Arabinogalactan, also known as galactoarabinan, larch arabinogalactan, and larch gum, is a

biopolymer Biopolymers are natural polymers produced by the cells of living organisms. Like other polymers, biopolymers consist of monomeric units that are covalently bonded in chains to form larger molecules. There are three main classes of biopolymers, ...

consisting of

arabinose Arabinose is an aldopentose – a monosaccharide containing five carbon atoms, and including an aldehyde (CHO) functional group. For biosynthetic reasons, most saccharides are almost always more abundant in nature as the "D"-form, or structural ...

and

galactose Galactose (, '' galacto-'' + ''-ose'', "milk sugar"), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a C-4 epimer of glucose. A galactose molecu ...

monosaccharides. Two classes of arabinogalactans are found in nature: plant arabinogalactan and microbial arabinogalactan. In plants, it is a major component of many gums, including

gum arabic Gum arabic, also known as gum sudani, acacia gum, Arabic gum, gum acacia, acacia, Senegal gum, Indian gum, and by other names, is a natural gum originally consisting of the hardened sap of two species of the ''Acacia'' tree, '' Senegalia se ...

and gum ghatti. It is often found attached to proteins, and the resulting

arabinogalactan protein Arabinogalactan-proteins (AGPs) are highly glycosylated proteins (glycoproteins) found in the cell walls of plants. Each one consists of a protein with sugar molecules attached (which can account for more than 90% of the total mass). They are mem ...

(AGP) functions as both an intercellular

signaling molecule In biology, cell signaling (cell signalling in British English) or cell communication is the ability of a Cell (biology), cell to receive, process, and transmit signals with its environment and with itself. Cell signaling is a fundamental property ...

and a glue to seal plant wounds. The microbial arabinogalactan is a major structural component of the

mycobacterial ''Mycobacterium'' is a genus of over 190 species in the phylum Actinomycetota, assigned its own family, Mycobacteriaceae. This genus includes pathogens known to cause serious diseases in mammals, including tuberculosis ('' M. tuberculosis'') a ...

cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mec ...

. Both the arabinose and galactose exist solely in the furanose configuration. The galactan portion of microbial arabinogalactan is linear, consisting of approximately 30 units with alternating β-(1-5) and β-(1-6)

glycosidic linkage A glycosidic bond or glycosidic linkage is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate. A glycosidic bond is formed between the hemiacetal or hemiketal group ...

s. The arabinan chain, which consists of about 30 residues, is attached at three branch points within the galactan chain, believed to be at residues 8, 10 and 12. The arabinan portion of the polymer is a complex branched structure, usually capped with

mycolic acids Mycolic acids are long fatty acids found in the cell walls of the Mycolata taxon, a group of bacteria that includes ''Mycobacterium tuberculosis'', the causative agent of the disease tuberculosis. They form the major component of the cell wall o ...

; the arabinan glycosidic linkages are α-(1-3), α-(1-5), and β-(1-2). The mycobacterial arabinogalactan is recognized by a putative immune lectin

intelectin Intelectins are lectins (carbohydrate-binding proteins) expressed in humans and other chordates. Humans express two types of intelectins encoded by ITLN1 and ITLN2 genes respectively. Several intelectins bind microbe-specific carbohydrate res ...

present in chordates.

Structure of microbial arabinogalactan

The reducing end of microbial arabinogalactan consists of the terminal sequence →5)-D-Gal''f''-(1→4)-L-Rha''p''-(1→3)-D-GlcNAc. A muramyl-6-P is also found within the

peptidoglycan Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like peptidoglycan layer outside the plasma membrane, the rigid cell wall (murein sacculus) characteristic of most ba ...

functional group. The mycolylarabinogalactan of mycobacteria is attached to the peptidoglycan by the actinomycete-specific diglycosylphosphoryl bridge, L-Rha''p''-(1→3)-D-GlcNAc-(1→P). Arabinogalactan contains a galactan chain, with alternating 5-linked β-D-galactofuranosyl (Gal''f'') and 6-linked β-D-Gal''f'' residues. The arabinan chains are attached to C-5 of some of the 6-linked Gal''f'' residues. There are three major structural domains for arabinan. The first is a domain consisting of linear 5-linked α-D-Ara''f'' residues. The second is a domain with branched 3,5 linked α-D-Ara''f'' residues substituted with 5-linked α-D-Ara''f'' units at both branched positions, and the third is A terminal non-reducing domain for end arabinan consisting of a 3,5-linked α-D-Ara''f'' residue substituted at both branched positions with the disaccharide β-D Ara''f''-(1→2)- α-D-Ara''f''. These three arabinan chains are attached to the galactan at residues 8, 10, and 12. The non-reducing end of arabinogalactan is

covalently A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...

attached to the mycolic acids of the outer membrane. The

hydrophobicity In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, t ...

of mycolic acids is a barrier to drug entry. Additionally, the mycolyl arabinogalactan peptidoglycan is responsible for aspects of disease

pathogenesis Pathogenesis is the process by which a disease or disorder develops. It can include factors which contribute not only to the onset of the disease or disorder, but also to its progression and maintenance. The word comes from Greek πάθος ''pat ...

and much of the

antibody response An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...

in infections. The mycolyl substituents are selectively and equally distributed on the 5-hydroxyl functions of terminal- and the penultimate 2-linked Ara''f'' residues. The mycolyl residues are clustered in groups of four on the non reducing terminal pentaarabinosyl unit (β-Ara''f''-(1→2)-α-Araf)₂-3,5-α-Ara''f ''. Thus, the majority (66%) of the pentaarabinosyl units are substituted by mycolic acids, leaving the minority (33%) available for interaction with the immune system. Approximately one of the three arabinosyl chains attached to the galactan chain contains

succinyl Succinic acid () is a dicarboxylic acid with the chemical formula (CH2)2(CO2H)2. The name derives from Latin ''succinum'', meaning amber. In living organisms, succinic acid takes the form of an anion, succinate, which has multiple biological r ...

groups. Although one succinyl group is most common, up to three succinyl groups per released arabinan fragment can be found on oligo-arabinans. However, arabinan fragments substituted with GalNH₂ are not succinylated. Importantly, in the case of ''M. tuberculosis'', and most likely in all slow growing organisms, both positive charge (protonated GalNH₂as GalNH₃⁺) and negative charge (succinyl) are present in the middle regions of the arabinan, specifically at O-2 of the inner 3,5-α-D-Ara''f'' units. The succinyl residues are on the non-mycolylated chain. Recently, a complete primary model of arabinogalactan has been proposed.

Commercial applications

It is used as a thickener in foods, in cosmetics, and is being studied for possible medical uses.

References

{{Bacteria Polysaccharides

Structure of microbial arabinogalactan

Commercial applications

See also

References