3.1.2  Glycosylation
Glycosylation and its related products, i.e., glycans, introduce changes to the viral envelope that make the virus fitted for interaction with the host cell membrane [5]. Generally, glycans are oligosaccharides linked to the dense decoration of the spike glycoprotein. In particular, these oligosaccharides have shown to influence the folding of the S protein and proteolytic process so that they facilitate the virus cell entry. Moreover, a virus with the glycosylated glycoprotein gains an extra feature for an escape from the immune responses. Therefore, glycans are a good target for vaccine design.
Two main types of glycans are N-linked and O-linked glycans. Both are released from glycoproteins. Whereas enzymes fulfill the construction of N-glycans, chemical methods perform the release of O-glycans. N-glycans are linked to the amino acid asparagine (Asn) residues (Asn-any amino acids except for proline- Ser or Thr) utilizing an N-glycosidic bond, mostly N-acetylglucosamine. O-glycans are attached to the amino acid serine (Ser) and threonine (Thr) residues by the addition of an N-acetyl galactosamine (GalNAc). For example, N-glycans exist in Hendra virus, SARS-CoV, influenza virus, hepatitis virus, HIV-1, and West Nile virus [6], and O-glycans have occurred in the Ebola virus.
The 2019-nCoV S protein includes 13 and 9N-linked glycosylation sequons in the S1 and S2 subunit, respectively [3]. All of these have previously occurred in the SARS-CoV S glycoprotein, except for four-linked glycosylation sequons in the S1. Also, due to the existence of an amino acid proline in the polybasic cleavage site, which makes the inserted sequence PRRA, there are three O-linked glycans introduced to the 2019-nCoV RBD residues S673, T678, and S686 [7].