Reactions for the synthesis of the small nucleotide-linked sugar substrates that are used in the synthesis of the N-glycan precursor and in the later steps of glycosylation are annotated here.
All these nucleotide-linked sugar donors are synthesized in the cytosol; however, to participate in the later reactions of N-glycan precursor biosynthesis (when the glycan is oriented toward the lumen of the endoplasmic reticulum (ER)), these substrates must be attached to a dolichyl-phosphate molecule and then flipped toward the luminal side of the ER, through a mechanism which is still not known but which involves a different protein than the one that mediates the flipping of the LLO itself (Sanyal et al. 2008). Two of the genes encoding enzymes involved in these reactions, MPI and PMM2, are known to be associated with Congenital Disorders of Glycosylation (CDG) diseases of type I. Of these, CDG-Ia, associated with defects in PMM2, is the most frequent CDG disease reported.