Selective fragmentation of the N-glycan moiety and protein backbone of ribonuclease B on an Orbitrap Fusion Lumos Tribrid mass spectrometer.

RATIONALE: The functional study and application of an intact glycoprotein require the structural characterization of both the protein backbone and the glycan moiety; the former has been successfully demonstrated with selective fragmentation of the protein backbone in CID and ExD; whether the latter can be achieved with selective fragmentation of the glycan moiety remains to be explored.
METHODS: RNase B solution was electrosprayed and its intact glycoforms of GlcNAc2 Mann (n = 5-9) with the highest abundance (charge state z = 16) were isolated individually and fragmented using CID, ETD, HCD, ETciD, and EThcD on the Orbitrap Fusion Lumos Tribrid mass spectrometer; the dissociation parameters were optimized for selective fragmentation of the N-glycan moiety and protein backbone as well as high sequence coverage. The obtained spectra were interpreted using the protein and N-glycan database search engines ProteinGoggle and GlySeeker, respectively.
RESULTS: With exploration of different dissociation parameters for all the five methods, selective fragmentation of the N-glycan moiety (the protein backbone staying intact) was observed in both HCD and EThcD at low collisional energies, but only a few matched product ions were observed; more comprehensive fragmentation was observed at high collisional energies (the protein backbone lost). Selective protein backbone fragmentation was observed in all the five dissociation methods.
CONCLUSIONS: For comprehensive structural characterization of intact N-glycoproteins using tandem mass spectrometry, the composition and topology of the N-glycan moiety can be identified using HCD and EThcD complementarily at low and high energies; while the amino acid sequence and glycosite can be identified using CID, ETD, HCD, ETciD, and EThcD with their optimal dissociation parameters.