Arabinogalactan

Two types of arabinogalactan:

Type I: (→1)-β-D-galactose(4→) (main chain), (→1)-α-L-arabinose(5→) (side chain) linked at main chain C(3) [3].

Type II: (→1)-β-D-galactopyranose(3→) (main chain) and (→1)-β-D-galactopyranose-(6→) (side chain on virtually all units in the main chain) or (→1)-α-L-arabinofuranose(3→) (less frequent) or (→1)-D-glucopyranosyl uronic acid(6→) (least frequent). The β-D-galactopyranose and α-L-arabinofuranose substituents are in turn substituted by (→1)-β-D-glucopyranose-(6→) and (→1)-β-L-arabinopyranose-(3→) respectively, to give two-unit side-chain on almost every main chain residue [3]. Arabinogalactan is devoid of repeating units, but contains motifs, linkage and composition is highly dependent on the source [4]. The arabinogalactan from Mycobacterium tuberculosis contains five domains [6].

Larch arabinogalactan contains a polymer of (1→3)-β-linked D-galactopyranosyl as the main chain. Some of the units are replaced by (1→6)-linked L-arabinopyranose residues or by (1→6)-linked D-galactopyranose residues. Some of the D-galactosyl side chains, in turn, are substituted with a (1→3)-linked L-arabinofuracyl unit [11]. The L-arabinose to D-galactose ratio is 1:6 [11].

In soybean, the arabinogalactan consists of a (1→4)-β-linked D-galactopyranose backbone with substitution, every fourth or fifth residue, by (1→5)-α-linked D-arabinofuranose.

Urkonan A, B and C are composed of L-arabinose, D-xylose, D-galactose, D-glucose, L-rhamnose and D-galacturonic acid in the ratios A - 12:4:12:1:10. B - 12:4:12:1:2:4. C - 8:3:6:14:2:3 [2].

Most arabinogalactan samples are diversible into two fractions: A with M~100,000 and B with M~16,000. The 100,000-weight fraction is 67-90% [1]. True arabinogalactans have considerably higher molecular weights: 180.000-900.000 [5].