PNGaseF-Generated N‑Glycans Adduct onto Peptides in the Gas Phase

Glycoproteomics has recently increased in popularity due to instrumental and methodological advances. That said, O-glycoproteomic analysis is still challenging for various reasons, including signal suppression, search algorithm limitations, and co-occupancy of N- and O-glycopeptides. To decrease sample complexity and simplify analysis, most O-glycoproteomic workflows include PNGaseF digestion, which is an endoglycosidase that removes most N-glycan structures. Here, we report that N-glycans released from PNGaseF digestion were identified during data acquisition and hampered detection of O-glycopeptides. Importantly, we noted instances where free glycans adducted to unmodified peptides in the gas phase and were misidentified by search algorithms as O-glycopeptides. We confirmed the presence of free glycans in other experiments performed in our laboratory, as well as from data generated by other groups. To overcome this limitation, we demonstrated that released N-glycans can be removed using a molecular weight cut off filter prior to (glyco)protease digestion, which improves O-glycoproteomic coverage.