In the latter compartments of the distal Golgi the N-Glycan is further modified, leading to the wide range of N-Glycans observed in multicellular organisms. The first step of N-Glycan elongation in the Golgi is the addition of a GlcNAc residue on the alpha 1,3 branch by the enzyme MGAT1 (GlcNAc-TI), which commits the elongation pathway to Complex or Hybrid N-Glycans from Oligomannose N-Glycans. At this point, the pathway bifurcates again to generate Complex or Hybrid N-Glycans. The addition of a GlcNAc in the middle of the two arms of the N-Glycan, catalyzed by MGAT3 (GNT-III), inhibits the removal of the mannoses on the alpha1,3 branches by MAN2 and the addition of a GlcNAc by MGAT2 (GlcNAc-TII), and commits the pathway toward the synthesis of hybrid N-Glycans. Alternatively, the removal of these mannoses and the action of MGAT2 leads to the synthesis of complex N-Glycans (Kornfeld and Kornfeld 1985).
The exact structure of the network of reactions leading to Complex or Hybrid N-Glycans is still not completely described and validated experimentally. Here we will annotate only one generic reaction for each of the enzymes known to participate in this process. For a better annotation on the reactions and genes involved in the synthesis of Complex and Hybrid N-Glycans we recommend the GlycoGene Database (Ito H. et al, 2010) (http://riodb.ibase.aist.go.jp/rcmg/ggdb/textsearch.jsp) for annotations on genes, and the Consortium for Functional Genomics (http://riodb.ibase.aist.go.jp/rcmg/ggdb/textsearch.jsp) for annotation of Glycan structures and reactions. Moreover, a computationally inferred prediction on the structure of this network is available through the software GlycoVis (Hossler P. et. al. 2006).