|Superclasses:||all carbohydrates → a carbohydrate → a glycan → a polysaccharide → an arabinogalactan|
Arabinogalactans are biopolymers consisting of arabinose and galactose residues. Two main classes of arabinogalactans are found in nature: plant arabinogalactan and microbial arabinogalactan.
Plant arabinogalactans (AGs) are widely spread throughout the plant kingdom. Many edible and inedible plants are rich sources of these polysaccharides. AGs are associated with the pectin cell-wall component by physical bonds, and some of them are covalently linked to the complex pectin molecule as neutral side chains [Ebringerova05].
AG of type I has a linear (1→4)-β-D-Galp backbone, bearing 20-40% of α-L-Araf residues (1→5)-linked in short chains, in general at position 3. It is commonly found in pectins from citrus, apple and potato. Type I AG has been isolated from the skin of Opuntia ficus indica pear fruits [Habibi04].
AG of type II, known as arabino-3,6-galactan, has a (1→3)-β-D-Galp backbone heavily substituted at position 6 by mono- and oligosaccharide side chains composed of arabinosyl and galactosyl units. It is more widespread than the AG type I and occurs in cell walls of dicots and cereals often linked to proteins (known as arabinogalactan proteins). AG type II, along with galactomannans and cellulose, comprises the predominating polysaccharides of green arabica coffee beans [Fischer01]. In some case type II AGt contains a small amount of β-glucuronate.
AG type II occurs as minor, water-soluble components in softwoods. It is very abundant in the wood of larch trees (members of the genus Larix) - certain parts of the western larch ( Larix occidentalis) contain up to 35% AG [Whistler93]. However, larch AG type II is located in the lumen of the tracheids and ray cells, and is thus not a cell-wall component and by definition, not part of the hemicellulose of these trees.
Alderwick05: Alderwick LJ, Radmacher E, Seidel M, Gande R, Hitchen PG, Morris HR, Dell A, Sahm H, Eggeling L, Besra GS (2005). "Deletion of Cg-emb in corynebacterianeae leads to a novel truncated cell wall arabinogalactan, whereas inactivation of Cg-ubiA results in an arabinan-deficient mutant with a cell wall galactan core." J Biol Chem 280(37);32362-71. PMID: 16040600
Bhamidi08: Bhamidi S, Scherman MS, Rithner CD, Prenni JE, Chatterjee D, Khoo KH, McNeil MR (2008). "The identification and location of succinyl residues and the characterization of the interior arabinan region allow for a model of the complete primary structure of Mycobacterium tuberculosis mycolyl arabinogalactan." J Biol Chem 283(19);12992-3000. PMID: 18303028
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