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

Galactose Glycopolymer-Grafted Silica Nanoparticles: Synthesis and Binding Studies with Lectin

Weak binding of carbohydrates with protein receptors possesses serious drawbacks in the advancement of therapeutics; however, the development of strategies for multipoint interactions between carbohydrates and protein can overcome these challenges. One such method is developed in this work where glycopolymer-grafted silica nanoparticles with a large number of carbohydrate units are prepared for the interactions with multiple binding sites of the protein. First, a glycomonomer, β-d-galactose-hydroxyethyl methacrylate (β-GEMA), was synthesized in a two-step process by coupling β-d-galactose pentaacetate and hydroxyethyl methacrylate (HEMA), followed by deacetylation for the preparation of poly(β-GEMA) glycopolymers (GPs). Further, the poly(β-GEMA) chains were grafted onto the silica nanoparticle (SiNP) surface by utilizing the "grafting-from" strategy of surface-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization to prepare p(β-GEMA)-grafted SiNPs (GNPs). Five different chain lengths ranging from 10 to 40 kDa of the GPs and the GNPs were prepared, and various characterization techniques confirmed the formation of GPs and grafting of the GPs on the SiNP surface. The particle size of GNPs and the number of GPs grafted on the SiNP surface showed a strong dependence on the chain length of the GPs. Further, the GNPs were subjected to a binding study with β-galactose-specific protein peanut agglutinin (PNA). A much stronger binding in the case of GNPs was observed with an association constant ∼320 times and ∼53 times than that of the monomeric methyl-β-d-galactopyranoside and the GPs, respectively. Additionally, the binding of the PNA with GNPs and GPs was also studied with varying chain lengths to understand the effects of the chain length on the binding affinity. A clear increase in binding constants was observed in the case of GNPs with increasing chain length of grafted GPs, attributed to the enhanced enthalpic and entropic contributions. This work holds its uniqueness in these improved interactions between carbohydrates and proteins, which can be used for carbohydrate-based targeted therapeutics.

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.