O-Search-Pattern: A Searching Tool Utilizing the Y-Ion Pattern to Enhance O-Glycopeptide Identification for the Analysis of O-GalNAc Glycoproteome

Different from N-linked glycosylation, the core structures of mucin type O-glycans are much more diverse, and the sensitive interpretation of O-glycopeptide spectra remains a challenge. The Y-ion pattern, a series of Y-ions with known mass gaps derived from the penta-saccharide core structure of N-linked glycosylation, is exploited to facilitate N-glycopeptide identification from their spectra. However, the pattern of Y ions in O-glycopeptides has not been well studied. In this study, we found that the Y-ion patterns were also frequently observed in the spectra of O-glycopeptides, and a special search approach is presented to identify O-glycopeptides by utilizing the Y-ion patterns. In this strategy, theoretical O-glycan Y-ion patterns are constructed to match the experimental Y-ions in O-glycopeptide spectra, which enables the determination of the mass of some glycans and results in the reduction of searching space. In addition, a Y-ion pattern-based deisotope process is also developed to correct the precursor m/z. The new search strategy was applied to search a human serum data set, and 15.4%–199.0% more O-glycopeptide-spectrum matches (OGPSMs) and 19.6%–107.1% more glycopeptide sequence identifications than other state-of-the-art software tools were observed. This search mode, the O-Search-Pattern, has been implemented into our database search software, MS-Decipher, and is recommended for searching the O-glycopeptide spectra acquired by sceHCD (stepped collision energy higher-energy collisional dissociation).