Pectins are a family of complex polysaccharides that contain 1,4-linked α-D-galactosyluronic residues. Three pectic polysaccharides, homogalacturonan, rhamnogalacturonan-I and substituted galacturonans, have been isolated from primary plant cell walls. Homogalacturonan (HG) is a linear chain of 1,4-linked α-D-galactosyluronic residues, in which some of the carboxyl groups are methyl esterified. They may also be O-acetylated at the C-2 and C-3 positions. Homogalacturonans have been isolated from sunflower heads and apple pectin but were obtained by extraction treatments likely to cleave covalent bonds so they may have been released from a heterogeneous pectic polysaccharide.
Pectin is not a homopolysaccharide however and has rhamnopyranosyl residues inserted in the galactosyluronic backbone at 1 to 4% substitution. The other major feature of these rhamnogalacturonan-I (RG-I) chains are large substituted side chains. Between 20 and 80% of the rhamnopyranosyl residues are, depending on plant source and method of isolation, substituted at C-4 with neutral and acidic oligosaccharide side chains. The predominant side chains contain large linear and branched L-arabinofuranosyl and/or D-galactopyranosyl residues and their relative proportion and chain lengths may differ depending on plant source. Other rarer side chains are also present and generally shorter. These are illustrated in illustration 1.
The final and much more minor component of the backbone is rhamnogalacturonan-II (RG-II). This is not structurally related to RG-I since its backbone is composed of 1,4-linked D-galactosyluronic residues like HG. At approximately 30 glycosyl residues long it has a non-saccharide and an octasaccharide side chain attached to C-2 of some of the backbone residues and two structurally different disaccharides attached to C-3 of the backbone. RG-II is of interest as it occurs in relatively high amounts in wine and other fruit juices and it has been demonstrated that it binds heavy metals and has immunomodulating activities.
It is possible to separate essentially pure galacturonan fractions from other high molecular weight pectin fractions by degrading purified pectins specifically in the galacturonan backbone either chemically or enzymatically . It appears that there is an intramolecular distribution in which the neutral sugars are concentrated in blocks of more highly substituted rhamnogalacturonan regions ('hairy' regions) which are separated in the polymer by D-galactosyluronic-rich regions ('smooth' regions). These smooth regions can be up to 100 units in length.