Fig. 2: N-glycan structures.

Schematics of representative N-glycan structures expressed in mammals. The N-glycan precursor oligosaccharide (Glu₃Man₉GlcNAc₂-pp-dolichol) is attached co- or post-translationally to proteins containing the consensus sequence Asn-Xaa-(Ser/Thr), where Xaa is any amino acid except proline. Substrate proteins are represented as gray ovals and example N-glycan structures are represented following established symbol nomenclature conventions.¹⁶³ Subsequent modification of N-glycans by the transfer or cleavage of monosaccharide units following attachment to the protein can produce high-mannose, hybrid, or complex N-glycans. The simplest possible N-glycan consists of Man₃GlcNAc₂-Asn, and this represents the N-glycan core structure that is present in all N-linked glycans. Some common substructures, such as core fucose or bisecting GlcNAc provide important spatial constraints within the glycan structure and can influence subsequent glycan processing steps. Structures depicted here are representative examples of N-glycan subtypes. Hundreds of possible N-glycan structures can be synthesized by mammals and the specific glycan structures synthesized by any particular cell is determined by the spatiotemporal colocalization of enzyme, nucleotide-sugar donor, and substrate protein.