N-linked glycosylation

The formation of glycosidic bond is energetically unfavourable, therefore the reaction is coupled to the hydrolysis of two ATP molecules.

N-linked glycans are almost always attached to the nitrogen atom of an asparagine (Asn) side chain that is present as a part of Asn–X–Ser/Thr consensus sequence, where X is any amino acid except proline (Pro).

Once the precursor oligosaccharide is formed, the completed glycan is then transferred to the nascent polypeptide in the lumen of the ER membrane.

The removal of the final third glucose residue signals that the glycoprotein is ready for transit from the ER to the cis-Golgi.

However, if the protein is not folded properly, the glucose residues are not removed and thus the glycoprotein can't leave the endoplasmic reticulum.

[citation needed] Golgi enzymes play a key role in determining the synthesis of the various types of glycans.

The order of action of the enzymes is reflected in their position in the Golgi stack: Similar N-glycan biosynthesis pathway have been found in prokaryotes and Archaea.

[6] However, compared to eukaryotes, the final glycan structure in eubacteria and archaea does not seem to differ much from the initial precursor made in the endoplasmic reticulum.

[8] The glycosylation patterns on the various immunoglobulins including IgE, IgM, IgD, IgA, and IgG bestow them with unique effector functions by altering their affinities for Fc and other immune receptors.

On the other hand, eukaryotic expression hosts such as yeast and animal cells, have different glycosylation patterns.

Non-human mammalian expression systems such as CHO or NS0 cells have the machinery required to add complex, human-type glycans.

Furthermore, animal cells can also produce glycoproteins containing the galactose-alpha-1,3-galactose epitope, which can induce serious allergenic reactions, including anaphylactic shock, in people who have Alpha-gal allergy.

These drawbacks have been addressed by several approaches such as eliminating the pathways that produce these glycan structures through genetic knockouts.

Furthermore, other expression systems have been genetically engineered to produce therapeutic glycoproteins with human-like N-linked glycans.

These include yeasts such as Pichia pastoris,[18] insect cell lines, green plants,[19] and even bacteria.

The different types of lipid-linked oligosaccharide (LLO) precursor produced in different organisms.
Biosynthesis pathway of N -linked glycoproteins: The synthesis of N -linked glycan starts in the endoplasmic reticulum, continues in the Golgi and ends at the plasma membrane, where the N -linked glycoproteins are either secreted or becomes embedded in the plasma membrane.
Step-by-step synthesis of the precursor oligosaccharide in the ER lumen during N -linked glycosylation: the diagram illustrates the steps occurring in both the Phase I and Phase II as described in the table.
Glycan processing in the ER and Golgi.
The three major types of glycans.
The difference between the glycan produced by humans and animal cells. Human cells lack the Neu5Gc cap.