Structure–Function Characterization of Terminalia catappa (TC) Seed Lectin by Chemical and Spectroscopic Approaches

Lectins are carbohydrate-binding proteins that play pivotal roles in numerous biological processes and have attracted considerable interest owing to their applications in glycoscience, diagnostics, and therapeutics. Despite the medicinal importance of Terminalia catappa, the structural basis underlying the carbohydrate-binding properties of its seed lectin remains largely unexplored. Selective chemical modification of amino acid residues was employed to investigate their functional significance in the lectin active site. Modification of tryptophan and carboxylate residues markedly reduced hemagglutination activity, whereas alteration of serine and histidine residues produced no significant effect. Fluorescence spectroscopy showed a maximum emission at 280 nm, indicating that the tryptophan residues are predominantly located within a hydrophobic microenvironment and participate in ligand binding. Fluorescence quenching studies showed no significant shift in the emission maximum following ligand interaction, suggesting that the overall protein conformation remained largely intact. Partial protection of tryptophan fluorescence by lactose further confirmed the direct involvement of tryptophan residues in the carbohydrate-binding site. Quenching studies using acrylamide, cesium chloride, and potassium iodide, supporting the localization of tryptophan residues within a negatively charged microenvironment. Kinetic analysis following N-bromosuccinimide (NBS) oxidation demonstrated that two tryptophan residues are essential for maintaining lectin activity. Overall, this study provides molecular insights into the structure–function relationship of T. catappa seed lectin by identifying specific amino acid residues and structural features responsible for its carbohydrate-binding properties.