Structural Characterization and Immunomodulatory Activity of Two Homogalacturonan-Type Pectic Polysaccharides from Fruits of Schisandra chinensis (Turcz.) Baill.

Two homogalacturonan (HG)-type pectic polysaccharides, SCP-A2 (M_w, 5.37 × 10³ g/mol) and SCP-B3 (M_w, 6.10 × 10³ g/mol), were isolated from Schisandra chinensis fruits. Their structures were determined to consist of repeating units of →1)-α-d-GalAp-(4→, exhibiting varying degrees of esterification based on multiple chromatographic analysis techniques. The possible distribution of methyl esters in the HG region of SCP-A2 and SCP-B3 was elucidated using endopolygalacturonase (endo-PG) hydrolysis combined with HILIC-ESI^--QTOF-MS/MS and UPSEC-ESI^--MS^E. Surface plasmon resonance analysis revealed that both SCP-A2 (K_D = 1.12 × 10^-4 and 2.27 × 10^-4 M for TLR2/4) and SCP-B3 (K_D = 4.80 × 10^-5 and 6.68 × 10^-5 M for TLR2/4) exhibited strong binding affinities to TLR2/4. However, SCP-B3 showed approximately twice the affinity for TLR2/4 than SCP-A2. The in vitro assessment of immunomodulatory activity further confirmed that SCP-B3 significantly promoted the secretion of NO and upregulated the levels of TNF-α and IL-6 more effectively than SCP-A2. These results indicate that SCP-B3 may display a more significant immune activation capability due to its higher affinity for TLR2/4.