of PLP bound to thaumatinhas no influence on the ability of PLP to modify the sensorycharacteristics of thaumatin. This suggests that the sensoryalteration caused by PLP cannot be ascribed to the changes inthe net charge of the protein, but is likely to be due to themodification of specific lysine residue(s) which are thus implicatedin the sweet site.     

Reductive alkylation of lysine residues in subtilisin DY

Only lysine epsilon-amino groups (and the N-terminal alpha-amino group) in native subtilisin DY were reductively alkylated by glyceraldehyde in the presence of sodium cyanoborohydride. The modified protein molecule was cleaved by TosPheCH2Cl-trypsin or cyanogen bromide and the two sets of peptides obtained were fractionated and purified by gel filtration and HPLC. For determination of the degree of modification of each lysine residue, selected peptides were subjected to sequence analysis combined with quantitative estimation of the containing PTH-Lys and PTH-epsilon-DHP-Lys. The data obtained showed that the lysine residues in positions 12, 15, 27, 43, 136, 141, 265 were entirely modified, those in positions 170, 184, 237 were partially modified, and Lys22 and Lys94 were unaccessible for the reagent. The caseinolytic activity decreased by 23% when the maximum number of lysine residues (8.6 of the total 12 residues) in subtilisin DY were modified. The CD-spectra of native and modified enzyme showed only slight differences. Both these experiments suggest that the lysine residues do not take part directly in the catalytic reaction but are responsible for maintaining the native three-dimensional enzyme structure. The data obtained for the accessibility of the different lysine residues in subtilisin DY correlated very well with the positions of these residues in a video model of the structure of subtilisin Carlsberg, thus suggesting that the spatial structures of these two enzymes are very similar.     

Alkylamine derivatives of cytochrome c. Comparison with other lysine-modified analogues illuminates structure/function relations in the protein

For investigations of the functional roles of the lysine residues of cytochrome c, analogues in which these residues are modified without charge loss are highly desirable. The 19 lysine residues of the horse heart protein have been modified by reductive alkylation. Two analogues were prepared, using formaldehyde and acetone as the dialkylating and monoalkylating reagent respectively. Modification was shown to be clean and quantitative. Characterisation of the alkylamine derivatives by physicochemical measurements and biological activity determinations was carried out. The potential of these analogues in structure/function studies of cytochrome c is discussed. It is illustrated by their use, in conjunction with other lysine-modified derivatives, to investigate the extent to which surface charge determines redox potential, and to study the physicochemical changes that accompany rising pH. In the latter case the observed phenomena are not as closely correlated as previously thought, suggesting that there is a complex set of rearrangements of structure underlying the functional changes. The data confirm that modification of the lysine residues influences these changes. These residues participate in numerous surface intramolecular links, so the lack of correlation may be explained if each of the changing parameters were under the influence of a different set of residues. However, neither a lysine residue, nor a histidine residue directly displaces methionine from the sixth coordination position of the haem iron at alkaline pH.     

The role of individual lysine residues in the basic patch on turnip cytochrome f for electrostatic interactions with plastocyanin in vitro.

The role of electrostatic interactions in determining the rate of electron transfer between cytochrome f and plastocyanin has been examined in vitro with mutants of turnip cytochrome f and mutants of pea and spinach plastocyanins. Mutation of lysine residues Lys58, Lys65 and Lys187 of cytochrome f to neutral or acidic residues resulted in decreased binding constants and decreased rates of electron transfer to wild-type pea plastocyanin. Interaction of the cytochrome f mutant K187E with the pea plastocyanin mutant D51K gave a further decrease in electron transfer rate, indicating that a complementary charge pair at these positions could not compensate for the decreased overall charge on the proteins. Similar results were obtained with the interaction of the cytochrome f mutant K187E with single, double and triple mutants of residues in the acidic patches of spinach plastocyanin. These results suggest that the lysine residues of the basic patch on cytochrome f are predominantly involved in long-range electrostatic interactions with plastocyanin. However, analysis of the data using thermodynamic cycles provided evidence for the interaction of Lys187 of cytochrome f with Asp51, Asp42 and Glu43 of plastocyanin in the complex, in agreement with a structural model of a cytochrome f-plastocyanin complex determined by NMR.     

Crystal structure of enteropeptidase light chain complexed with an analog of the trypsinogen activation peptide.

Enteropeptidase is a membrane-bound serine protease that initiates the activation of pancreatic hydrolases by cleaving and activating trypsinogen. The enzyme is remarkably specific and cleaves after lysine residues of peptidyl substrates that resemble trypsinogen activation peptides such as Val-(Asp)4-Lys. To characterize the determinants of substrate specificity, we solved the crystal structure of the bovine enteropeptidase catalytic domain to 2.3 A resolution in complex with the inhibitor Val-(Asp)4-Lys-chloromethane. The catalytic mechanism and contacts with lysine at substrate position P1 are conserved with other trypsin-like serine proteases. However, the aspartyl residues at positions P2-P4 of the inhibitor interact with the enzyme surface mainly through salt bridges with the Nzeta atom of Lys99. Mutation of Lys99 to Ala, or acetylation with acetic anhydride, specifically prevented the cleavage of trypsinogen or Gly-(Asp)4-Lys-beta-naphthylamide and reduced the rate of inhibition by Val-(Asp)4-Lys-chloromethane 22 to 90-fold. For these reactions, Lys99 was calculated to account for 1.8 to 2.5 kcal mol(-1) of the free energy of transition state binding. Thus, a unique basic exosite on the enteropeptidase surface has evolved to facilitate the cleavage of its physiological substrate, trypsinogen.     

Identification of secretin,vasoactive intestinal peptide and glucagon binding sites: from chimaeric receptors to point mutations

We have identified two basic residues that are important for the recognition of secretin and vasoactive intestinal peptide (VIP) by their respective receptors. These two peptides containing an Asp residue at position 3 interacted with an arginine residue in transmembrane helix 2 (TM2) of the receptor, and the lysine residue in extracellular loop 1 (ECL1) stabilized the active receptor conformation induced by the ligand. The glucagon receptor possesses a Lys instead of an Arg in TM2, and an Ile instead of Lys in ECL1; it markedly prefers a Gln side chain in position 3 of the ligand. Our results suggested that, in the wild-type receptor, the Ile side chain prevented access to the TM2 Lys side chain, but oriented the glucagon Gln(3) side chain to its proper binding site. In the double mutant, the ECL1 Lys allowed an interaction between negatively charged residues in position 3 of glucagon and the TM2 Arg, resulting in efficient receptor activation by [Asp(3)]glucagon as well as by glucagon.     

Accessibility and mobility of lysine residues in beta-lactoglobulin

N epsilon-[2H6]Isopropyllysyl-beta-lactoglobulin was prepared by reductive alkylation of beta-lactoglobulin with [2H6]acetone and NaBH4 to provide a 2H (NMR) probe for the study of lysine involvement in lipid-protein interactions. Amino acid analysis showed 80% of the protein's 15 lysine residues to be labeled. Unmodified lysine residues were located through peptide maps produced from CNBr, tryptic, and chymotryptic digests of the labeled protein. Lys47 was not modified; Lys135,138,141, located along an amphipathic helical rod, were each partially unmodified. All other lysine residues were at least 90% modified. Average correlation times calculated from 2H NMR spectra were 20 and 320 ps for 8.7 and 3.3 residues, respectively, in 6 M guanidine hydrochloride; in nondenaturing solution, values of 70 and 320 ps were obtained for 6.5 and 3.2 residues, respectively, with the remaining 2.3 modified residues not observed, suggesting that side chains of lysine residues in unordered or flexible regions were more mobile than those in stable periodic structures. 2H NMR spectra of the protein complexed with dipalmitoylphosphatidylcholine confirmed the extrinsic membrane protein type behavior of beta-lactoglobulin previously reported from 31P NMR studies of the phospholipids complexed with beta-lactoglobulin. Although no physiological function has yet been identified, comparison of these results with the X-ray structure [Papiz et al. (1986) Nature (London) 324, 383-385] supports the hypothesis that residues not accessible for modification may help to stabilize the cone-shaped beta-barrel thought to contain binding sites for small lipid-soluble molecules.     

Probing the sweet determinants of brazzein: wild-type brazzein and a tasteless variant, brazzein-ins(R18a-I18b), exhibit different pH-dependent NMR chemical shifts

Brazzein is a small, intensely sweet protein. As a probe of the functional properties of its solvent-exposed loop, two residues (Arg-Ile) were inserted between Leu18 and Ala19 of brazzein. Psychophysical testing demonstrated that this mutant is totally tasteless. NMR chemical shift mapping of differences between this mutant and brazzein indicated that residues affected by the insertion are localized to the mutated loop, the region of the single alpha-helix, and around the Cys16-Cys37 disulfide bond. Residues unaffected by this mutation included those near the C-terminus and in the loop connecting the alpha-helix and the second beta-strand. In particular, several residues of brazzein previously shown to be essential for its sweetness (His31, Arg33, Glu41, Arg43, Asp50, and Tyr54) exhibited negligible chemical shift changes. Moreover, the pH dependence of the chemical shifts of His31, Glu41, Asp50, and Tyr54 were unaltered by the insertion. The insertion led to large chemical shift and pKa perturbation of Glu36, a residue shown previously to be important for brazzein's sweetness. These results serve to refine the known sweetness determinants of brazzein and lend further support to the idea that the protein interacts with a sweet-taste receptor through a multi-site interaction mechanism, as has been postulated for brazzein and other sweet proteins (monellin and thaumatin).     

PI-PfuI and PI-PfuII, intein-coded homing endonucleases from Pyrococcus furiosus. II. Characterization Of the binding and cleavage abilities by site-directed mutagenesis.

PI- Pfu I and PI- Pfu II from Pyrococcus furiosus are homing endonucleases, as shown in the accompanying paper. These two endonucleases are produced by protein splicing from the precursor protein including ribonucleotide reductase (RNR). We show here that both enzymes specifically interact with their substrate DNA and distort the DNA strands by 73 degrees and 67 degrees, respectively. They have two copies of the amino acid sequence motif LAGLIDADG, which is present in the majority of homing endonucleases and provides some of the catalytic residues necessary for DNA cleavage activity. Site-specific mutagenesis studies showed that two acidic residues in the motifs, Asp149 and Glu250 in PI- Pfu I, and Asp156 and Asp249 in PI- Pfu II, were critical for catalysis. The third residues of the active site triads, as predicted from the structure of PI- Sce I, were Asn225 in PI- Pfu I and Lys224 in PI- Pfu II. Substitution of Asn225 in PI- Pfu I by Ala did not affect catalysis. The cleavage activity of PI- Pfu II was 50-fold decreased by the substitution of Ala for Lys224. The binding affinity of the mutant protein for the substrate DNA also decreased 6-fold. The Lys in PI- Pfu II may play a direct or indirect role in catalysis of the endonuclease activity.