Binding of 4-methylumbelliferyl n-acetyl-chitooligosaccharides to wheat-germ agglutinin. A reinvestigation of equilibrium studies

Folding and homodimerization of wheat germ agglutinin

Wheat germ agglutinin (WGA) is emblematic of proteins that specialize in the recognition of carbohydrates. It was the first lectin reported to have a capacity for discriminating between normal and malignant cells. Since then, it has become a preferred model for basic research and is frequently considered in the development of biomedical and biotechnological applications. However, the molecular basis for the structural stability of this homodimeric lectin remains largely unknown, a situation that limits the rational manipulation and modification of its function. In this work we performed a thermodynamic characterization of WGA folding and self-association processes as a function of pH and temperature by using differential scanning and isothermal dilution calorimetry. WGA is monomeric at pH 2, and one of its four hevein-like domains is unfolded at room temperature. Under such conditions, the agglutinin exhibits a fully reversible thermal unfolding that consists of three two-state transitions. At higher pH values, the protein forms weak, nonobligate dimers. This behavior contrasts with that observed for the other plant lectins studied thus far, which form strong, obligate oligomers, indicating a distinctly different molecular basis for WGA function. For dimer formation, the four domains must be properly folded. Nevertheless, depending on the solution conditions, self-association may be coupled with folding of the labile domain. Therefore, dimerization may proceed as a rigid-body-like association or a folding-by-binding event. This hybrid behavior is not seen in other plant lectins. The emerging molecular picture for the WGA assembly highlights the need for a reexamination of existing ligand-binding data in the literature.

Lactotriaose-containing carbosilane dendrimers: Syntheses and lectin-binding activities

Carbosilane dendrimers periphery-functionalized with lactotriaose (GlcNAcbeta1-3Galbeta1-4Glc) with valencies of three, four, six, and twelve were prepared for use in a lectin-binding assay. A lactotriaose derivative was prepared from D-glucosamine and D-lactose derivatives. The N-Troc-protected glucosamine glycosyl donor and 3'-O-unprotected lactose glycosyl acceptor were condensed in the presence of silver trifluoromethanesulfonate and methylsulfenyl bromide to provide corresponding trisaccharide with new beta-1-3 linkages in 92% yield. The protection group of the trisaccharide was transformed into an acetyl group. The 4-pentenyl glycoside was prepared from the acetate via glycosyl bromide. The alkene was converted into acetyl sulfide by addition of thioacetic acid under radical conditions. The lactotriaose unit was linked with carbosilane dendrimers to afford acetyl-protected glycodendrimers. De-O-acetylation of the dendrimers was carried out in the presence of sodium methoxide and then aq NaOH to give the desired lactotriaose clusters using a carbosilane dendrimer backbone. Their biological activities toward WGA were evaluated by fluorescence methods. The binding constants of free lactotriaose and trivalent, tetravalent, hexavalent, and dodecavalent glycodendrimers to WGA were determined to be 1.1x10(3), 4.4x10(4), 5.1x10(4), 2.8x10(6), and 1.3x10(6) M-1, respectively. The hexavalent glycodendrimer showed a 2500-fold larger binding effect than that of free lactotriaose.

Cytoagglutination and cytotoxicity of Wheat Germ Agglutinin isolectins against normal lymphocytes and cultured leukemic cell lines--relationship between structure and biological activity

The relationships between degree of lectin-cell binding, cytotoxicity and cytoagglutinating activity of three Wheat Germ Agglutinin isolectins (WGA-1, WGA-2, WGA-3) against normal lymphocytes and cultured leukemic cell lines (Jurkat, MOLT-4, Raji, Daudi, K-562) were studied. All WGA-isolectins interacted in a similar degree with normal lymphocytes, while in the case of leukemic cells, the degree of isolectin-cell binding increased in the order: WGA-1< or =WGA-3<WGA-2 at isolectin concentrations 0.5 microM and higher, and WGA-3<WGA-2< or =WGA-1 at 0.25 microM isolectin concentration. The WGA interacted in higher degree with Jurkat, Raji, Daudi and K-562, followed by MOLT-4 and normal lymphocytes. The velocity of cytoagglutination in the presence of 0.25 microM WGA-isolectins increased in the order: WGA-3<WGA-2< or =WGA-1, and was better expressed in Jurkat, Raji, Daudi and K-562, followed by MOLT-4 and normal lymphocytes. The cytotoxicity of isolectins was very well expressed against Jurkat, MOLT-4, Raji and Daudi, and less expressed against K-562 and normal lymphocytes. In the case of leukemic cells, the cytotoxic effect of WGA-isolectins increased in the order: WGA-3<WGA-2=WGA-1. A very good positive correlation was determined between velocity of cytoagglutination and degree of lectin-cell binding (r=0.77, P<0.001). A good inverse correlation was found between cytotoxicity and degree of lectin-cell binding (r=-0.34, P<0.001), and poor correlation was observed between cytotoxicity and cytoagglutinating activity of WGA-isolectins (r=0.16, P<0.01). The results suggest that the WGA-isolectins, structurally distinguishable in only several amino acid sequences, interacted in different degrees with leukemic cells and manifested different cytoagglutinating and cytotoxic activity.

Interactions of wheat-germ agglutinin with GlcNAc beta 1,6Gal sequence

The interactions of wheat-germ agglutinin (WGA) with the GlcNAc beta 1,6Gal sequence, a characteristic component of branched poly-N-acetyllactosaminoglycans, were investigated using isothermal titration calorimetry and X-ray crystallography. GlcNAc beta 1,6Gal exhibited an affinity greater than GlcNAc beta 1,4GlcNAc to all WGA isolectins, whereas Gal beta 1,6GlcNAc showed much less affinity than GlcNAc beta 1,4GlcNAc. X-ray structural analyses of the glutaraldehyde-crosslinked WGA isolectin 3 crystals in complex with GlcNAc beta 1,6Gal, GlcNAc beta 1,4GlcNAc and GlcNAc beta 1,6Gal beta 1,4Glc were performed at 2.4, 2.2 and 2.2 A resolution, respectively. In spite of different glycosidic linkages, GlcNAc beta 1,6Gal and GlcNAc beta 1,4GlcNAc exhibited basically similar binding modes to each other, in contact with side chains of two aromatic residues, Tyr64 and His66. However, the conformations of the ligands in the two primary binding sites were not always identical. GlcNAc beta 1,6Gal showed more extensive variation in the parameters defining the glycosidic linkage structure compared to GlcNAc beta 1,4GlcNAc, demonstrating large conformational flexibility of the former ligand in the interaction with WGA. The difference in the ligand binding conformation was accompanied by alterations of the side chain conformation of the amino acid residues involved in the interactions. The hydrogen bond between Ser62 and the non-reducing end GlcNAc was always observed regardless of the ligand type, indicating the key role of this interaction. In addition to the hydrogen bonding and van der Waals interactions, CH--pi interactions involving Tyr64, His66 and Tyr73 are suggested to play an essential role in determining the ligand binding conformation in all complexes. One of the GlcNAc beta 1,6Gal ligands had no crystal packing contact with another WGA molecule, therefore the conformation might be more relevant to the interaction mode in solution.

Site-directed selection of oligonucleotide antagonists by competitive elution

Oligonucleotide ligands that bind a protein or a small molecule of interest are readily isolated by in vitro selection and amplification of rare sequences from combinatorial libraries of sequence-randomized oligonucleotides (Gold et al., 1995). Classic systematic evolution of ligands by exponential enrichment (SELEX) protocols are affinity based (Tuerk and Gold, 1990), but because many problems and applications require antagonists, protocols for selecting inhibitors are both desirable and valuable. A widely applicable approach for isolating inhibitors is competitive elution with a molecule that binds the targeted molecule's active or binding site. We have used this approach to isolate antagonists of wheat germ agglutinin (WGA) from a library of 2'NH2-pyrimidine, 2'OH-purine oligonucleotides by elution with N N' N"-triacetylchitotriose, (GlcNAc)3. The highest affinity aptamers have equilibrium dissociation constants of 1 nM-20 nM for WGA, a 10(3)-10(4)-fold improvement relative to (GlcNAc)3, and unlike the carbohydrate, are highly specific. In addition to competing for binding with (GlcNAc)3, aptamers inhibit WGA-mediated agglutination of sheep erythrocytes, demonstrating that they are able to compete with natural ligands presented on the surfaces of cells. These results illustrate the feasibility of isolating high-affinity, high-specificity antagonists by competitive elution with low molecular weight, relatively low-affinity, and low-specificity small molecules.

Fast and slow tracks in lysozyme folding: insight into the role of domains in the folding process

The folding of lysozyme involves parallel events in which hydrogen exchange kinetics indicate the development of persistent structure at very different rates. We have monitored directly the kinetics of formation of the native molecule by the binding of a fluorescently labelled inhibitor, MeU-diNAG (4-methylumbelliferyl-N,N'-diacetyl-beta-D-chitobioside). The data show that native character monitored in this way also develops with different timescales. Although the rate determining step on the slow pathway (approximately 75% of molecules at pH 5.5, 20 degrees C) can be attributed to the need to reorganise structure formed early in the folding process, the data indicate that the rate determining step on the fast track (involving approximately 25% of molecules) involves the docking of the two constituent domains of the protein. In the fast folding track the data are consistent with a model in which each domain forms persistent structure prior to their docking in a locally cooperative manner on a timescale comparable to the folding of small single domain proteins.

Site-directed mutagenesis and sugar-binding properties of the wheat germ agglutinin mutants Tyr73Phe and Phe116Tyr

Wheat germ agglutinin is a dimeric lectin composed of two identical subunits. Each subunit consists of four homologous hevein-like domains of 42 or 43 amino acids each. Amino acid residues at the same position in each domain involved in sugar binding are thought to play a similar role in sugar binding. In order to clarify the role of the amino acid residue at domain position 30 of wheat germ agglutinin isolectin 2 (WGA2) in sugar binding, two WGA2 variants each containing a mutation, either Tyr73-->Phe (domain B) or Phe116-->Tyr (domain C), were produced. The binding activity for (GlcNAc)3 and the three-dimensional structure of these mutants were characterized by comparing with the properties of wild-type WGA2. Equilibrium dialysis experiments using (GlcNAc)3 indicated that the mutation Tyr73-->Phe reduced the overall sugar-binding activity at both pH 5.9 and pH 4.7. In addition, positive cooperativity toward (GlcNAc)3 binding was observed at pH 4.7. In contrast, the mutation of Phe116-->Tyr increased the overall sugar-binding activity at pH 5.9, but reduced this activity at pH 4.7 without changing the number of sugar-binding sites. Positive cooperativity was not observed at pH 5.9 or pH 4.7. X-ray crystallographic analysis of mutant WGA2 revealed that the mutation of Tyr73-->Phe caused a side chain movement of the Glu115 residue of the opposite subunit that formed a hydrogen bond with Tyr73 in wild-type WGA2. No changes were observed in the backbone structure and the disposition of the benzene ring of Phe73. The mutation Phe116-->Tyr caused the formation of a new hydrogen bond between Tyr116 and Glu72 of the opposite subunit. The changes in the sugar-binding properties in WGA2 mutants are discussed in relation to the structural change at the binding site.

Conversion of egg-white lysozyme to a lectin-like protein with agglutinating activity analogous to wheat germ agglutinin

Oligomers made of Asp-52-esterified lysozyme or native one showed agglutination for human, mouse, rat and chicken erythrocytes, and also for Hep G2 cells (liver carcinoma-derived cell line) [1] not weaker than wheat germ agglutinin (WGA). Oligomers of Asp-52-esterified lysozyme have about 4-fold stronger agglutinating activity than those of the native one. These results indicate that some enzymes can be converted to lectin-like proteins (neolectin) with binding characteristics similar to their substrate specificity.

Tertiary interactions in the folding pathway of hen lysozyme: kinetic studies using fluorescent probes

The refolding kinetics of hen lysozyme have been studied using a range of fluorescent probes. These experiments have provided new insight into the nature of intermediates detected in our recent hydrogen-exchange labeling studies [Radford, S.E., et al. (1992) Nature 358, 302-307], which were performed under the same conditions. Protection from exchange results primarily from the development of stabilizing side-chain interactions, and the fluorescence studies reported here have provided a new perspective on this aspect of the refolding process. The intrinsic fluorescence of the six tryptophan residues and its susceptibility to quenching by iodide have been used to monitor the development of hydrophobic structure, and these studies have been complemented by experiments involving binding to a fluorescent hydrophobic dye 1-anilino-naphthalenesulfonic acid (ANS). Formation of fixed tertiary interactions of aromatic residues has been monitored by near-UV circular dichorism, while development of a competent active site has been probed by binding to a competitive inhibitor bearing a fluorescent label, 4-methylumbelliferyl-N,N'-diacetyl-beta-chitobiose. The combination of these techniques has enabled us to monitor the development both of the hydrophobic core of the protein and of interactions between the two folding domains. If the behavior of the tryptophans is representative of the hydrophobic residues of the protein in general, it seems that collapse is already substantial in species formed within the first few milliseconds of refolding and is highly developed in later intermediates which nonetheless appear to lack many fixed tertiary interactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural and functional changes associated with cyanogen bromide treatment of wheat germ agglutinin

The lectin, wheat germ agglutinin (WGA), has been shown to have a significant larvicidal effect on the European corn borer, a major insect pest of corn. In order to characterize this toxic effect, we have undertaken structure-function studies on WGA. To this extent, the effect of cyanogen bromide (CNBr) on the conformation, subunit interactions, and biological activity of WGA has been investigated. The CNBr-modified lectin exhibits no toxicity to the ECB, cannot dimerize, does not bind to N-acetylglucosamine or its polymers, has no or vastly reduced hemagglutinating activity against red blood cells of different animals, and shows loss of an antigenic determinant by immunodiffusion. The CD spectrum of CNBr-WGA is not significantly different from that of native WGA, although the intrinsic fluorescence shows about 30% quenching. Our results suggest that the integrity of the N-terminal domain of WGA is essential for dimer formation. Furthermore, toxicity of WGA to ECB may be intrinsically related to its ability to dimerize and bind to sugar residues.

Physicochemical studies of binding of 4-methylumbelliferyl beta-D-galactopyranoside to cold agglutinin

The fluorescence of 4-methylumbelliferyl beta-D-galactopyranoside (MeUmbGalp) was quenched in the presence of cold agglutinin, showing that there was binding between MeUmbGalp and cold agglutinin. That binding was saccharide-specific. By using this quenching phenomenon, the association constants (Ka) of the binding of cold agglutinin at different temperatures (10 degrees C and 15 degrees C) to MeUmbGalp and also the number of binding sites were calculated. The Ka values were found to be 2.63 x 10(3) M-1 at 10 degrees C and 1.58 x 10(3) M-1 at 15 degrees C. Though there is a change in Ka values, the number of binding sites was calculated to be six at both temperatures (10 degrees C and 15 degrees C). From the Ka values the thermodynamic parameters (free energy, enthalpy and entropy) of the binding were derived, and analysis of the data indicated that the binding is spontaneous, exothermic and hydrophobic in nature.

The binding of N-trifluoroacetyl chito-oligosaccharides to wheat-germ agglutinin: a fluorescence investigation

We describe the synthesis of N-trifluoroacetyl chito-oligosaccharides and their use as ligands to probe the binding sites of wheat-germ agglutinin, a lectin specific for N-acetylglucosamine. The binding is monitored using intrinsic protein fluorescence, which is due to tryptophan side-chains. We present arguments purporting to show the presence of a fluorophore close to each of the four sites. The binding of chito-oligosaccharides to wheat-germ agglutinin is complex and can only be approximately described by an independent and equivalent sites model. This model applies when the ligand concentration range is restricted to higher values. The possible role of ligand-mediated protein aggregation and of site inequivalence is discussed. We find that the affinity of trifluoroacetylated chito-oligosaccharides for wheat-germ agglutinin is higher than that of the N-acetylated parent compounds, the difference increasing with chain length. Our results are in agreement with a model of the binding site previously proposed by Clegg et al. (Biochemistry 22 (1983) 4797-4804).

Comparative binding of wheat germ agglutinin and its succinylated form on lymphocytes

We have compared the binding parameters of native wheat germ agglutinin (WGA) and its succinylated form (SWGA) to rat lymphocytes. Scatchard plots were obtained with the fluoresceinated lectins in a concentration range of 10 nM to 0.1 mM. Association and dissociation rate parameters were also measured. The following differences were observed: at low concentration of WGA, binding is positively cooperative with a Hill coefficient of 1.75, whereas binding of SWGA is not. The numbers of high-affinity sites are respectively (2.5 +/- 0.8) X 10(6) and (6.4 +/- 1.3) X 10(5) for WGA and SWGA. Association constants were found to be (4.7 +/- 1.7) X 10(6) l mol-1 for WGA and (1.42 +/- 0.36) X 10(7) l mol-1 for SWGA, which is 35 times higher than for native WGA. Neuraminidase treatment decreases the Hill coefficient as well as the number of sites involved in the cooperative binding of native WGA. Equilibrium data were obtained at three temperatures to determine the thermodynamic parameters (delta H degree and delta S degree). These results are indicative of an oligomerization process dynamically formed at the membrane level before tight binding of the lectin to its receptors could occur.

Thermodynamic and kinetic studies on saccharide binding to soya-bean agglutinin

The fluorescence of N-dansylgalactosamine [N-(5-dimethylaminonaphthalene-1-sulphonyl)galactosamine] was enhanced 11-fold with a 25 nm blue-shift in the emission maximum upon binding to soya-bean agglutinin (SBA). This change was used to determine the association constants and thermodynamic parameters for this interaction. The association constant of 1.51 X 10(6) M-1 at 20 degrees C indicated a very strong binding, which is mainly due to a relatively small entropy value, as revealed by the thermodynamic parameters: delta G = -34.7 kJ X mol-1, delta H = -37.9 kJ X mol-1 and delta S = -10.9 J X mol-1 X K-1. The specific binding of this sugar to SBA shows that the lectin can accommodate a large hydrophobic substituent on the C-2 of galactose. Binding of non-fluorescent ligands, studied by monitoring the fluorescence changes when they are added to a mixture of SBA and N-dansylgalactosamine, indicates that a hydrophobic substituent at the anomeric position increases the affinity of the interaction. The C-6 hydroxy group also stabilizes the binding considerably. Kinetics of binding of N-dansylgalactosamine to SBA studied by stopped-flow spectrofluorimetry are consistent with a single-step mechanism and yielded k+1 = 2.4 X 10(5) M-1 X s-1 and k-1 = 0.2 s-1 at 20 degrees C. The activation parameters indicate an enthalpicly controlled association process.

Interaction of rice (Oryza sativa) lectin with N-acetylglucosaminides. Fluorescence studies

The interaction of lectin isolated from rice (Oryza sativa) embryos with N-acetylglucosaminides was studied by equilibrium dialysis and fluorescence. Equilibrium dialysis with 4-methylumbelliferyl-(GlcNac)2 showed that rice lectin (Mr 38000) contains four equivalent saccharide-binding sites. Addition of the N-acetylglucosaminides GlcNac, (GlcNac)2 and (GlcNac)3 enhanced the intrinsic fluorescence of rice lectin and this was accompanied by a 10nm blue-shift of its maximum fluorescence with (GlcNac)2 and (GlcNac)3. These changes in intensity allowed determination of the association constants, which increased with the number of saccharide units: at 20 degrees C, Ka = (1.3 +/- 0.1) X 10(3), (5.1 +/- 0.4) X 10(4) and (2.6 +/- 0.1) X 10(5) M-1 for GlcNac, (GlcNac)2 and (GlcNac)3 respectively. The binding enthalpy, delta H0, for the three glucosaminides were very low and ranged from -12.1 to -20.6 kJ X mol-1. The results are compared with those obtained with wheat-germ agglutinin, another GlcNac-specific gramineaous lectin.

Studies of the cellulolytic system of Trichoderma reesei QM 9414. Reaction specificity and thermodynamics of interactions of small substrates and ligands with the 1,4-beta-glucan cellobiohydrolase II

The 1,4-beta-glucan cellobiohydrolase II (CBH II) from Trichoderma reesei QM 9414 catalyses the hydrolysis of the 4-methylumbelliferyl beta-D-glycosides derived from cellotriose, cellotetraose and cellopentaose [MeUmb(Glc)n; n = 3 - 5]. The reaction has been followed by quantitative high-performance liquid chromatography. Specific activity for cellobiose removal at apparent substrate saturation were determined as (0.8 +/- 0.2) min-1 for MeUmb(Glc)3 and (9 +/- 2) min-1 for MeUmb(Glc)4. The enzyme showed a deviant specificity with MeUmb(Glc)5 as substrate. Two chromophoric products were formed simultaneously [MeUmb(Glc)3 and MeUmb(Glc)2] with turn-over numbers (17 +/- 4) min-1 and (21 +/- 6) min-1, respectively. Methylumbelliferyl beta-glucoside (MeUmbGlc) and the corresponding cellobioside [MeUmb(Glc)2] were used in equilibrium binding experiments. Both ligands yielded one binding site per molecule of Mr = 54000 upon forced flow dialysis (diafiltration). The association constants found were in fair agreement with those determined from MeUmb fluorescence quenching titrations. Quenching was total at all temperatures investigated for MeUmb(Glc)2, whereas for MeUmbGlc it increased from 80% to 100% between 2 degrees C and 20 degrees C. The association constants fitted linear van't Hoff plots in both cases. MeUmb(Glc)2 and MeUmbGlc were also used as indicator ligands to determine the association constants and thermodynamic parameters of several non-chromophoric ligands of CBH II. The binding of glucose increased the affinity for MeUmb(Glc)2 whereas it displaced MeUmbGlc from its complex. A putative binding site of the CBH II containing four subsites can be proposed. The thermodynamic data for methyl beta-D-glucopyranoside and cellobiose as ligands also point at an extended binding site.

Thermodynamics of wheat germ agglutinin-sialyloligosaccharide interactions by proton nuclear magnetic resonance

The thermodynamic parameters that characterize the binding of wheat germ agglutinin isolectin I (WGA I) to the alpha 2-3 isomer of (N-acetylneuraminyl)lactose have been determined by 360-MHz proton nuclear magnetic resonance spectroscopy. The chemical exchange of the ligand between the free and bound sites resulted in a broadening and upfield shifting of the N-acetyl methyl resonance [Kronis, K.A., & Carver, J.P. (1985) Biochemistry (preceding paper in this issue)] which has allowed the determination of the equilibrium constant, KD, and the dissociation rate constant, kD. In this paper, the analysis of the temperature dependence of the KD values between 25.4 and 51.6 degrees C yielded equilibrium parameters indicative of a large entropy barrier to binding: delta H degree = -13.3 +/- 1.0 kcal mol-1 and delta S degree = -31.9 +/- 2.4 cal mol-1 K-1. The Arrhenius plot of the effect of temperature on the dissociation rate (kD) and the plot of 1n (kD/T) vs. 1/T indicated that the transition complex represented an unfavorable energy state compared to the dissociated molecules with an activation energy (EA) of +18.0 kcal mol-1 and enthalpy and entropy of dissociation (delta HD not equal to and delta SD not equal to) values of +17.4 +/- 0.3 kcal mol-1 and +13.4 +/- 1.2 cal mol-1 K-1, respectively. The driving force for this binding reaction is the large negative delta H degree with a small enthalpic barrier to association (delta HA = +4.1 kcal mol-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Structural comparison of the two distinct sugar binding sites in wheat germ agglutinin isolectin II

The two unique sugar binding sites in wheat germ agglutinin, located in the subunit/subunit interface of the dimer molecule and termed primary and secondary binding sites, are compared in the light of the newly obtained chemical amino acid sequence and a high-resolution electron density map (1.8 A). Homology was found in the three amino acid residues directly involved in sugar binding: Tyr73II, Ser62II, Glu115I in the primary site, and Tyr159I, Ser148I, Asp29II in the secondary site (subscripts refer to promoters I and II). Thirteen corresponding side-chain atoms of these three homologous residues in the two sites could be superimposed with a root-mean-square difference of 1.39 A. The three sugar binding residues are located in subsite 1 of each extended binding location and contribute to binding of the terminal, non-reducing N-acetyl-D-glucosamine and N-acetyl-D-neuraminic acid residues only, and they provide three hydrogen bonds for complex stabilization. Two hydrogen bonds are made with the carbonyl and amido portions of the N-acetyl group and the third with the C-3 OH group of the sugar ring. It is suggested that small differences in the sugar binding affinities at these two unique sites exist, due to the different numbers of van der Waals' interactions made at these sites, which contribute to stabilizing, for instance, the wheat germ agglutinin/N,N'-diacetyl-chitobiose complex. The single tryptophan residue is located at a distance of approximately 13 A from the primary site and is thought to have no affect on sugar binding. In addition, the disposition of the four saccharide binding sites of the dimer with respect to three local, pseudo 2-fold symmetry axes, relating domains of opposite protomers, is discussed.

Nature of the interaction between Ricinus communis agglutinin and blood cells

Binding of Ricinus communis agglutinin (RCA 120) to carbohydrate receptors of human lymphocytes and erythrocytes is enthalpically driven. As in the case of simple saccharides, the delta S contribution is always unfavorable to the interaction. This result is different from that observed for other lectins and might indicate that hydrophobic interactions do not play a dominant role in binding of RCA 120 to cell surfaces.

4-Methylumbelliferyl-glycosides as fluorescence probes of sugar-binding sites on lectin molecules: spectral properties and dependence of fluorescence on polarity and viscosity

The spectral properties of 4-methylumbelliferyl-glycosides (MeUmb-glycosides) were investigated in order to assess their usefulness as probes of the microenvironment of sugar binding sites on lectin molecules. It was shown that the abnormally high values for fluorescence polarization of free MeUmb-glycosides (from 0.07 to 0.251) were due neither to their molecular size nor to the blockade of their movement, but to the short lifetimes (less than 0.55 ns) of the excited state of these compounds. Working essentially with two MeUmb-monosaccharides and one MeUmb-disaccharide (MeUmb-alpha-D-galactopyranoside, MeUmb-beta-D-galactopyranoside, and MeUmb-2-acetamido-2-deoxy-3-O-(beta-D-galactopyranosyl)-beta-D- galactopyranoside) which were solubilized in various solvents, it was demonstrated that solvent polarity and viscosity definitely affected the fluorescence intensity of MeUmb-glycosides. A low-polarity medium reduced this intensity, and high viscosity enhanced it. The implications of these findings are discussed in relation to the variations in the fluorescence intensity of MeUmb-glycosides when these compounds were bound to lectins.

The binding of monosaccharides to wheat germ agglutinin: Fluorescence and NMR investigations

The synthesis of N-acetyl- and N-trifluoroacetyl-glucosaminides was reported. The interaction of these compounds with wheat germ agglutinin, a plant lectin specific for N-acetyl-glucosamine and sialic acid, was investigated by two complementary approaches: 1H and 19F NMR, and fluorescence spectroscopy. This last technique relies on the existence of a competitive equilibrium involving the protein, the ligand and O-(methylumbelliferyl)-N-acetyl-glucosaminide, a fluorescent saccharide. The binding constants and the chemical shifts in the complex were determined and were related to the protein structure.

Binding of 4-methylumbelliferyl beta-D-galactosyl-(1 leads to 3)-N-acetyl-beta-D-galactosaminide to peanut agglutinin. Characterisation and application in substitution titrations

Binding of 4-methylumbelliferyl-2-acetamido-2-deoxy-3-O-(beta-D-galactopyranosyl) beta-D-galactopyranoside [MeUmb beta Gal(beta 1 leads to 3)GalNAc] to peanut agglutinin was characterized by equilibrium dialysis and by measurement of the increase in ultraviolet absorption or fluorescence of the chromophoric glycoside upon continuous titration with excess of the lectin. All data in the 4-30 degrees C range correspond to delta G = -(26.5 +/- 0.1) kJ mol-1, delta H = -(58.4 +/- 2) kJ mol-1 and delta S = -(107 +/- 8)J mol-1 K-1. Values of the association constants are e.g. K = 2.5 X 10(5) M-1 at 4 degrees C and K = 4.5 X 10(4) M-1 at 25 degrees C. MeUmb beta Gal(beta 1 leads to 3)GalNAc was used as an indicator ligand to determine K values for nonchromophoric carbohydrates by continuous displacement titrations, measuring either fluorescence or difference in absorption of the indicator. The data were analyzed in terms of the general expression for a non-ideal indicator system (as detailed in the appendix). Thus, the values of K are not underestimated. They are K = 4.8 X 10(3) M-1 for methyl alpha-D-galactopyranoside [Me alpha Gal], 2.0 X 10(3) M-1 for methyl beta-D-galactopyranoside [Me beta Gal] and 4.7 X 10(3) M-1 for lactose [Gal(beta 1 leads to 4)Glc], all at 14.5 degrees C. The MeUmb difference absorption spectra resulting from binding of the lectin with MeUmb beta Gal(beta 1 leads to 3)GalNAc and MeUmb beta Gal(beta 1 leads to 4)Glc are larger than for MeUmb beta Gal and MeUmb alpha Gal. These observations are consistent with the extended nature of the combining site of peanut agglutinin.

Protein-sugar interactions: environmental effect on the fluorescence of O-(4-methylumbelliferyl)-glycosides

We have investigated the effect of 12 solvents and several amino acids on the fluorescence of O-(4-methylumbelliferyl)-glycosides. We showed that: i) the fluorescence quenching is not related to the dielectric constant of the solvents: the fluorescence intensity was maximal in water (d = 80) and in acetic acid (d = 6.2) and was at least ten times lower in acetone (d = 21) and in dioxane (d = 2.2); ii) the fluorescence of O-(4-methylumbelliferyl)-N-acetyl-beta-glucosaminide is not quenched in the presence of various amino acids including arginine, asparagine, aspartate, histidine, leucine, phenylalanine and proline; iii) the fluorescence of O-(4-methylumbelliferyl)-glycoside is quenched by sulfur, phenol and indole amino acids or derivatives containing sulfur, phenol or indole groups. The changes in fluorescence intensities of O-(4-methylumbelliferyl)-glycosides upon binding to concanavalin A, wheat germ agglutinin and lysozyme are discussed with regard to the amino acid content of their binding sites.

Identification and characterization of the direct folding process of hen egg-white lysozyme

Refolding kinetics of hen egg-white lysozyme (HEWL) have been studied by means of the stopped-flow method with guanidinium chloride as the denaturant. We show here that the three-species model U1 in equilibrium or formed from U2 in equilibrium or formed from N (U1 and U2 = unfolded; N = native) now established for pancreatic ribonuclease A is also valid for HEWL on the basis of the following lines of evidence: (1) refolding kinetics outside the transition region are biphasic; (2) dependence of the fractional amplitude for the fast phase on the ratio of the time constants of the two phases agrees with theory; (3) unfolding kinetics outside the transition region are of single phase; (4) direct evidence for the U2 leads to U1 transformation is obtained by double-jump experiments; (5) the time constant of the binding reaction of a substrate analogue, 4-methylumbelliferyl N,-N'-diacetyl-beta-chitobioside, to HEWL molecules during refolding reaction agrees with the time constant of the direct refolding phase U2 leads to N. The characteristic properties of the nucleation-controlled reaction of refolding of small globular proteins are discussed in general. The results of the discussion are used to suggest that the direct folding process is nucleation controlled from the experimental results of the temperature dependence of the refolding rate.

Binding of 4-methylumbelliferyl beta-D-galactopyranoside to Momordica charantia lectin: fluorescence-quenching studies

The binding of 4-methylumbelliferyl beta-D-galactopyranoside (MeUmb-Galp), to Mormordica charantia lectin was studied by equilibrium dialysis and quenching of ligand fluorescence. The fluorescence of MeUmb-Galp decreases as a function of solvent polarity. On binding to M. charantia lectin, its fluorescence was nearly 100% quenched, showing that the binding of the glycoside takes place in hydrophobic environment. The binding of the fluorescent sugar was saccharide-specific as evidenced by reversal of MeUmb-Galp fluorescence quenching by lactose. The association constant is independent of the experimental method used and at 25 degrees C the value is (1.96 +/- 0.05) X 10(4) M-1. The number of binding sites as determined by equilibrium dialysis and fluorescence quenching agree very well with each other; n being equal to 1.98 +/- 0.02. The Ka value for the glycoside was also determined by competition studies employing reversal of fluorescence quenching of MeUmb-Galp by lactose. The value of ka obtained for lactose is 1.21 X 10(4) M-1 at 30 degrees C. The internal consistency of the association constant and number of binding site values at low and high saturation indicates the absence of additional subsite on M. charantia lectin. The thermodynamic parameters do not differ greatly with change in temperature; the values of - delta H degrees and - delta S degrees are equal to 30 +/- 9.63 kJ mol-1 and 21 +/- 0.3 J mol-1 K-1 respectively in the range of 15--35 degrees C indicting that the binding of M. charantia lectin to saccharide is exothermic in nature.

Fluorescence-polarization studies on binding of 4-methylumbelliferyl beta-D-galactopyranoside to Ricinus communis (castor-bean) agglutinin

The binding of Ricinus communis (castor-bean) agglutinin 1 to saccharides was studied by equilibrium dialysis and fluorescence polarization by using the fluorescently labelled sugar 4-methylumbelliferyl beta-D-galactopyranoside. No appreciable change in ligand fluorescence of 4-methylumbelliferyl beta-D-galactopyranoside was considerably polarized on its binding to the lectin. The association constants obtained by Scatchard analysis of equilibrium-dialysis and fluorescence-polarization data do not differ much from each other, and at 25 degrees C, Ka = 2.4 (+/- 0.2) X 10(4)M-1. These values agree reasonably well with that reported in the literature for Ricinus agglutinin 1. The number of binding sites obtained by the different experimental procedures is 1.94 +/- 0.1 per molecule of 120 000 daltons and is equal to the reported value of 2. The consistency in the values of Ka and number of binding sites indicate the absence of additional subsites on Ricinus agglutinin 1 for its specific sugars. In addition, the excellent agreement between the binding parameters obtained by equilibrium dialysis and fluorescence polarization indicate the potential of ligand-fluorescence-polarization measurements in the investigation of lectin-sugar interactions.

Binding of 4-methylumbelliferyl-beta-D-ribopyranoside to beta-D-xylosidase from Bacillus pumilus

The determination of the binding parameters of 4-methylumbelliferyl-beta-D-ribopyranoside, a fluorescent ligand of beta-D-xylosidase (exo-1,4-beta-xylosidase, EC 3.2.1.37) from Bacillus pumilus, is described. A single binding site per polypeptide chain (60 000 daltons) was found and the homogeneity of the binding sites in the dimeric or tetrameric forms of the enzyme were shown. The association constants, as a function of temperature and ionic strength, were obtained from equilibrium binding experiments and compared to the kinetically determined inhibition constants. The apparent discrepancies are attributed to a temperature, ionic strength and concentration dependent shift in the dimer-tetramer equilibrium of the enzyme and different affinities of the ligand for both oligomeric forms. Sedimentation velocity experiments seem to corroborate this hypothesis.

Cooperativity of lectin binding to lymphocytes, and its relevance to mitogenic stimulation

The relationship between the binding patterns of soybean agglutinin, peanut agglutinin (both in their native (unaggregated) form and in their polymerized form), and of Phaseolus vulgaris leucoagglutinin, to neuraminidase-treated lymphocytes from different sources, and the mitogenic activity of these lectins, was studied. In all cases investigated, binding of a lectin to lymphocytes which resulted in stimulation was a positive cooperative process. Our findings support the assumption that clustering of receptors and conformational changes in membrane structure are prerequisites for mitogenic stimulation.