Branching of N-linked glycans

Mammalian N-glycans can carry up to four GlcNAc branches, thereby yielding tetra-antennary glycans. The third and fourth branches are initiated by MGAT4 and MGAT5 GlcNAc-transferases. The MGAT3 enzyme adds a bisecting GlcNAc, which cannot be elongated. The addition of the bisecting β1-4 linked GlcNAc by MGAT3 and the addition of the β1-6 linked GlcNAc by MGAT5 are mutually exclusive. In fishes and birds, additional branches can be added by GlcNAc-transferases of the MGAT family.

The extent of N-glycan branching depends on the availability of the donor substrate UDP-GlcNAc. The intracellular concentration of UDP-GlcNAc approximates 5 mM, which is rather high for a nucleotide-activated sugar. The first branching enzymes, MGAT1 and MGAT2, have K_m values for UDP-GlcNAc in the sub-millimolar range, meaning that these branching steps occur readily under any condition. By contrast, the later enzymes MGAT4 and MGAT5 have K_m values around 5 to 10 mM for UDP-GlcNAc, indicating that the extent of tri- and tetra-antennary branching depends on the intracellular availability of the donor substrate. The intracellular concentration of UDP-GlcNAc is regulated by the rate-limiting enzyme glutamine:fructose-6-P acetyltransferase (GFAT). High UDP-GlcNAc levels are achieved when substrates like fructose-1,6-bisphosphate, glutamine, acetyl-CoA and uridine are elevated in cells. Accordingly, the degree of N-glycan branching is proportional to the energy status of cells: high levels of nutrients lead to high concentrations of UDP-GlcNAc and thereby to extensive branching of N-glycans.