Glycosylation is the most versatile and one of the most abundant of all co- and post-translational modifications. It results from the addition of sugar residues to a protein backbone to form a glycoprotein. Glycosylation plays an important role in many biological functions, including immune defense, fertilization, viral replication, parasitic infection, cell growth, inflammation, and cell-cell adhesion.
In vivo, glycosylation is tissue-dependent and can vary significantly with cell state. Proteins are glycosylated by the actions of a series of glycosidases and glycosyltransferases that act sequentially on the growing glycan as it passes through the lumen of the endoplasmic reticulum and the Golgi apparatus. Since the various enzymatic reactions may not all reach completion, a variety of glycan structures are commonly attached at each glycosylation site. Consequently, under a given set of conditions, different populations of glycosylation forms (glycoforms) may be generated for a single protein.
The number of glycoforms and their relative abundance within a cell are affected by the intrinsic structural properties of the individual protein, as well as the repertoire of glycosylation enzymes available (including their type, concentration, kinetic characteristics, compartmentalization). This repertoire has been shown to change upon changes in cell state (e.g., oncogenic transformation).
In vitro, glycosylation strongly depends on growth conditions—e.g., type of cell, nutrient concentrations, pH, cell density, and age of culture—all of which affect the glycosylation patterns of glycoproteins.