Galactosyltransferases

Galactosyltransferases are a broad class of enzymes within the glycosyltransferase superfamily that catalyze the transfer of galactose residues to acceptor molecules. In most systems, the activated donor is UDP-galactose, though other donor substrates exist for specific pathways. The transferred galactose forms glycosidic bonds with proteins, lipids, or other sugars, contributing to the assembly of complex glycans found on glycoproteins, glycolipids, and, in some pathways, proteoglycans.

Most galactosyltransferases reside in the Golgi apparatus as type II transmembrane proteins, with a short cytosolic tail, a single transmembrane segment, and a lumenal catalytic domain. They operate within the secretory pathway, participating in N- and O-glycosylation and glycolipid biosynthesis. Many examples require accessory factors or altered substrate specificity under particular conditions, such as lactose synthase, a complex of beta-1,4-galactosyltransferase (B4GALT1) and alpha-lactalbumin that synthesizes lactose in mammary tissue.

In humans, several gene families encode galactosyltransferases, including the beta-1,4-galactosyltransferases (B4GALT1–7) and beta-1,3-galactosyltransferases (B3GALT families), which contribute to diverse glycan structures. Other notable enzymes include alpha-1,3-galactosyltransferases (e.g., GGTA1), which establish specific glycan epitopes. The activity of galactosyltransferases is essential for proper glycoprotein and glycolipid maturation, and aberrant function can contribute to congenital disorders of glycosylation and altered cellular recognition. They are also a focus of glycoengineering efforts to tailor glycan structures for therapeutics.