Purification and physicochemical characterization of two galactose-specific isolectins from the seeds of Trichosanthes cordata.

A galactose-specific lectin has been purified from the seeds of Trichosanthes cordata by affinity chromatography on crosslinked guar gum. The affinity-eluted lectin could be resolved into two isolectins, TCA-I and TCA-II by ion-exchange chromatography on DEAE cellulose. The molecular weights of the isolectins were determined as 59 and 52 kDa by SDS-PAGE. TCA-I is a heterodimer in which the two subunits with masses of 32 and 27 kDa, are covalently connected by disulfide bonds. TCA-I and TCA-II are glycoproteins with 6.2% and 6.8% covalently bound neutral sugar, respectively. CD spectroscopic studies indicate that the two isolectins are very similar in secondary structure and contain about 8 to 10% alpha-helix, 37-38% beta-sheet, 20% beta-turns, and 32-33% unordered structures. These isolectins have similar carbohydrate specificities as revealed by hemagglutination-inhibition assays. Carbohydrate specificity, subunit size and composition, and secondary structure of TCA isolectins suggest close similarity to type-II ribosome inactivating proteins. The agglutination activity of TCA-I was found to be highest in the pH range 7.0-8.0. The lectin activity was unaffected between 0 and 40 degrees C, but decreased dramatically above 40 degrees C. Association constant for the interaction of TCA-I with lactose was determined by monitoring ligand-induced changes in the protein intrinsic fluorescence characteristics as 7.42 x 10(3) M(-1) at 25 degrees C. The exposure and accessibility of the tryptophan residues of TCA-I and the effect of ligand binding on them have been probed by quenching studies employing neutral and ionic quenchers.

Tryptophan environment, secondary structure and thermal unfolding of the galactose-specific seed lectin from Dolichos lablab: fluorescence and circular dichroism spectroscopic studies.

Fluorescence and circular dichroism spectroscopic studies were carried out on the galactose-specific lectin from Dolichos lablab seeds (DLL-II). The microenvironment of the tryptophan residues in the lectin under native and denaturing conditions were investigated by quenching of the intrinsic fluorescence of the protein by a neutral quencher (acrylamide), an anionic quencher (iodide ion) and a cationic quencher (cesium ion). The results obtained indicate that the tryptophan residues of DLL-II are largely buried in the hydrophobic core of the protein matrix, with positively charged side chains residing close to at least some of the tryptophan residues under the experimental conditions. Analysis of the far UV CD spectrum of DLL-II revealed that the secondary structure of the lectin consists of 57% alpha-helix, 21% beta-sheet, 7% beta-turns and 15% unordered structures. Carbohydrate binding did not significantly alter the secondary and tertiary structures of the lectin. Thermal unfolding of DLL-II, investigated by monitoring CD signals, showed a sharp transition around 75 degrees C both in the far UV region (205 nm) and the near UV region (289 nm), which shifted to ca. 77-78 degrees C in the presence of 0.1 M methyl-beta-D-galactopyranoside, indicating that ligand binding leads to a moderate stabilization of the lectin structure.

Fluorescence quenching and time-resolved fluorescence studies on Trichosanthes dioica seed lectin.

Fluorescence quenching and time-resolved fluorescence studies have been carried out on the Trichosanthes dioica seed lectin (TDSL). The emission lambdamax of native TDSL, seen at 328nm, shifts to 343nm upon denaturation with 6M guanidinium chloride. Quenching titrations were performed with neutral (acrylamide and succinimide) and ionic (I(-) and Cs(+)) quenchers in order to probe the exposure and accessibility of tryptophan residues of the protein. Maximum quenching was observed with acrylamide, followed by succinimide, iodide and Cs(+). Dramatic increase in the extent of quenching and other quenching parameters by all the quenchers were observed upon denaturation of TDSL, suggesting that all the tryptophan residues in native TDSL are buried in the hydrophobic core of the protein. Increase in the extent of quenching upon denaturation of TDSL was maximum with I(-) and minimum with Cs(+), suggesting the presence of positively charged residue(s), near at least one tryptophan residue. Addition of saccharide ligands such as methyl-beta-d-galactopyranoside and lactose led to a small, but reproducible decrease in the fluorescence intensity of the lectin. The presence of lactose provided a partial protection against quenching by I(-), Cs(+) and succinimide, but not acrylamide. In time-resolved fluorescence measurements the fluorescence decay curves could be best fitted to biexponential patterns with lifetimes of 4.09 and 1.53ns for native lectin, 3.40 and 1.65ns for the lectin in presence of 0.1M lactose and 3.50 and 1.40ns for denatured lectin.

Energetics of carbohydrate binding to Momordica charantia (bitter gourd) lectin: an isothermal titration calorimetric study.

Physico-chemical and carbohydrate binding studies have been carried out on the Momordica charantia (bitter gourd) seed lectin (MCL). The lectin activity is maximal in the pH range 7.4-11.0, but decreases steeply below pH 7.0. The lectin activity is mostly unaffected in the temperature range 4-50 degrees C, but a sharp decrease is seen between 50 and 60 degrees C, which could be correlated to changes in the structure of the protein as seen by circular dichroism and fluorescence spectroscopy. Isothermal titration calorimetric studies show that the tetrameric MCL binds two sugar molecules and the binding constants (Kb), determined at 288.15 K, for various saccharides were found to vary between 7.3 x 10(3) and 1.52 x 10(4)M(-1). The binding reactions for all the saccharides investigated were essentially enthalpy driven, with the binding enthalpies (DeltaHb) at 288.15 K being in the range of -50.99 and -43.39 kJ mol(-1), whereas the contribution to the binding reaction from the entropy of binding was negative, with values of binding entropy (DeltaSb) ranging between -99.2 and -72.0 J mol(-1)K(-1) at 288.15 K. Changes in heat capacity (DeltaCp) for the binding of disaccharides, lactose and lactulose, were significantly larger in magnitude than those obtained for the monosaccharides, methyl-beta-D-galactopyranoside, and methyl-alpha-D-galactopyranoside, and could be correlated reasonably well with the surface areas of these ligands. Enthalpy-entropy compensation was observed for all the sugars studied, suggesting that water structure plays an important role in the overall binding reaction. CD spectroscopy indicates that carbohydrate binding does not lead to significant changes in the secondary and tertiary structures of MCL, suggesting that the carbohydrate binding sites on this lectin are mostly preformed.

Purification, physicochemical characterization, saccharide specificity, and chemical modification of a Gal/GalNAc specific lectin from the seeds of Trichosanthes dioica.

A new galactose-specific lectin has been purified from the extracts of Trichosanthes dioica seeds by affinity chromatography on cross-linked guar gum. The purified lectin (T. dioica seed lectin, TDSL) moved as a single symmetrical peak on gel filtration on Superose-12 in the presence of 0.1 M lactose with an M(r) of 55 kDa. In the absence of ligand, the movement was retarded, indicating a possible interaction of the lectin with the column matrix. In SDS-PAGE, in the presence of beta-mercaptoethanol, two non-identical bands of M(r) 24 and 37 kDa were observed, whereas in the absence of beta-mercaptoethanol, the lectin yielded a single band corresponding to approximately 55,000 Da, indicating that the two subunits of TDSL are connected by one or more disulfide bridges. TDSL is a glycoprotein with about 4.9% covalently bound neutral sugar. Analysis of near-UV CD spectrum by three different methods (CDSSTR, CONTINLL, and SELCON3) shows that TDSL contains 13.3% alpha-helix, 36.7% beta-sheet, 19.4% beta-turns, and 31.6% unordered structure. Among a battery of sugars investigated, TDSL was inhibited strongly by beta-d-galactopyranosides, with 4-methylumbelliferyl-beta-d-galactopyranoside being the best ligand. Chemical modification studies indicate that tyrosine residues are important for the carbohydrate-binding and hemagglutinating activities of the lectin. A partial protection was observed when the tyrosine modification was performed in the presence of 0.2 M lactose. The tryptophan residues of TDSL appear to be buried in the protein interior as they could not be modified under native conditions, whereas upon denaturation with 8 M urea two Trp residues could be selectively modified by N-bromosuccinimide. The subunit composition and size, secondary structure, and sugar specificity of this lectin are similar to those of type-2 ribosome inactivating proteins, suggesting that TDSL may belong to this protein family.