Identification of the High Mannose N -Glycan Isomers Undescribed by Conventional Multicellular Eukaryotic Biosynthetic Pathways.

N -linked glycosylation is one of the most important post-translational modifications of proteins. Current knowledge of multicellular eukaryote N -glycan biosynthesis suggests high mannose N -glycans are generated in the endoplasmic reticulum and Golgi apparatus through conserved biosynthetic pathways. According to conventional biosynthetic pathways, four Man 7 GlcNAc 2 isomers, three Man 6 GlcNAc 2 isomers, and one Man 5 GlcNAc 2 isomer are generated during this process. In this study, we applied our latest mass spectrometry method, logically derived sequence tandem mass spectrometry (LODES/MS n ), to re-examine high mannose N -glycans extracted from various multicellular eukaryotes which are not glycosylation mutants. LODES/MS n identified many high mannose N -glycan isomers previously unreported in plantae, animalia, cancer cells, and fungi. A database consisting of retention time and CID MS n mass spectra was constructed for all possible Man n GlcNAc 2 ( n = 5, 6, 7) isomers that include the isomers by removing arbitrary numbers and positions of mannose from canonical N -glycan, Man 9 GlcNAc 2 . Many N -glycans in this database are not found in current N -glycan mass spectrum libraries. The database is useful for rapid high mannose N -glycan isomeric identification.