Structure elucidation of glycoproteins by direct nanoESI MS and MS/MS analysis of proteolytic glycopeptides

Bovine ribonuclease B (RNAse B) and asialofetuin (FETUA) were subjected to in-capillary tryptic digest (Pohlentz et al. Proteomics. 2005, 5, 1758-1763) and the obtained glycopeptides were analyzed, respectively, by nanoelectrospray ionization mass spectrometry and collision-induced dissociation (CID) during the ongoing digest. For RNAse, B glycans of the high-mannose type (Man(4) to Man(9)) attached to either a tetra- or a hexapeptide containing the sole N-glycosylation site of the protein were detected. Glycopeptides derived from all three N-glycosylation sites of FETUA were observed, and the corresponding CID spectra proved the respective glycans to be oligosaccharides of the triantennary complex type. Moreover, an O-glycopeptide carrying Gal-GalNAc at T(280) could be unambiguously identified. An in-solution tryptic/chymotryptic digest of human transferrin (TRFE) was analyzed directly for glycopeptides subsequent to the addition of methanol and formic acid. Disialylated diantennary glycans were observed in glycopeptides of both N-glycosylation sites of TRFE. These results demonstrate the feasibility of direct structure determination of glycopeptides in proteolytic mixtures without any further refurbishment.