Review of Glycosylation Engineering of Biopharmaceuticals: Methods and Protocols: A book edited by Alain Beck

The glycoform profile of a glycoprotein is non-templated, i.e., is not encoded within the genome or otherwise predetermined; however, it is estimated that ~50% of human genes having an open reading frame encode a –N-X-S/T- amino acid sequence, where X represents any amino acid other than proline, that comprises a potential site (sequon) for N-linked glycosylation of the translated protein. N-linked glycosylation is both a co- and post-translational modification. The complex oligosaccharide GlcNAc2Man9Glu3 may be added at a –N-X-S/T- sequon as the polypeptide chain emerges from the ribosome tunnel. Local secondary structure determines whether oligosaccharide is added and the extent of addition. Higher occupancy is observed for –N-X-T- sequons than at –N-X-S- sequons, and the efficiency of addition can be further influenced by adjacent amino acid residues.