Tryptophan exposure and accessibility in the chitooligosaccharide-specific phloem exudate lectin from pumpkin (Cucurbita maxima). A fluorescence study

Effect of temperature and pH on the structure and stability of tumor-specific lectin jacalin and insights into the location of its tryptophan residues: CD, DSC and fluorescence studies

Jacalin, the jackfruit seed lectin, exhibits high specificity for the tumor-specific T-antigen and is used in various biomedical and biotechnological applications. Here, we report biophysical studies on the thermal unfolding of jacalin and the effect of pH and temperature on its secondary structure. Differential scanning calorimetric (DSC) studies revealed that native jacalin unfolds at ∼60 °C and that carbohydrate binding stabilizes the protein structure. Circular dichroism spectroscopic studies indicated that the secondary structure of jacalin remains mostly unaffected over pH 2.0-9.0, whereas considerable changes were observed in the tertiary structure. DSC experiments demonstrated that jacalin exhibits two overlapping transitions between pH 2 and 5, which could be attributed to dissociation of the tetrameric protein into subunits and their unfolding. Interestingly, only one transition between pH 6 and 9 was observed, suggesting that the subunit dissociation and unfolding occur simultaneously. While quenching of the protein intrinsic fluorescence by acrylamide increased significantly upon carbohydrate binding, quenching by succinimide is essentially unaffected. We attribute this difference to increased exposure of Trp-123 in the α-chain as it is involved in carbohydrate binding. Both acrylamide and succinimide gave biphasic Stern-Volmer plots, consistent with differential accessibility of the two tryptophan residues of jacalin to them.

Subunit association, and thermal and chemical unfolding of Cucurbitaceae phloem exudate lectins. A review

Detailed characterization of protein (un)folding intermediates is crucial for understanding the (un)folding pathway, aggregation, stability and their functional properties. In recent years, stress-inducible lectins are being investigated with much interest. In plants phloem proteins PP1 and PP2 are major components of the phloem fluid. While PP1 is a structural protein, PP2 exhibits lectin activity, and was proposed to play key roles in wound sealing, anti-pathogenic activity, and transportation of various molecules including RNA within the plant. Cucurbitaceae fruits contain high concentrations of PP2 lectins, which recognize chitooligosaccharides with high specificity. Although the presence of PP2 lectins in the phloem exudate of Cucurbitaceae species was documented over 40 years ago, so far only a few proteins from this family have been purified and characterized in detail. This review summarizes the results of biophysical studies aimed at investigating the oligomeric status of these lectins, their thermal stability, structural perturbations caused by changes in pH and addition of chaotropic agents and characterization of intermediates observed in the unfolding process. The implications of these results in the functional roles played by PP2 type lectins in their native environment are discussed. Finally, perspectives for future biophysical research on these proteins are given.

Macromolecular Crowding Significantly Affects the Conformational Features and Carbohydrate Binding Properties of CIA17, a PP2-Type Lectin from Coccinia indica

The effect of macromolecular crowding on the conformational features and carbohydrate binding properties of CIA17, a PP2-type lectin, was investigated employing polymeric dextrans D6, D40, and D70 (M_r ∼ 6, 40, and 70 kDa, respectively) as crowders. While the secondary structure of CIA17 was significantly affected by D6, with a considerable decrease in the number of β-sheets and β-turns with a corresponding increase in the number of unordered structures, relatively smaller changes were induced by D40 and D70. However, differential scanning calorimetry (DSC) studies revealed that the thermal stability of the protein remains unchanged in the presence of crowders. While the larger dextrans, D70 and D40, induced modest quenching (∼10%) of the protein fluorescence by a static pathway, the smaller D6 induced a higher degree of quenching (37%), which involved both static and collisional quenching processes. The results of fluorescence correlation spectroscopy measurements together with DSC studies suggested that CIA17 forms larger oligomers in the presence of D40 and D70 but D6 prevents the formation of higher-order oligomers. The association constant for the CIA17-chitooligosaccharide interaction increased by ∼30% and 160% in the presence of D40 and D70, respectively, but decreased by ∼30% in the presence of D6. The higher binding affinity can be attributed to the excluded volume effect, i.e., an increased effective concentration of the protein in the presence of D40 and D70, whereas D6, being smaller, possibly penetrates into the protein interior, disrupting the water structure around the protein and also inducing conformational changes, resulting in weaker binding. These observations demonstrate that molecular crowding significantly affects the carbohydrate binding characteristics of lectins, which can modulate their physiological function.

Low-pH Molten Globule-Like Form of CIA17, a Chitooligosaccharide-Specific Lectin from the Phloem Exudate of Coccinia indica, Retains Carbohydrate-Binding Ability

pH-induced changes in the conformation, structural dynamics, and carbohydrate-binding activity of Coccinia indica agglutinin (CIA17), a PP2-type lectin, were investigated employing biophysical approaches. The secondary structure of CIA17 remains nearly unaltered over a wide pH range (2.0-8.5), while the tertiary structure of the protein exhibits considerable changes. A decrease in the fluorescence intensity and excited-state lifetime at low pH indicated perturbation in the local conformation (near Trp residues) of CIA17, which was further supported by enhancement in the Trp accessibility toward charged quenchers under acidic conditions. Fluorescence correlation spectroscopic studies indicated that at pH 2.0, CIA17 exists as a monomer over the concentration range of 10-200 nM and forms dimers at higher concentrations (K_D ∼ 387 nM) but could not form higher oligomers even at ∼150-fold higher concentrations, unlike under native conditions at pH 7.4. Thermal unfolding of the low pH intermediate involves two distinct steps: dissociation of a dimer to a monomer, followed by the unfolding of the monomer. These results strongly suggest that the acid-induced unfolding pathway of CIA17 involves the formation of a monomeric molten globule-like intermediate, which retains appreciable carbohydrate-binding ability. These observations are of great physiological significance since the PP2 proteins are involved in plant defense responses.

A Novel Biocompatible Ternary Nanoparticle with High Antibacterial Activity: Synthesis, Characterization, and Its Application in Beef Preservation

Edible nanoparticles containing antibacterial agents are one of the effective strategies to control foodborne diseases. Herein, novel ternary nanoparticles (TNP) were prepared from rosemary essential oil (REO), nisin and Lycium barbarum polysaccharides (LBP) through hydrophobic and electrostatic interaction. The average particle size of TNP was 211.5 nm, and its encapsulation efficiency reached 86.6%. After the addition of LBP, the physical stability, thermal stability and storage stability of TNP were significantly improved. In vitro, compared with the control group, the population of S. aureus and E. coli O157:H7 in the TNP-treated group was reduced by 2.386 log CFU/mL and 1.966 log CFU/mL, respectively, on the fifth day. The free radical scavenging rate of TNP was 63.15%. The application of TNP on beef presented favorable preservation effects without affecting its color and texture. Therefore, the synthesis strategy of TNP has important reference significance for the research and development of new food antibacterial agents.

Protective effect of β-cyclodextrin on stability of nisin and corresponding interactions involved

Nisin has been extensively employed in food industry as an antimicrobial peptide, but the proteolytic degradation and interference by food components resulted in the decrease of its antimicrobial activity. This study intends to explore the protective effect of β-cyclodextrin (β-CD) on nisin and corresponding interactions involved. The antibacterial experiments indicated that the combination of nisin and β-CD could provide a better antibacterial effect on cooked pork meat. Spectral analysis with UV-vis and fluorescence spectroscopy showed that the micro-environment of nisin could be influenced by β-CD. Tricine-SDS-PAGE results exhibited that the combination of nisin and β-CD could delay the degradation of nisin and improve its stability in the presence of trypsin. In sum, the study provided a potential approach to improve the stability of nisin, especially in the presence of trypsin.

The microbial transglutaminase immobilization on carboxylated poly(N-isopropylacrylamide) for thermo-responsivity

Microbial transglutaminase (mTG) is widely utilized in the PEGylation of pharmaceutical proteins. mTG immobilization can be achieved via covalent bonding on solid supports. However, the catalytic efficiency of mTG immobilized on solid supports was significantly reduced by mass transfer limitation. To overcome this limitation, mTG was covalently immobilized on the thermo-responsive carboxylated poly(N-isopropylacrylamide) (pNIPAM). The pNIPAM-mTG conjugate exhibited reversibly solubility in aqueous solution with a low critical solution temperature (LCST) at 39°C, i.e., it was insoluble above 39°C and soluble below 39°C. The pH dependence of the pNIPAM-mTG conjugate was similar with that of the native mTG. Upon conjugation to pNIPAM, the optimal temperature of mTG shifted down from 50-55°C to 40-45°C, and the thermal stability of the conjugate was elevated. The easy separation of the pNIPAM-mTG conjugate with its substrate and the catalytic efficiency of the pNIPAM-mTG conjugate were demonstrated by employing the pNIPAM-mTG conjugate to cross-link bovine serum albumin (BSA) and catalyze PEGylation of therapeutic protein, cytochrome c (Cyt C), respectively. The thermo-responsive mTG is suitable to modify proteins in food processing and biomedical engineering.

Novel unfolding sequence of banana lectin: Folded, unfolded and natively unfolded-like monomeric states in guanidine hydrochloride

The sequence of unfolding events of dimeric banana lectin (Banlec), as induced by guanidine hydrochloride (GdnHCl), has been investigated by size-exclusion HPLC, fluorescence, far-UV CD, low temperature phosphorescence and selective chemical modification. 8-Anilino-1-naphthalenesulfonate (ANS) binding indicates a structured unfolding intermediate which has been characterized as dissociated monomer by size-exclusion chromatography. Interestingly, the unfolding elution pattern reveals two distinct unfolded states. One is a usual random coil. The other represents a novel species having elution behavior and structural compactness (Stokes radius) similar to dissociated monomer but showing no regular secondary structure as determined by far-UV CD, thus resembling a natively unfolded state. N-Bromosuccinimide (NBS) oxidation shows that single tryptophan residue remains unmodified in dissociated monomer intermediate while the same is oxidized in natively unfolded-like species. Such difference in tryptophan environment in these species is supported by acrylamide quenching studies, and phosphorescence results at 77 K which show a blue-shift of (0,0) band from 414.8 nm to 409.2 nm. The present results reveal subtlety of structural characteristics of unfolded states of Banlec in GdnHCl, which provide important insight in protein unfolding reaction.

Mutational analysis of the carbohydrate binding activity of the tobacco lectin

At present the three-dimensional structure of the tobacco lectin, further referred to as Nictaba, and its carbohydrate-binding site are unresolved. In this paper, we propose a three-dimensional model for the Nictaba domain based on the homology between Nictaba and the carbohydrate-binding module 22 of Clostridium thermocellum Xyn10B. The suggested model nicely fits with results from circular dichroism experiments, indicating that Nictaba consists mainly of β-sheet. In addition, the previously identified nuclear localization signal is located at the top of the protein as a part of a protruding loop. Judging from this model and sequence alignments with closely related proteins, conserved glutamic acid and tryptophan residues in the Nictaba sequence were selected for mutational analysis. The mutant DNA sequences as well as the original Nictaba sequence have been expressed in Pichia pastoris and the recombinant proteins were purified from the culture medium. Subsequently, the recombinant proteins were characterized and their carbohydrate binding properties analyzed with glycan array technology. It was shown that mutation of glutamic acid residues in the C-terminal half of the protein did not alter the carbohydrate-binding activity of the lectin. In contrast, mutation of tryptophan residues in the N-terminal half of the Nictaba domain resulted in a complete loss of carbohydrate binding activity. These results suggest that tryptophan residues play an important role in the carbohydrate binding site of Nictaba.

The effects of formins on the conformation of subdomain 1 in actin filaments

In this study we investigated the effects of formins on the conformation of actin filaments by using the method of fluorescence quenching. Actin was labelled with IAEDANS at Cys(374) and the quencher was acrylamide. The results showed that formin binding induced structural changes in the subdomain 1 of actin protomers which were reflected by greater quenching constants (K(SV)). Simultaneously the fraction of the fluorophore population accessible for the quencher (alpha) decreased. These observations suggest that the conformational distribution characteristic for the actin protomers became broader after the binding of formins, for which the structural framework was provided by a more flexible protein matrix in the microenvironment of the label. The effects of formins depended on the formin:actin molar ratio, and also on the ionic strength of the medium. These observations are in agreement with previous results and underline the importance of the intramolecular conformational changes induced by formins in the structure of actin filaments.