Rhamnogalacturonans

Rhamnogalacturonans are pectic polysaccharides that occur in the primary cell walls of land plants. They are grouped into two major classes, RG-I and RG-II, which differ in backbone structure and side chains.

Rhamnogalacturonan I (RG-I) has a backbone of repeating disaccharide units built from galacturonic acid and rhamnose. Attached to the rhamnose residues are diverse side chains rich in neutral sugars, especially galactose and arabinose, which contribute to the heterogeneity of RG-I between species and tissues. The side chains can influence cell wall porosity, hydration, and mechanical properties.

Rhamnogalacturonan II (RG-II) is smaller but more chemically complex. It features a relatively conserved backbone and an extensive set of side chains that include rare sugars. RG-II is notable for its ability to form borate cross-links between molecules, promoting dimerization and contributing to wall strength and integrity.

Analytical approaches include enzymatic degradation, chromatographic methods, and immunochemical detection using specific antibodies. Functionally, RG-I and RG-II support cell adhesion and wall architecture and influence the physical properties of pectin gels. In plants, the relative abundance and branching patterns of RG-I, RG-II, and their substitutions regulate wall porosity, extensibility, and responses to environmental conditions.