**2. N- and O-glycosylation**

Several proteins undergoes post translational modification by N-glycosylation that denote with the attachment of the N-acetylglucosamine (GlcNAc) to the nitrogen atom present in the Asn side chain of the amino acid by a β-1 N linkage [2]. These GlcNAc2 mannose (Man)3 core containing glycoconjugates shows the tendency to add/remove several monosaccharides. These conjugation reactions include galactosylation, sialylation, GlcNAclyation and fucosylation [2]. Moreover, glycosylation occur on hydroxyl functional group of the amino acids Ser and Thr. The most common sugars showing conjugation with Ser and Thr are GlcNAc and N-acetylgalactosamine (GalNAc) [5]. GalNAc associated glycans, often known as mucin-type O-glycans are abundantly found in extracellular spaces and secreted proteins like mucin [6]. Mucin is characterized with high number of Pro, Ser and Thr residues that makes it susceptible to the O-linked glycosylation. The participating sugars conjugate with the protein as it moves through the cis, medial and trans Golgi apparatus. The glycopeptide O-glycans are post transnationally modified by glycosyltransferases [5].
