N-glycosylation
N-glycosylation
The process of N-glycosylation consists of a covalent linkage of a specific oligosaccharide (Glc3Man9GlcNAc2) on a nascent protein. N-glycosylation is ubiquitous in eukaryotes. Once the oligosaccharide is transferred, several subsequent steps of maturation will occur along the secretory pathway.
First steps of N-glycosylation are conserved through eukaryotes (from yeast to human). They take place in the endoplasmic reticulum. The following and last steps of maturation, occurring in the Golgi apparatus and leading to polymannosylated, complex or hybrid structures, will determine the differences between species.
Diversity - heterogeneity
A great diversity of glycan structures exists between species, and also between tissues in same species. For example, in human, a given glycoprotein can be differently glycosylated in brain and kidney; such a glycoprotein could then exist in 2 different states of glycosylation, named glycoforms.
Cellular environment (traduction rate, Dol-P-oligosaccharide disponibility, etc.) strongly affects the N-glycosylation site occupancy. Each site will be mainly in one state: fully, partially or unglycosylated. 10% to 30% of potential N-glycosylation sites are estimated to be free.
N-glycosylation Roles
Stability
N-glycosylation affects physical and chemical properties of glycoproteins, modifying molecular weight, solubility and electrical charges. N-glycosylation also enhances the establishment of physiological protein conformation, increases thermostability and protects against proteolysis. These properties, involved together in the modulation of half-life time, will have high impact on therapeutic proteins effects.
Recognition
Many glycan structures can be identified by particular proteins called lectins, which are involved in recognition mechanisms, such as:
- Protein-protein, protein-cell, or cell-cell targeting,
- Interactions between host and foreign compounds (virus or bacteria).
Activity
Different glycoforms may also have different activities: a specific motif can modulate positively or negatively enzymatic activity. The heterogeneity of N-glycosylation reflects the cellular environment of the protein.
Diagnosis
In addition to its role in stability and activity, variations of N-glycosylation could be useful for diagnosis. For example, hypoglycosylation is associated with some congenital deficits of glycosylation (CDGs).
