Figure 1.

N-linked glycan biosynthetic pathway and defects of protein N-glycosylation.

Mannose-6-phosphate (Man-6-P) is generated from the fructose-6-phosphate (Fru-6-P) and converted to mannose-1-phosphate (Man-1-P). Activated mannose (Man) and activated N-acetylglucosamine (GlcNAc) are used to build the lipid-linked oligosaccharide (LLO) precursor, which is assembled on the activated dolichol on the endoplasmic reticulum (ER) membrane and flipped into the ER lumen. Further extension yields oligosaccharide consisting of 2 GlcNAc, 9 Man and 3 glucoses (Glc) which is transferred onto asparagines residues of the polypeptide. Additional trimming and processing of oligosaccharide continues in the Golgi apparatus (GA). CDGs are named according to novel nomenclature, with the abbreviated old names give in brackets. Fru – fructose; Man – mannose; GDP – guanidine diphosphate; UDP – uridine diphosphate; OST – oligosaccharyltransferase.