| Health / Medical Topics |
Lactose Synthesis Pathway
The synthesis of glycosidic bonds between monosaccharides requires energy input, energy that is provided by joining sugar monomers to a nucleotide such as UDP. Glycosyltransferases join two sugar monomers together, with UDP as the leaving group that drives the reaction forward. One glycosyltransferases is galactosyltransferase. Galactosyltransferase 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. Galactosyltransferase 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 galactosyltransferase alone does not have the substrate specificity required to produce lactose. The complete lactose synthase complex includes galactosyltransferase and another non-catalytic subunit, alpha-lactalbumin. In the presence of alpha-lactalbumin, galactosyltransferase 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. (NCI Thesaurus/BIOCARTA)
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