sialyltransferase

Sialyltransferases are a family of glycosyltransferase enzymes that catalyze the transfer of sialic acid, typically N-acetylneuraminic acid (Neu5Ac), from the donor substrate CMP-Neu5Ac to acceptor sugars on glycoproteins and glycolipids. This enzymatic activity generates sialylated glycans that influence cell–cell recognition, receptor function, and signaling. Most sialyltransferases reside in the Golgi apparatus as type II transmembrane proteins with a short cytoplasmic tail, a single transmembrane segment, and a lumenal catalytic domain.

They act on diverse glycan acceptors, including terminal galactose or GalNAc residues, producing various sialic acid linkages. The main linkage types are alpha2-3, alpha2-6, and alpha2-8, which determine interactions with selectins, pathogens, and other glycan-binding proteins. The donor substrate is CMP-Neu5Ac, and many sialyltransferases require divalent cations for activity.

Several gene families encode human sialyltransferases, with distinct but overlapping specificities. The ST3GAL family creates Neu5Ac alpha2-3 linkages to galactose; ST6GAL enzymes install Neu5Ac alpha2-6 linkages to galactose; ST6GALNAC enzymes sialylate GalNAc residues, contributing to sialyl-Tn and related antigens; polysialyltransferases of the ST8SIA family form long alpha2-8-linked chains on substrates such as NCAM. Humans express multiple members, including ST3GAL1-6, ST6GAL1-2, ST6GALNAC1-4, and ST8SIA1-6, with tissue-specific and developmentally regulated expression patterns.

Biologically, sialylation modulates immune interactions, cell adhesion, and pathogen binding. It influences cancer progression and metastasis, inflammatory responses, and neural development. Defects in sialylation pathways, such as CMP-sialic acid transport or specific sialyltransferases, can contribute to congenital disorders of glycosylation and related diseases, making these enzymes important in research and potential therapeutic targeting.