galactosamine

Galactosamine is an amino sugar, scientifically named 2-amino-2-deoxy-D-galactose. In most biological contexts the term refers to its N-acetylated derivative, N-acetylgalactosamine (GalNAc). The unsubstituted form exists as the D- and L-enantiomers, with the D-enantiomer predominating in mammalian glycoconjugates.

GalNAc is a major component of glycoproteins, proteoglycans, and glycosaminoglycans. It plays a central role in mucin-type O-glycosylation, where GalNAc is transferred to serine or threonine residues by a family of GalNAc-transferase enzymes, producing the Tn antigen (GalNAc-O-Ser/Thr) as an early intermediate. In glycosaminoglycans such as chondroitin sulfate and dermatan sulfate, GalNAc residues alternate with uronic acids to form repeating disaccharide units.

In blood group chemistry, GalNAc contributes to antigen formation; the A antigen is generated by an alpha1-3 linked GalNAc on a galactose moiety, yielding GalNAcα1-3Gal. This activity is mediated by specific glycosyltransferases and underlies the expression of blood group A.

Biosynthesis and metabolism are centered on UDP-GalNAc as the donor molecule. Enzymes known as UDP-GalNAc transferases attach GalNAc to proteins (and lipids) at serine or threonine residues during glycosylation. Free galactosamine is also available commercially in salt forms for research use. GalNAc-containing glycans are widely studied in biology and medicine, including investigations into development, immunology, and diseases such as cancer where mucin-type glycosylation patterns can be altered.