Glycosylation refers to an enzymatic process involving the linking of saccharides to generate glycans (Haltiwanger  Lowe, 2004). This carbohydrates are usually linked to proteins and lipids in the cell. This is an important process in the body as it produces glycans which are significant biopolymers that are found within cells. The glycans that are generated have several structural and functional functions within membranes as well as secreted proteins especially in the folding of proteins. Most of the proteins that are produced in the rough Endoplasmic reticulum undergo this process of glycosylation. Glycosylation is also occurs in the nucleus in addition to the cytoplasm. There are five types of glycans that are produces as a result of glycosylation
N-linked glycans which are attached to arginine side chains.
O-linked glycans which are attached to hydroxyproline side chains.
C-linked glycans which are attached to tryptophan side chain.
Phospho-glycans which are connected through a phosphate of phospho-serines.
Glypiates which are formed as a result of the addition of a GPI anchor.

Glycosylation is both a co-translational as well as post-translational modification. It is an enzymatic controlled process that is site specific. Under this process, a donor molecule is usually added which in most cases are activated nucleotide sugars known as oligosaccharides. The process of glycosylation can take place on different portions of the same protein where the oligosaccharides attach themselves (Schachter, 2004). After attachment of the oligosaccharide and the protein is properly folded, the oligosaccharide is then removed from the protein which is then taken to the Endoplasmic Reticulum. N-linked glycosylation and O-linked glycosylation are the most common forms of glycosylation.

N-linked glycosylation
This process involves the addition of a 14-sugar molecule to the hydrogen of an asparagine located within the polypeptide sequence of the intended protein. This 14-sugar molecule is composed of 2 N-acetylglucosamine, 3 glucose, and 9 mannose, molecules.

O-linked glycosylation
O-linked glycosylation involves the addition of N-acetyl-galactosamine and which normally takes place at a later phase of protein processing within the Golgi apparatus (Hang  Bertozzi, 2005).


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