**1. Introduction**

Glycosylation refers to the post translational modification of proteins, lipids and nucleic acids in the presence of enzymes. The common post translational modifications of biological importance include N-linked glycosylation, O-linked glycosylation, phospho–serine glycosylation, as well as C-mannosylation and glypation (addition of glycophosphatidylinositol. Glycosylation reaction is mediated by glycosyltransferases that conjugate carbohydrate to proteins, lipids and nucleic acids. Glycosylation is a biological mechanism that aids the protein folding and help in protein signaling along with cell-cell interaction. Recently mass spectrometry technique is recently gained attention in quantitative estimation of glycoproteomics. In COVID-19 infection, the SARS-CoV-2 viral proteins including spike(S), envelope (C), membrane (M) and nucleocapsid (N) undergo post translational modification of glycosylation and phosphorylation that plays important role in virulence and infectivity.

Defined in the broadcast sense, glycosylation is the conjugation process of joining carbohydrate to the protein's backbone via enzymatic reaction. Post modification after this reaction, the protein is termed as glycoprotein. In our body, the most common glycosylation reaction refers to the N-glycosylation and O-glycosylation. Among these two post modification process, the N-glycosylation is frequently occurring mechanism. This mechanism of post translation modification comes under the domain of glycobiology, which refers to the study of biosynthesis, structures and biology of saccharides (also known as sugar or carbohydrates). Glycosylation is a critical mechanism of the biosynthetic-secretory pathway that occurs in endoplasmic reticulum (ER) and Golgi apparatus [1]. I was a known fact that nearly half of the eukaryotic proteins undergo modification, which presents the covalent conjugation of sugar moieties to particular amino acids of interest [2]. Membrane-bound and soluble proteins along with secreted proteins, ligands, surface proteins that are transported from the Golgi to the cytoplasm also become glycosylated [2].

Lipids and proteoglycans are also among the susceptible targets of the glycosylation that significantly contribute in increasing the number of substrate for the post translation modification. Carbohydrate plays an important role in the regulation of multicellular organs and organism's functions as a resultant several biological macromolecules possess covalently attached saccharides including monosaccharides and oligosaccharides which are commonly known as glycans [3]. These glycans contribute a major role in modulating cell-to-cell interaction, development and function of cell, interaction between cell and host [4]. Protein bound glycans are abundant in the nucleus and cytoplasm and serve as the key regulator element there [4].
