Glycosylation is the reaction in which a carbohydrate, i.e. a
glycosyl donor, is attached to a hydroxyl or other functional group of another molecule. In biology
glycosylation refers to the enzymatic process that attaches
glycans to
proteins,
lipids, or other
organic molecules. This enzymatic process produces one of the fundamental
biopolymers found in cells. Glycosylation is a form of co-translational and
post-translational modification. Glycans serve a variety of structural and functional roles in membrane and secreted proteins.
The majority of proteins synthesized in the rough
ER undergo glycosylation. It is an
enzyme-directed site-specific process, as opposed to the non-enzymatic chemical reaction of
glycation. Glycosylation is also present in the
cytoplasm and nucleus as the O-GlcNAc modification. Five classes of glycans are produced: