Fluorescence studies on the interaction of hydrophobic ligands with Momordica charantia (bitter gourd) seed lectin

Spectroscopic and thermodynamic characterization of the interaction between sugar-stabilised silver nanoparticles and wheat germ agglutinin (WGA), a chitin binding lectin

Nanomaterials have lately been investigated in agriculture as eco-friendly and effective antifungal agents. Many nanomaterials, notably metal nanoparticles, have strong antifungal properties. Among metal nanoparticles, Ag nanoparticles have received the most attention as antifungal agents. Many plant lectins have been identified as antifungal agents. Conjugating AgNPs with antifungal lectins is thus expected to improve Ag nanoparticle antifungal efficacy. Understanding the molecular interactions and physical features of lectin-sugar-stabilised nanoparticle conjugates is critical for future applications. WGA has traditionally been used as an anti-tumor and antifungal agent. To investigate the prospect of developing an effective biocompatible antifungal system with applications in medicine and agriculture, fluorescence spectroscopy was used to investigate the interaction between sugar-stabilised silver nanoparticles and WGA. During the association, protein intrinsic fluorescence emission is suppressed by about ∼15 % at saturation, with no significant shift in fluorescence emission maxima. Binding tests reveal a strong bond. Stern-Volmer analysis of the quenching data indicates that the interaction happens via a static quenching process that induces complex formation. The study of hemagglutination activity and interaction experiments in the presence of particular sugar shows that the lectin's sugar-binding site is separate from the nanoparticle-binding site, and cell recognition is conserved in the lectin-nanoparticle complex. The Van't Hoff plot thermodynamic parameters suggest that the contact is hydrophobic. The fact that ΔGo is negative shows that the binding is a spontaneous process. CD spectroscopy experiments reveal that the lectin's secondary structure is not affected while binding to the nanoparticle. Our findings suggest that a stable WGA-silver nanoparticle combination may emerge for a variety of applications.

Interaction of sugar stabilised silver nanoparticles with Momordica charantia seed lectin, a type II ribosome inactivating protein

Sugar-stabilised nanomaterials have received a lot of attention in cancer therapy in recent years due to their pronounced application as specific targeting agents and maximizing their therapeutic potential while bypassing off-target effects. Lectins, the carbohydrate-binding proteins, are capable of binding to receptors present on the target cell/tissue and interact with transformed glycans better than normal cells. Besides some of the lectins exhibit anticancer activity. Conjugating sugar-stabilised NPs with lectins there for is expected to multiply the potential for the early diagnosis of cancer cells and the specific release of drugs into the tumor site. Because of the prospective applications of lectin-sugar-stabilised nanoparticle conjugates, it is important to understand their molecular interaction and physicochemical properties. Momordica charantia Seed Lectin (MCL) is a type II RIP and has been known as an anti-tumor agent. Investigation of the interaction between sugar-stabilised silver nanoparticles and MCL has been performed by fluorescence spectroscopy to explore the possibility of creating an effective biocompatible drug delivery system against cancer cells. In this regard interaction between lectin and NPs should be well-preserved, while recognizing the specific cell surface sugar. Therefore experiments were carried out in the presence and absence of specific sugar galactose. Protein intrinsic fluorescence emission is quenched at ~ 20% at saturation during the interaction without any significant shift in fluorescence emission maximum. Binding experiments reveal a good affinity. Tetrameric MCL binds to a single nanoparticle. Stern-Volmer analysis of the quenching data suggests that the interaction is via static quenching leading to complex formation. Hemagglutination experiments together with interaction studies in the presence of specific sugar show that the sugar-binding site of the lectin is distinct from the nanoparticle-binding site and cell recognition is very much intact even after binding to AgNPs. Our results propose the possibility of developing MCL-silver nanoparticle conjugate with high stability and multiple properties in the diagnosis and treatment of cancer.

Purification, biochemical/biophysical characterization and chitooligosaccharide binding to BGL24, a new PP2-type phloem exudate lectin from bottle gourd (Lagenaria siceraria)

Phloem Protein 2 (PP2), highly abundant in the sieve elements of plants, plays a significant role in wound sealing and anti-pathogenic responses. In this study, we report the purification and characterization of a new PP2-type lectin, BGL24 from the phloem exudate of bottle gourd (Lagenaria siceraria). BGL24 is a homodimer with a subunit mass of ~24 kDa and exhibits high specificity for chitooligosaccharides. The isoelectric point of BGL24 was estimated from zeta potential measurements as 5.95. Partial amino acid sequence obtained by mass spectrometric studies indicated that BGL24 exhibits extensive homology with other PP2-type phloem exudate lectins. CD spectroscopic measurements revealed that the lectin contains predominantly β-sheets, with low α-helical content. CD spectroscopic and DSC studies showed that BGL24 exhibits high thermal stability with an unfolding temperature of ~82 °C, and that its secondary structure is essentially unaltered between pH 3.0 and 8.0. Fluorescence titrations employing 4-methylumbelliferyl-β-D-N,N',N″-triacetylchitotrioside as an indicator ligand revealed that the association constants for BGL24-chitooligosaccharide interaction increase considerably when the ligand size is increased from chitotriose to chitotetraose, whereas only marginal increase was observed for chitopentaose and chitohexaose. BGL24 exhibited moderate cytotoxicity against MDA-MB-231 breast cancer cells, whereas its effect on normal splenocytes was marginal.

Ligand binding and retention in snake gourd seed lectin (SGSL). A crystallographic, thermodynamic and molecular dynamics study

Snake gourd seed lectin (SGSL) is a non-toxic homolog of type II ribosome-inactivating proteins (RIPs) which contain a catalytic domain and a lectin domain. Isothermal titration calorimetry (ITC) measurements of the interactions of the protein with LacNAc, Lac, Gal, Me-α-Gal were carried out and the crystal structures of the native protein and its complex with Lac were determined. The crystal structure of the Me-α-Gal complex has already been determined. While the crystal structure showed the presence of two-sugar-binding sites, one on each of the two domains of the lectin chain, ITC measurements indicated the presence of only one binding site. In order to resolve this anomaly, molecular dynamics (MD) simulations were carried out on the native protein and on its complexes with Me-α-Gal and Lac. Simulations were also performed on the protein after reducing the inter-chain disulfide bridge between the two chains. The crystal structures and the simulations confirmed the robustness of the protein structure, irrespective of the presence or absence of the disulfide bridge. The simulations indicated that although two sites can bind sugar, only the ligand at one site is retained in a dynamic situation. The studies thus bring out the subtle relationship between binding and retention of the ligand.

Structural studies on a non-toxic homologue of type II RIPs from bitter gourd: Molecular basis of non-toxicity, conformational selection and glycan structure

The structures of nine independent crystals of bitter gourd seed lectin (BGSL), a non-toxic homologue of type II RIPs, and its sugar complexes have been determined. The four-chain, two-fold symmetric, protein is made up of two identical two-chain modules, each consisting of a catalytic chain and a lectin chain, connected by a disulphide bridge. The lectin chain is made up of two domains. Each domain carries a carbohydrate binding site in type II RIPs of known structure. BGSL has a sugar binding site only on one domain, thus impairing its interaction at the cell surface. The adenine binding site in the catalytic chain is defective. Thus, defects in sugar binding as well as adenine binding appear to contribute to the non-toxicity of the lectin. The plasticity of the molecule is mainly caused by the presence of two possible well defined conformations of a surface loop in the lectin chain. One of them is chosen in the sugar complexes, in a case of conformational selection, as the chosen conformation facilitates an additional interaction with the sugar, involving an arginyl residue in the loop. The N-glycosylation of the lectin involves a plant-specific glycan while that in toxic type II RIPs of known structure involves a glycan which is animal as well as plant specific.

Correlation of membrane binding and hydrophobicity to the chaperone-like activity of PDC-109, the major protein of bovine seminal plasma

The major protein of bovine seminal plasma, PDC-109 binds to choline phospholipids present on the sperm plasma membrane upon ejaculation and plays a crucial role in the subsequent events leading to fertilization. PDC-109 also shares significant similarities with small heat shock proteins and exhibits chaperone-like activity (CLA). Although the polydisperse nature of this protein has been shown to be important for its CLA, knowledge of other factors responsible for such an activity is scarce. Since surface exposure of hydrophobic residues is known to be an important factor which modulates the CLA of chaperone proteins, in the present study we have probed the surface hydrophobicity of PDC-109 using bisANS and ANS. Further, effect of phospholipids on the structure and chaperone-like activity of PDC-109 was studied. Presence of DMPC was found to increase the CLA of PDC-109 significantly, which could be due to the considerable exposure of hydrophobic regions on the lipid-protein recombinants, which can interact productively with the nonnative structures of target proteins, resulting in their protection. However, inclusion of DMPG instead of DMPC did not significantly alter the CLA of PDC-109, which could be due to the lower specificity of PDC-109 for DMPG as compared to DMPC. Cholesterol incorporation into DMPC membranes led to a decrease in the CLA of PDC-109-lipid recombinants, which could be attributed to reduced accessibility of hydrophobic surfaces to the substrate protein(s). These results underscore the relevance of phospholipid binding and hydrophobicity to the chaperone-like activity of PDC-109.

Misconceptions over Förster resonance energy transfer between proteins and ANS/bis-ANS: Direct excitation dominates dye fluorescence

Our aim was to disprove the widespread misconception that Förster resonance energy transfer (FRET) is the only explanation for observing fluorescence from ANS (8-anilino-1-naphthalenesulfonic acid) and bis-ANS (4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid, dipotassium salt) following excitation at 280nm in the presence of protein. From ultraviolet (UV) absorption spectra and fluorescence emission spectra of bis-ANS and ANS in buffer and ethanol, direct excitation at 280nm was found to be the dominant mechanism for the resulting dye fluorescence. Furthermore, Tyr/Trp quenching studies were performed for solutions of N-acetyl-l-tryptophanamide, heat-stressed immunoglobulin G (IgG), and bovine serum albumin (BSA) by monitoring changes in steady state fluorescence spectra and time-resolved fluorescence decays as a function of dye concentration. Stronger quenching of the intrinsic BSA and IgG fluorescence in steady state than in time-resolved fluorescence by bis-ANS and ANS pointed toward static quenching being the dominant mechanism in addition to dynamic quenching and/or FRET. In conclusion, one should consider the role of direct excitation of ANS and bis-ANS at 280nm to ensure a proper interpretation of fluorescence signals resulting from dye-protein interactions. When ANS or bis-ANS is to be used for protein characterization, we recommend selectively exciting the dyes at the higher absorption wavelength maximum (370 or 385nm, respectively).