Figure 1. The glycosyl residues that are known to be present in the three pectic polysaccharides that have been isolated from plant cell walls.

Figure 2. The structure of homogalacturonan. Homogalacturonan is composed of 1,4-linked -d-galactopyranosyluronic acid residues (GalpA). The carboxyl groups of the GalpA residues are often methyl-esterified. Some of the hydroxyl groups may be O-acetylated.

Figure 3. Selected examples of the structures of oligosaccharides that are attached to the backbone of RG-I. The rhamnosyl residue in A–C originates from the RG-I backbone. Structure E is O-(2-O-trans-feruloyl)--l-Araf-(1,5)-l-Ara.

Figure 4. The glycosyl sequence of rhamnogalacturonan II (RG-II). The structures of the side-chains (A–D) have been determined. The positions of these side-chains relative to each other along the backbone are not known and their positions have been assigned arbitrarily (denoted by ?”). In the plant cell wall, two RG-II molecules are crosslinked together by a single borate-diol ester to form a dimer (see Figure 5). OAc = O-acetyl ester, Me = O-methyl ether.

Figure 5. Borate ester crosslinking of rhamnogalacturonan II (RG-II). In the plant cell wall, two RG-II molecules are crosslinked together by a single borate-diol ester to form a dimer. The borate ester is located on C2 and C3 of two of the four 3¢-linked apiofuranosyl (Apif) residues of the dimer. The borate ester is believed to crosslink the Apif residue in each of the two 2-O-Me-Xyl-containing side-chains (R₁, see A in Figure 4) but not the Apif residue in each of the two aceric acid-containing side-chains (R₂, see D in Figure 4).

Figure 6. Mode of action of exo- and endopolygalacturonases. Exopolygalacturonases cleave the glycosidic bond of a terminal nonreducing GalpA residue (a), whereas endopolygalacturonases cleave the glycosidic bonds of internal GalpA residues (b).

Figure 7. Mode of action of endopectate lyases and endopectin lyases. Endopectate lyases cleave glycosidic bonds adjacent to an internal non-methyl-esterified GalpA residue (a). Endopectin lyases cleave internal glycosidic bonds adjacent to a methyl-esterified GalpA residue (b). Both enzymes cleave the glycosidic bond by -elimination and generate oligosaccharides terminated at their nonreducing end with a 4,5 unsaturated residue (4-deoxy--l-threo-enopyranosyluronic acid).

Figure 8. Mode of action of rhamnogalacturonan lyase. The enzyme cleaves the glycosidic bond between a 2-linked rhamnosyl residue and a 4-linked galactosyluronic acid residue by a -elimination reaction and generates oligosaccharides terminated at their nonreducing end with a 4-deoxy--l-threo-enopyranosyluronic acid residue. R = H or an oligosaccharide side-chain.