Quaternary association and reactivation of dimeric concanavalin A

Alteration of Surface Glycoproteins After Photodynamic Therapy

BACKGROUND

Cell membranes have been identified as an important intracellular cancer treatment target, since the glycoconjugates present on the cell surface are involved in numerous cell functions. Photodynamic therapy (PDT) is a therapeutic modality employed in the treatment of tumors that uses visible light to activate a photosensitizer.

OBJECTIVE

This study analyzed the expression of surface carbohydrates after PDT with two different photosensitizers, 5-aminolevulinic acid (ALA) and Photosan-3.

METHODS

Mice were injected subcutaneously with 2 × 10⁵ B16 cells. After 7-10 days, the presence of a tumor with a diameter of 3.6 mm was observed. Photosan-3^® and 5-aminolevulinic acid-ALA were used in the PDT treatment. Control animals (not submitted to either laser treatment or photosensitizer injection) and treated animals were euthanized 15 days post-treatment. The tumors were irradiated with a red diode laser, λ = 655 nm, energy density of 10 J.cm^-2, and power density of 45 mW.cm^-2. After 2 weeks of treatment with PDT, the mice were euthanized, the tumors were collected, and the cell surfaces were labeled with lectins concanavalin A (ConA) and wheat germ agglutinin (WGA).

RESULTS

Fluorescence microscopy analysis of the cell surfaces with lectins ConA and WGA showed the presence of α-mannose and α-glucose.

CONCLUSIONS

The combined effects of either Photosan-3 or ALA and red laser light on melanoma suggest an inhibitory glycosylation action from PDT on the surface of B16-F10 cells.

Conserved Activity of Reassociated Homotetrameric Protein Subunits Released from Mesoporous Silica Nanoparticles

Mesoporous silica nanoparticles (MSN) with enlarged pores were prepared and characterized, and reversibly dissociated subunits of concanavalin A were entrapped in the mesopores, as shown by multiple biochemical and material characterizations. When loaded in the MSN, we demonstrated protein stability from proteases and, upon release, the subunits reassociated into active proteins shown through mannose binding and o-phthalaldehyde fluorescence. We have demonstrated a versatile and facile method to load homomeric proteins into MSN with potential applications in enhancing the delivery of large therapeutic proteins.

Cetyltrimethylammonium bromide (CTAB) promote amyloid fibril formation in carbohydrate binding protein (concanavalin A) at physiological pH

Low concentration of CTAB provoked cross β-sheet formation whereas high concentrations of CTAB direct to alpha helix induction in Con A.

Thermal, chemical and pH induced unfolding of turmeric root lectin: modes of denaturation

Curcuma longa rhizome lectin, of non-seed origin having antifungal, antibacterial and α-glucosidase inhibitory activities, forms a homodimer with high thermal stability as well as acid tolerance. Size exclusion chromatography and dynamic light scattering show it to be a dimer at pH 7, but it converts to a monomer near pH 2. Circular dichroism spectra and fluorescence emission maxima are virtually indistinguishable from pH 7 to 2, indicating secondary and tertiary structures remain the same in dimer and monomer within experimental error. The tryptophan environment as probed by acrylamide quenching data yielded very similar data at pH 2 and pH 7, implying very similar folding for monomer and dimer. Differential scanning calorimetry shows a transition at 350.3 K for dimer and at 327.0 K for monomer. Thermal unfolding and chemical unfolding induced by guanidinium chloride for dimer are both reversible and can be described by two-state models. The temperatures and the denaturant concentrations at which one-half of the protein molecules are unfolded, are protein concentration-dependent for dimer but protein concentration-independent for monomer. The free energy of unfolding at 298 K was found to be 5.23 Kcal mol⁻¹ and 14.90 Kcal mol⁻¹ for the monomer and dimer respectively. The value of change in excess heat capacity upon protein denaturation (ΔC_p) is 3.42 Kcal mol⁻¹ K⁻¹ for dimer. The small ΔC_p for unfolding of CLA reflects a buried hydrophobic core in the folded dimeric protein. These unfolding experiments, temperature dependent circular dichroism and dynamic light scattering for the dimer at pH 7 indicate its higher stability than for the monomer at pH 2. This difference in stability of dimeric and monomeric forms highlights the contribution of inter-subunit interactions in the former.

Localization and environment of tryptophans in different structural states of concanavalin A

We have investigated the localization and environment of tryptophan residues in different quaternary and conformational states (tetrameric, dimeric, monomeric and unfolded) of metallized and demetallized concanavalin A (ConA) by selective chemical modification, fluorescence, and phosphorescence. ConA has four tryptophan residues (Trp 40, Trp 88, Trp 109 and Trp 182) per subunit. The pattern of oxidation by N-bromosuccinimide (NBS) shows that NBS modifies, in dimer, only Trp 182 which remains inaccessible in tetramer, two (Trp 88 along with Trp 182) in monomer, all four in unfolded form in presence of EDTA, and three (possibly Trp 40 along with Trp 88 and Trp 182) in unfolded form from native or remetallized ConA. Utilizing wavelength-selective fluorescence approach, we have observed a red edge excitation shift (REES) of 6-8 nm for tetramer and dimer. A more pronounced REES (11 nm) is observed for oxidized monomer compared to REES (3 nm) for unoxidized species. Acrylamide quenching shows the Stern-Volmer constant (K(SV)) for dimer, monomer, unfolded ConA and unfolded apo-ConA being 3.8, 5.2, 12.8, 14.0 M(-1), respectively. Phosphorescence studies at 77 K give more structured spectra, with two (0,0) bands at 406.2 (weak) and 413.2 nm for tetramer. However, a single (0,0) band appears at 413.2 for dimer and 412.6 nm for monomer, while the (0,0) band of the oxidized monomer is red shifted to 414.4 nm. These results may provide important insight into subtlety of organization and environment of tryptophans in the context of folding and structural studies of oligomeric proteins including lectins.

Effects of succinylation on thermal induced amyloid formation in Concanavalin A

We have recently shown that upon slight thermal destabilization the legume lectin Concanavalin A may undergo two different aggregation processes, leading, respectively, to amyloid fibrils at high pH and amorphous aggregates at low pH. Here we present an experimental study on the amyloid aggregation of Succinyl Concanavalin A, which is a dimeric active variant of Concanavalin. The results show that, as for the native protein, the fibrillation process appears to be favoured by alkaline pH, far from the isoelectric point of the protein. Moreover, it strongly depends on temperature and requires large conformational changes both at secondary and tertiary structure level. With respect to the native protein, the succinyl derivative forms amyloid fibrils in considerably longer times and with a minor exposure of hydrophobic regions. At physiological conditions, Concanavalin A still displays a sizeable tendency to form amyloid fibril, while the succinyl variant does not. A close correlation was observed between the progress of amyloid formation and a narrowing of the tryptophans fluorescence emission band, indicating a reduction of protein conformational heterogeneity in amyloid fibrils.

Amyloid fibrils formation and amorphous aggregation in concanavalin A

We here report an experimental study on the thermal aggregation process of concanavalin A, a protein belonging to the legume lectins family. The aggregation process and the involved conformational changes of the protein molecules were followed by means of fluorescence techniques, light scattering, circular dichroism, zeta potential measurements and atomic force microscopy. Our results show that the aggregation process of concanavalin A may evolve through two distinct pathways leading, respectively, to the formation of amyloids or amorphous aggregates. The relative extent of the two pathways is determined by pH, as amyloid aggregation is favored at high pH values ( approximately 9), while the formation of amorphous aggregates is favored at low pH ( approximately 5). At difference from amorphous aggregation, the formation of amyloid fibrils requires significant conformational changes on the protein, both at secondary and tertiary structural level. To our knowledge, this is the first observation of amyloid fibrils from concanavalin A.

Kinetic stability plays a dominant role in the denaturant-induced unfolding of Erythrina indica lectin

The urea-induced denaturation of dimeric Erythrina indica lectin (EIL) has been studied at pH 7.2 under equilibrium and kinetic conditions in the temperature range of 40-55 degrees C. The structure of EIL is largely unaffected in this temperature range in absence of denaturant, and also in 8 M urea after incubation for 24 h at ambient temperature. The equilibrium denaturation of EIL exhibits a monophasic unfolding transition from the native dimer to the unfolded monomer as monitored by fluorescence, far-UV CD, and size-exclusion FPLC. The thermodynamic parameters determined for the two-state unfolding equilibrium show that the free energy of unfolding (DeltaGu, aq) remains practically same between 40 and 55 degrees C, with a value of 11.8 +/- 0.6 kcal mol(-1) (monomer units). The unfolding kinetics of EIL describes a single exponential decay pattern, and the apparent rate constants determined at different temperatures indicate that the rate of the unfolding reaction increases several fold with increase in temperature. The presence of probe like external metal ions (Mn2+, Ca2+) does not influence the unfolding reaction thermodynamically or kinetically; however, the presence of EDTA affects only kinetics. The present results suggest that the ability of EIL to preserve the structural integrity against the highly denaturing conditions is linked primarily to its kinetic stability, and the synergic action of heat and denaturant is involved in the unfolding of the protein.