Figure 1.

N-Glycan synthesis and glucose trimming in the ER. The synthesis of N-glycans begins on the cytoplasmic face of the ER membrane. The enzymes that catalyze each step in N-glycan biosynthesis are encoded by ALG genes. Firstly, GlcNAc-P is attached to the membrane-bound dolichol phosphate from UDP-GlcNAc by GlcNAc-1-phosphotransferase (ALG 7) and UMP is released. The addition of GlcNAc and mannose residues is catalyzed by ALG13/14, and ALG 1, 2, and 11 sequentially. The partially synthesized N-glycan (GlcNAc₂Man₅) is flipped across the ER membrane to the luminal side by an ATP-independent flippase. Four mannose and three glucose residues are added to generate the final mature N-glycan which is transferred to the nascent polypeptide chain by oligosaccharyltransferase (OST). The terminal two glucoses can be removed by glucosidase I and glucosidase II separately. Glucosidase II also removes the last glucose, which can be re-attached by UGGT.