rhamnogalacturonan

Rhamnogalacturonan refers to two major structural motifs of pectin, a polysaccharide that is a key component of plant primary cell walls: rhamnogalacturonan I (RG-I) and rhamnogalacturonan II (RG-II). Together with homogalacturonan, RG-I and RG-II contribute to the walls’ porosity, hydration, and mechanical properties.

Rhamnogalacturonan I is built on a repeating disaccharide backbone of α-D-galacturonic acid (GalA) linked to α-L-rhamnose (Rha), written as [-4)-GalA-(1-2)-Rha-(1-]. The rhamnose residues commonly carry side chains that extend into the cell wall, typically consisting of galactan and arabinan polysaccharides, and in some cases other sugars. The degree and pattern of substitution vary among plant species and tissues, giving RG-I considerable structural flexibility.

Rhamnogalacturonan II is a structurally distinct, highly conserved domain of pectin with a more compact architecture and elaborate, branched side chains that include a variety of rare sugars. RG-II is often described as having a homogalacturonan-like backbone with extensive, complex substituents. A notable feature is its ability to form boron-mediated cross-links between RG-II molecules, which can enhance cell wall stability in the presence of borate.

Biosynthesis occurs in the Golgi apparatus, where specific glycosyltransferases assemble RG-I and RG-II, before they are secreted to the cell wall. Degradation involves pectinolytic enzymes such as polygalacturonases and rhamnogalacturonan hydrolases, which modify wall properties during growth, ripening, and defense.

Analytical approaches include monosaccharide composition and linkage analysis, NMR, and antibody-based methods targeting RG-I and RG-II epitopes to study distribution and structure in different plants.