Oligosaccharide chains of glycoproteins, glycolipids and glycosaminoglycans are synthesized by glycosyltransferases by the transfer of specific glycosyl moieties from activated sugar-nucleotide donors to specific acceptors. Structural studies on several of these enzymes have shown that one or two flexible loops at the substrate-binding site of the enzymes undergo a marked conformational change from an open to a closed conformation on binding the donor substrate. This conformational change, in which the loop acts as a lid covering the bound donor substrate, creates an acceptor-binding site. After the glycosyl unit is transferred from the donor to the acceptor, the saccharide product is ejected and the loop reverts to its native conformation, thereby releasing the remaining nucleotide moiety. The specificity of the sugar donor is determined by a few residues in the sugar-nucleotide-binding pocket of the enzyme that are conserved among the family members from different species.