NacetylDgalactosamine

N-acetyl-D-galactosamine, written in common usage as N-acetylgalactosamine and abbreviated GalNAc, is an amino sugar that is the N-acetylated derivative of D-galactosamine. It is a hexosamine that typically exists in the pyranose ring form and can adopt alpha or beta anomers. In biology, GalNAc is most notable as a component of mucin-type O-glycans, where it is added to serine or threonine residues of proteins by polypeptide N-acetylgalactosaminyltransferases, forming GalNAc-O- linkages that initiate O-glycosylation.

The donor nucleotide sugar for these transfers is UDP-GalNAc, which is produced in cells from UDP-N-acetylglucosamine (UDP-GlcNAc) through epimerization and related steps. GalNAc residues are also central to the composition of glycosaminoglycans and proteoglycans, appearing in repeating units such as those in chondroitin sulfate and dermatan sulfate as N-acetylgalactosamine.

In addition to its structural roles, GalNAc is used in research and clinical contexts to study glycosylation patterns. The presence of GalNAc residues can be detected using specific lectins and antibodies, and truncated forms such as the Tn antigen (GalNAc linked to Ser/Thr) are of interest in cancer biology due to altered glycosylation in tumors.

Overall, N-acetyl-D-galactosamine is a fundamental monosaccharide derivative that serves as a key building block in diverse glycan structures and glycoprotein modification pathways across organisms.