Figure 1. Biosynthetic pathway of nucin core 2-associated sialyl Lewis a (sLe^a) antigen.

Biosynthesis of mucin O-glycans is initiated by the addition of GalNAc to ser or thr of the peptide as catalyzed by polypeptide N-acetylgalactosaminyltransferases (GALNTs). This is followed by the addition of Gal in β1-3 linkage to GalNAc as catalyzed by C1GALT1 enzyme and produces core-1. Core 2 is generated by adding GlcNAc in β1-6 linkage to GalNAc by GCNTs. To synthesize sLe^a, Gal is transferred to GlcNAc in β1-3 linkage to form type 1 chain as catalyzed by B3GALTs, followed by the addition of α2-3NeuAc to Gal as catalyzed by ST3GALTs and then α4-fucose (Fuc) to GlcNAc as catalyzed by α3/4FUTs.