Glycosyltransferases

These enzymes transfer glycosyl residues from nucleotide-activated phosphoglycosides to other carbohydrates or aglycans. They are involved in post-translational modification of proteins ( glycosylation). More than 150 different glycosyltransferases are known but there may be at least double this number to be characterized. For example 6 N-Acetylglucosaminyltransferases (GlcNAc-T) are involved in the addition of GlcNAc to the growing complex type oligosaccharide chains. They require Mn (2+) ions.

Glycosyltransferases are localized in the membranes of the ER and GA ( Golgi Apparatus).

They are very specific for both the donor sugar nucleotide and the acceptor molecule. One enzyme catalyzes the formation of only one glycosidic linkage.

Only small changes (such as deoxygenation or methylation) are permitted at some hydroxyl groups of an acceptor oligosaccharide. Other hudroxyls are critical for the acceptor to remain a substrate (particularly the hydroxyl to which the donor sugar is transferred). Epimers are not tolerated at any hydroxyl group.

The sequences of different glycosyltransferases show the very little homology ( even within the enzymes recognizing the same acceptor and donor). Some glycosyltransferases have a "sialylmotif" but it is not known if this corresponds to a substrate binding domain.