Preliminary Exploration of Structure-Immunostimulatory Activity Correlation of Spherical Pectin from Chrysanthemum Tea Infusion

The spherical pectin is an important bioactive component of chrysanthemum tea infusion, but its biological function, primary structure, and structure-activity relationship remain unclear. The present study evaluated the immunostimulatory activity of spherical pectin from Chrysanthemummorifolium Ramat. ‘Hangbaiju’ tea infusion in RAW264.7 cells and preliminarily investigated its structure-immunostimulatory activity relationship. The rhamnogalacturonan-I (RG-I) domain played a key role in the immunostimulatory activity of spherical pectin. Terminal and branched arabinose residues together accounted for 73.8% of the total arabinose residues in spherical pectin, indicating that the arabinan chains of spherical pectin were highly branched. The backbone of these arabinan chains consisted of →5)-α-Araf-(1→ repeats, and additional →5)-α-Araf-(1→ branches were linked to the backbone via α-1,3-glycosidic linkages. The spherical pectin rich in highly branched arabinan chains activated RAW264.7 cells via recognition by toll-like receptor 4 (TLR4). Molecular docking analysis revealed that →5)-α-Araf-(1→ branches in spherical pectin could bind to toll-like receptor 4/myeloid differentiation protein-2 (TLR4/MD-2) complexes and stabilize the dimer structure, which represents an important mechanism for its immunostimulatory activity. This study provides new insights into the structure-function relationship of spherical pectin.