Higher-Energy Collisional Dissociation Mass Spectrometry Fragmentation Enables Distinguishing O‑GlcNAc from Tn Antigen in Cancer Cells

Protein glycosylation plays essential roles in various biological processes, and thus determining the glycan structure present on the protein is essential to comprehensively understand these events. However, distinguishing saccharide stereoisomers is challenging, especially when their structures are very similar and their molecular weight and potential glycosylation sites are identical. One representative example is O-linked β-N-acetylglucosamine (O-GlcNAc) and O-linked α-N-acetylgalactosamine (Tn antigen). Traditional biochemistry approaches used in separating O-GlcNAc- and Tn antigen-modified peptides mainly include chemical derivatizations, lectins, and antibodies. However, subsequent mass spectrometry (MS) analysis is still required if one aims to determine the exact glycosylation site. Herein, a straightforward approach using the ratio of relative abundance (RA) of two fragment ions (RA126.055/RA138.055) in higher-energy collisional dissociation (HCD) MS without relying on the traditional biochemistry technique is reported to discriminate between O-GlcNAc and Tn antigen. This ratio was verified by synthetic glycopeptides and proteomic analysis in HeLa cells, where 10 proteins were found to be O-GlcNAcylation and 4 proteins were found to be Tn antigen-modified. Overall, this method can be extensively employed in liquid chromatography–mass spectrometry (LC-MS)-based proteomic studies and thus is of importance in biological and biomedical research.