Pectin has been shown to inhibit the actions of
galectin-3, a β-galactoside-
binding protein associated with
cancer progression. The structural features of
pectin involved in this activity remain unclear. We investigated the effects of different ginseng
pectins on
galectin-3 action. The
rhamnogalacturonan I-rich
pectin fragment, RG-I-4, potently inhibited galectin-3-mediated hemagglutination,
cancer cell adhesion and homotypic aggregation, and binding of
galectin-3 to T-cells. RG-I-4 specifically bound to the
carbohydrate recognition domain of
galectin-3 with a dissociation constant of 22.2 nm, which was determined by surface plasmon resonance analysis. The structure-activity relationship of RG-I-4 was investigated by modifying the structure through various enzymatic and chemical methods followed by activity tests. The results showed that (a)
galactan side chains were essential to the activity of RG-I-4, whereas
arabinan side chains positively or negatively regulated the activity depending on their location within the RG-I-4 molecule. (b) The activity of
galactan chain was proportional to its length up to 4 Gal residues and largely unchanged thereafter. (c) The majority of
galactan side chains in RG-I-4 were short with low activities. (d) The high activity of RG-I-4 resulted from the cooperative action of these side chains. (e) The backbone of the molecule was very important to RG-I-4 activity, possibly by maintaining a structural conformation of the whole molecule. (f) The isolated backbone could bind
galectin-3, which was insensitive to
lactose treatment. The novel discovery that the side chains and backbone play distinct roles in regulating RG-I-4 activity is valuable for producing highly active
pectin-based
galectin-3 inhibitors.