2.2 The development history and current situation of N-GlcNAc in
China
N-glycosylation occurs on the amide nitrogen of the asparagine side
chain, and the sugar chain is covalently linked to the free NH2 group of
the aspartic acid of the protein. The synthesis of N-linked sugar chains
starts in the endoplasmic reticulum (ER) and completes in the Golgi
apparatus. The first step in the synthesis of N-glycans is to add a
14-glycan core oligosaccharide to the asparagine residue of the newly
formed polypeptide chain with the characteristic sequence of
Asn-Xaa-Ser/ Thr/Cys (Xaa can be any amino acid other than proline),
asparagine acts as a sugar chain receptor. The core oligosaccharide is
composed of 2 molecules of N-acetylglucosamine, 9 molecules of mannose
and 3 molecules of glucose. The first N-acetylglucosamine binds to the
phosphate group of the phosphopolyterpene alcohol on the ER bilayer
membrane. When a new polypeptide is synthesized on the ER membrane, the
entire sugar chain is transferred together. After transferring the
oligosaccharide to the nascent peptide, it is further processed in the
ER, and 3 molecules of glucose and 1 molecule of mannose are removed in
turn. The glycoprotein formed by ER has similar sugar chains. After
entering the Golgi apparatus from the Cis surface, most of the mannose
on the original sugar chain is removed during the transport between the
membrane vesicles, but a variety of glycosyltransferases are added in
turn. Different types of sugar molecules form oligosaccharide chains
with different structures. The glycan formed by N-glycosylation has only
one glycosyl group, but has multiple branches. Proteins in body fluids
such as plasma are mostly N-glycosylated, so N-glycoprotein is also
known as plasma glycoprotein. Almost all such glycosylation
modifications in animal cells are N-acetylglucosamine GlcNAc, and all
are β-configuration[18].