Galactosaminogalactan (commonly abbreviated as GAG or GG), is an exopolysaccharide composed of galactose and N-acetylgalactosamine (GalNAc). It is commonly found in the biofilm and

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 mech ...

of various fungal species. Although the sugar residues are arranged in no particular/discrete order, and thus a heteroglycan, the residues are all linked by α-1,4

glycosidic bonds 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 grou ...

. Galactosaminogalactan is typically extracted by ethanol precipitation from liquid culture or by alkaline treatment from the

. Once extracted, galactosaminogalactan becomes highly insoluble. In ''

Aspergillus fumigatus ''Aspergillus fumigatus'' is a species of fungus in the genus ''Aspergillus'', and is one of the most common ''Aspergillus'' species to cause disease in individuals with an immunodeficiency. ''Aspergillus fumigatus'', a saprotroph widespread in ...

'', a causative agent of aspergillosis, galactosaminogalactan is required for adherence to host tissue, to mask

PAMPs Pathogen-associated molecular patterns (PAMPs) are small molecular motifs conserved within a class of microbes. They are recognized by toll-like receptors (TLRs) and other pattern recognition receptors (PRRs) in both plants and animals. A vast arra ...

like β-1,3-glucans and to mediate virulence in several animal models. While its role in pathogenesis is still being defined, galactosaminogalactan has been found in histological sections of lungs of patients with aspergillosis. Besides its role in fungal virulence, certain fractions of laboratory purified galactosaminogalactan has been shown to induce neutrophil

apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...

and reduce inflammation.

Synthesis

Similar to other fungal cell wall polysaccharides, galactosaminogalactan is synthesized by polymerization of nucleotide sugars. Although the actual glycosyltransferase responsible for polymerization has not been reported, the synthesis of precursor nucleotide sugars has been studied. The galactose component originates from UDP-galactose and the GalNAc component originates from UDP-N-acetylgalactosamine. These nucleotide sugars are not physiologically favored and must to be converted from UDP-glucose and Uridine diphosphate N-acetylglucosamine (GlcNAc), respectively. The UDP-glucose 4-epimerase Uge3 is responsible for these conversions.Lee MJ, Gravelat FN, Cerone RP, Baptista SD, Campoli PV, Choe SI, Kravtsov I, Vinogradov E, Creuzenet C, Liu H, Berghuis AM, Latgé JP, Filler SG, Fontaine T, Sheppard DC. Overlapping and distinct roles of Aspergillus fumigatus UDP-glucose 4-epimerases in galactose metabolism and the synthesis of galactose-containing cell wall polysaccharides. J Biol Chem. 2014 Jan 17;289(3):1243-56.

References

{{Reflist Polysaccharides Galactose