Asparagine-linked glycosylation involves the sequential assembly of an oligosaccharide onto a polyisoprenyl donor, followed by the en bloc transfer of the glycan to particular asparagine residues within acceptor proteins. These N-linked glycans play a critical role in a wide variety of biological processes, such as protein folding, cellular targeting and motility, and the immune response. In the last decade, research in the field of N-linked glycosylation has achieved major advances, including the discovery of new carbohydrate modifications, the biochemical characterization of the enzymes involved in glycan assembly, and the biological impact of these glycans on target proteins. It is now firmly established that this enzyme-catalyzed modification occurs in all three domains of life. However, despite similarities in the overall logic of N-linked glycoprotein biosynthesis amongst the three kingdoms, the structures of the appended glycans are markedly different and thus influence the functions of elaborated proteins in various ways. Though nearly all eukaryotes produce the same nascent tetradecasaccharide (Glc3Man9GlcNAc2), heterogeneity is introduced into this glycan structure after transfer to protein through a complex series of glycosyl trimming and addition steps. In contrast, bacteria and archaea display diversity within their N-linked glycan structures through the use of unique monosaccharide building blocks during the assembly process. In a review, recent progress toward gaining a deeper biochemical understanding of this modification across all three kingdoms has been explained. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3101296/

In eukaryotes, N-linked glycosylation occurs at the membrane of the endoplasmic reticulum (ER), and involves the transfer of a tetradecasaccharide (Glc3Man9GlcNAc2) from a dolichyldiphosphate carrier onto the amide side chain nitrogen of an acceptor protein. Although a majority of the genetic and biochemical characterization of this pathway has been achieved in Saccharomyces cerevisiae, the process is remarkably conserved in all eukaryotes, from yeast to man. In S. cerevisiae, glycan assembly is carried out by a series of membrane-bound glycosyltransferases in the Alg family (asparagine-linked glycosylation), which catalyze the transfer of each monosaccharide onto a dolichyldiphosphate carrier. Dolichol, a long chain α-saturated polyisoprene that varies in size between 14 and 21 units depending on the cell type and species, is phosphorylated by a kinase (Sec59 in S. cerevisiae) prior to glycan assembly. In addition to its role as a membrane anchor, dolichylphosphate may serve other functions in the glycosylation process as well, such as promoting membrane fluidity, facilitating translocation across the ER membrane, and potentially recruiting key enzymes to the site of glycosylation https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3101296/

N-linked glycosylation, is the attachment of an oligosaccharide, a carbohydrate consisting of several sugar molecules, sometimes also referred to as glycan, to a nitrogen atom (the amide nitrogen of an asparagine (Asn) residue of a protein), in a process called N-glycosylation, studied in biochemistry. This type of linkage is important for both the structure and function of some eukaryotic proteins. The N-linked glycosylation process occurs in eukaryotes and widely in archaea, but very rarely in bacteria. The nature of N-linked glycans attached to a glycoprotein is determined by the protein and the cell in which it is expressed. It also varies across species. Different species synthesize different types of N-linked glycan. https://en.m.wikipedia.org/wiki/N-linked_glycosylation

There are three conditions to fulfill before a glycan is transferred to a nascent polypeptide:

Asparagine must be located in a specific consensus sequence in the primary structure (Asn–X–Ser or Asn–X–Thr or in rare instances Asn–X–Cys).

Asparagine must be located appropriately in the three dimensional structure of the protein (Sugars are polar molecules and thus need to be attached to asparagine located on surface of the protein and not buried within the protein)

Asparagine must be found in the luminal side of the endoplasmic reticulum for N-linked glycosylation to be initiated. Target residues are either found in secretory proteins or in the regions of transmembrane protein that faces the lumen. https://en.m.wikipedia.org/wiki/N-linked_glycosylation

Chinaza Godswill Awuchi Thank you for your detailed answer and information. It is much clear how the asparagine side chain amide can attach the glycan but glutamine also has uncharged polar amide, why glycosylation doesn't occur on glutamine?