The synthesis of glycosidic bonds between monosaccharides requires energy input, energy that is provided by joining sugar monomers to a nucleotide such as UDP. Glycosyl transferases join two sugar monomers together, with UDP as the leaving group that drives the reaction forward. One glycosyl transferase is galactosyl transferase. Galactosyl transferase catalyzes the formation of a glycosidic bond between galactose and N-acetyl-D-glucosamine to create N-acetyllactosamine (D-galactosyl-N-acetyl-D-glucosamine), found in the oligosaccharides of glycoproteins. Galactosyl transferase is broadly expressed. This enzyme also catalyzes the synthesis of the disaccharide lactose, the sugar found in milk. Although this enzyme is broadly expressed, lactose production only occurs in mammary glands. Lactose production is restricted to the mammary glands because galactosyl transferase alone does not have the substrate specificity required to produce lactose. The complete lactose synthase complex includes galactosyl transferase and another non-catalytic subunit, alpha-lactalbumin. In the presence of alpha-lactalbumin, galactosyl transferase substrate specificity is switched to use glucose, resulting in lactose production rather than N-acetyllactosamine. The folding of alpha-lactalbumin, which is related to lysozyme, has been studied extensively as a model of the mechanisms involved in protein folding.
