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.

Theoretical studies on the conformation of beta-D-N-acetyl neuraminic acid (sialic acid)

The possible conformations of sialic acid were analysed using semi-empirical potential functions. The solid state conformation has approx. 0.2 kcal/mol higher energy than the minimum energy conformation. These studies suggest that in solution sialic acid may exist preponderantly in two different conformations which differ in the orientation of the terminal hydroxymethyl group of glycerol side-chain. The present model is consistent with 1H- and 13C-NMR data, but differs from the earlier models.

The modes of binding methyl-alpha (and beta)-D-glucopyranosides and some of their derivatives to concanavalin A--a theoretical approach

The probable modes of binding of Methyl--alpha (and beta)-D-glucopyranosides and some of their derivatives to concanavalin A have been proposed from theoretical studies. Theory predicts that beta-MeGlcP can bind to ConA in three different modes whereas alpha-MeGlcP can bind only in one mode. beta-MeGlcP in its most favourable mode of binding differs from alpha-MeGlcP in its alignment in the active-site of the lectin where it binds in a flipped or inverted orientation. Methyl substitution at the C-2 atom of the alpha-MeGlcP does not significantly affect the possible orientations of the sugar in the active-site of the lectin. Methyl substitution at C-3 or C-4, however, affects the allowed orientations drastically leading to the poor inhibiting power of Methyl-3-O-methyl-alpha-D-glucopyranoside and the inactivity of Methyl-4-O-methyl-alpha-D-glycopyranoside. These studies suggest that the increased activity of the alpha-MeGlcP over beta-MeGlcP may be due to the possibility of formation of better hydrogen bonds and to hydrophobic interactions rather than to steric factors as suggested by earlier workers. These models explain the available NMR and other binding studies.

Fluorescence polarization as a tool to study lectin-sugar interaction. An investigation of the binding of 4-methylumbelliferyl beta-D-galactopyranoside to Abrus precatorious agglutinin

Polarization of ligand fluorescence was used to study the binding of 4-methylumbelliferyl beta-D-galactopyranoside (MeUmb-Galp) to Abrus precatorious agglutinin. The binding of the fluorescent sugar to the lectin led to considerable polarization of the MeUmb-Galp fluorescence, which was also quenched by about 30% on binding to the lectin. The binding of the fluorescent sugar was carbohydrate-specific, as evidenced by inhibition of both fluorescence polarization and quenching when lectin was preincubated with lactose. The association constant as determined by fluorescence polarization is 1.42 x 10(4) M-1 at 25 degrees C and is in excellent agreement with those determined by fluorescence quenching (Ka = 1.51 x 10(4) M-1) and equilibrium dialysis (Ka = 1.62 x 10(4) M-1) at 25 degrees C. The numbers of binding sites as determined by fluorescence polarization, quenching and equilibrium dialysis agree very well with one another, n being equal to 2.0 +/- 0.05. The consistency between the association constant value determined by fluorescence polarization, quenching and equilibrium dialysis shows the validity of this approach to study lectin-sugar interaction.

Role of hydroxyl group on the mesomorphism of alkyl glycosides:synthesis and thermal behavior of alkyl 6-deoxy-beta-D-glucopyranosides

A homologous series of alkyl 6-deoxy-beta-D-glucopyranoside amphiphiles was prepared,in an effort to identify the role of hydroxyl group in the mesomorphic behavior of alkyl glycosides. Synthesis was performed by a chlorination of the sugar moiety in alkyl-beta-D-glucopyranosides with methylsulfonyl chloride in DMF, followed by a metal mediated dehalogenation to secure alkyl 6-deoxy-beta-D-glucopyranosides, wherein the alkyl chain length varied from C-9 to C-16. The mesomorphic behavior of these 6-deoxy alkyl glycosides was assessed using polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction method. Whereas the lower homologues exhibited a monotropic SmA phase till sub-ambient temperatures, the higher homologues formed a plastic phase. A partial interdigitized bilaye structure of SmA phase is inferred from experimental d-spacing and computationally derived lengths of the molecules. The results were compared with those of normal alkyl glucopyranosides, retained with hydroxyl groups at C-2-C-6 carbons, and alkyl 2-deoxy-glucopyranosides, devoid of a hydroxyl group at C-2 and the comparison showed important differences in the mesomorphic behavior.(C)2010 Elsevier Ireland Ltd. All rights reserved.

Solid state structure of p-bromo phenyl 4,5,7-tri-O-benzyl-beta-D-glycero-D-talo-septanoside and an analysis of non-covalent interactions

The solid state structure of a new seven-membered sugar oxepane derivative, namely, p-bromo phenyl 4,5,7-tri-O-benzyl-beta-D-glycero-D-talo-septanoside is discussed, as determined through single crystal X-ray structural determination and in relation to their conformational features. The molecule adopts twist-chair as the preferred conformation, with conformational descriptor (TC2,3)-T-0,1. The solid state packing of molecules is governed by a rich network of non-covalent bonding originating from O-H center dot center dot center dot O, C-H center dot center dot center dot pi, C-H center dot center dot center dot Br and aromatic pi center dot center dot center dot pi interactions that stabilize the packing of molecules in the crystal. (C) 2015 Elsevier Ltd. All rights reserved.

Determination of distances of sugar protons from Mn2+ in concanavalin A

1H NMR spin-lattice relaxation time (T1) measurements have been carried out with various sugars, viz. methyl alpha-D-glucopyranoside (alpha-MeGluP), methyl beta-D-lucopyranoside (beta-MeGluP), methyl alpha--annopyranoside (alpha-MeManP), maltose (4-O-alpha-D-glucopyranosyl--glucose), nigerose (3-O-alpha-D-glucopyranosyl-D-glucose), p-nitrophenyl alpha-maltoside (PNP-alpha-maltoside) and p-nitrophenyl beta-maltoside (PNP-beta-maltoside) to determine the distances of sugar protons from Mn2+ in concanavalin A (Con A). With a rotational correlation time of 1.58 x 10(-10) s determined, distances were calculated using Solomon-Bloembergen equation. The data obtained indicated differences in disposition of different groups in the binding site of Con A. An average value of about 10 A was obtained for the distances of sugar protons from Mn2+ in Con A. In the case of mono and disaccharides, the non-reducing end sugar unit was found to be closer to Mn2+ than the reducing end one.

Purification and characterization of a beta-glucosidase and endocellulase from Humicola insolens

A .beta.-glucosidase and an endocellulase were purified from the culture filtrates of a thermophilic cellulolytic fungus Humicola insolens. Both the preparations were homogeneous by PAGE, ultracentrifugation and gel filtration (Mr 45,000). Ouchterlony immunodiffusion showed complete cross reactivity between the antibodies and the two enzyme antigens, indicating the presence of a common epitope on the two enzyme proteins. The two enzymes, however, differ in their amino acid composition and their substrate specificity. .beta.-Glucosidase acts on p-nitrophenyl .beta.-D-glucopyranoside and hydrolyses cellulose to release mainly glucose and small amounts of cellobiose from the non-reducing end. On the other hand, endocellulase hydrolyses cellulose to release cellopentaose, cellotetraose, cellotriose along with cellobiose and glucose and also hydrolyses larch wood xylan.

The effects of beta-mercaptoethanol and sodium dodecyl sulfate on the Humicola insolens beta-glucosidase

Hydrolysis of p-nitrophenyl-beta-D-glucoside by the beta-glucosidase of a thermophilic and cellulolytic fungus, Humicola insolens was stimulated by two-fold in the presence of high concentrations of beta-mercaptoethanol. This enzyme did not have any free sulfhydryl groups and high concentrations of beta-mercaptoethanol (5% v/v) reduced all of the three disulfide bonds present in the enzyme. In contrast, the hydrolysis of cellobiose and cellulose polymers was inhibited by 50% under the same conditions. Sodium dodecyl sulfate (1% w/v) even in combination with beta-mercaptoethanol did not show any significant effects on this enzyme. These unusual properties suggest that this enzyme may be of significant importance for understanding the structure of the enzyme.

Azospirillum lipoferum tRNAs: Fractionation and identification of tRNA species and the nucleotide sequence of tRNA(Asn) (QUU)

Transfer RNAs of Azospirillum lipoferum were separated by two- dimensional gel electrophoresis and identified by aminoacylation. Thirty-six tRNA spots were resolved by this technique and twenty-six tRNA species have been identified. There are five tRNAs for Leu, four for Val, three for Pro, two each for Arg, Ile, Lys and Tyr, and one each for Ala, Asp, His, Phe, Ser and Thr. The tRNA(Asn) (QUU) was purified and its nucleotide sequence was determined. The A. lipoferum tRNA(Asn) (QUU) is 92% similar to B. subtilis tRNA(Asn) gene and two hypermodified nucleosides, queuosine (Q) and N-(9-beta-D Ribofuranosylpurine-6-YL) carbamoyl)-threonine (t(6)A) are present in this tRNA.

The key enzyme in galactose metabolism, UDP-galactose-4-epimerase, affects cell-wall integrity and morphology in Candida albicans even in the absence of galactose

The enzyme UDP-galactose-4-epimerase (GAL10) catalyzes a key step in galactose metabolism converting UDP-galactose to UDPglucose which then can get metabolized through glycolysis and TCA cycle thus allowing the cell to use galactose as a carbon and energy source. As in many fungi, a functional homolog of GAL10 exists in Candida albicans. The domainal organization of the homologs from Saccharomyces cerevisiae and C albicans show high degree of homology having both mutarotase and an epimerase domain. The former is responsible for the conversion of beta-D-galactose to alpha-D-galactose and the hitter for epimerization of UDP-galactose to UDP-glucose. Absence of C albicans GAL10 (CaGAL10) affects cell-wall organization, oxidative stress response, biofilm formation and filamentation. Cagal10 mutant cells tend to flocculate extensively as compared to the wild-type cells. The excessive filamentation in this mutant is reflected in its irregular and wrinkled colony morphology. Cagal10 strain is more susceptible to oxidative stress when tested in presence of H2O2. While the S. cerevsiae GAL10 (ScGAL10), essential for survival in the presence of galactose, has not been reported to have defects in the absence of galactose, the C albicans homolog shows these phenotypes during growth in the absence of galactose. Thus a functional CaGal10 is required not only for galactose metabolism but also for normal hyphal morphogenesis, colony morphology, maintenance of cell-wall integrity and for resistance to oxidative stress even in the absence of galactose. (c) 2006 Elsevier Inc. All rights reserved.

Aggregation and mesomorphic properties of 'double-headed' carbohydrate amphiphiles

Sugar-based amphiphiles, consisting of two sugar head groups and an alkylene chain within the molecules, are synthesized and their aggregation and mesomorphic properties are evaluated. The hydrophilic sugar head groups, constituted with beta-D-glucopyranoside units, and the lyophilic alkylene units, are coupled to a glycerol backbone to afford the 'double-headed' sugar amphiphiles. Aggregation studies in aqueous solutions provided their critical micellar concentrations and the aggregation numbers. Mesophase characterizations by polarizing optical microscopy and differential scanning calorimetry (DSC) revealed the phase-transition behaviour of these new 'double-headed' glycolipids.

Thermodynamics of the binding of galactopyranoside derivatives to the basic lectin from winged bean (Psophocarpus tetrogonolobus)

The thermodynamics of the binding of D-galactopyranoside (Gal), 2-acetamido-2-deoxygalactopyranoside (GalNAc), methyl-alpha-D-galactopyranoside, and methyl-beta-D-galactopyranoside to the basic agglutinin from winged bean (WBAI) in 0.02 M sodium phosphate and 0.15 M sodium chloride buffer have been investigated from 298.15 to 333.15 K by titration calorimetry and at the denaturation temperature by differential scanning calorimetry (DSC). WBAI is a dimer with two binding sites. The titration calorimetry yielded single-site binding constants ranging from 0.56 +/- 0.14 x 10(3) M-1 for Gal at 323.15 K to 7.2 +/- 0.5 x 10(3) M-1 for GalNAc at 298.15 K and binding enthalpies ranging from -28.0 +/- 2.0 kJ mol-1 for GalNAc at 298.15 K to -14.3 +/- 0.1 kJ mol-1 for methyl-beta-D-galactopyranoside at 322.65 K. The denaturation transition consisted of two overlapping peaks over the pH range 5.6-7.4. Fits of the differential scanning calorimetry data to a two-state transition model showed that the low temperature transition (341.6 +/- 0.4 K at pH 7.4) consisted of two domains unfolding as a single entity while the higher temperature transition (347.8 +/- 0.6 K at pH 7.4) is of the remaining WBAI dimer unfolding into two monomers. Both transitions shift to higher temperatures and higher calorimetric enthalpies with increase in added ligand concentration at pH 7.4. Analysis of the temperature increase as a function of added ligand concentration suggests that one ligand binds to the two domains unfolding at 341.6 +/- 0.6 K and one ligand binds to the domain unfolding at 347.8 +/- 0.6 K.

Solution structure of O-glycosylated C-terminal leucine zipper domain of human salivary mucin (MUC7)

Solution structures of a 23 residue glycopeptide II (KIS* RFLLYMKNLLNRIIDDMVEQ, where * denotes the glycan Gal-beta-(1-3)-alpha-GalNAc) and its deglycosylated counterpart I derived from the C-terminal leucine zipper domain of low molecular weight human salivary mucin (MUC7) were studied using CD, NMR spectroscopy and molecular modeling. The peptide I was synthesized using the Fmoc chemistry following the conventional procedure and the glycopeptide II was synthesized incorporating the O-glycosylated building block (N alpha-Fmoc-Ser-[Ac-4,-beta-D-Gal-(1,3)-Ac(2)alpha-D-GalN(3)]-OPfp) at the appropriate position in stepwise assembly of peptide chain. Solution structures of these glycosylated and nonglycosylated peptides were studied in water and in the presence of 50% of an organic cosolvent, trifluoroethanol (TFE) using circular dichroism (CD), and in 50% TFE using two-dimensional proton nuclear magnetic resonance (2D H-1 NMR) spectroscopy. CD spectra in aqueous medium indicate that the apopeptide I adapts, mostly, a beta-sheet conformation whereas the glycopeptide II assumes helical structure. This transition in the secondary structure, upon glycosylation, demonstrates that the carbohydrate moiety exerts significant effect on the peptide backbone conformation. However, in 50% TFE both the peptides show pronounced helical structure. Sequential and medium range NOEs, C alpha H chemical shift perturbations, (3)J(NH:C alpha H) couplings and deuterium exchange rates of the amide proton resonances in water containing 50% TFE indicate that the peptide I adapts alpha-helical structure from Ile2-Val21 and the glycopeptide II adapts alpha-helical structure from Ser3-Glu22. The observation of continuous stretch of helix in both the peptides as observed by both NMR and CD spectroscopy strongly suggests that the C-terminal domain of MUC7 with heptad repeats of leucines or methionine residues may be stabilized by dimeric leucine zipper motif. The results reported herein may be invaluable in understanding the aggregation (or dimerization) of MUC7 glycoprotein which would eventually have implications in determining its structure-function relationship.