Large-scale Identification of N-linked Intact Glycopeptides in Human Serum using HILIC Enrichment and Spectral Library Search

The human serum N-linked glycoproteome has been determined through LC-MS/MS. The intact glycopeptides were identified through a spectral library search method embedded in the pMatchGlyco software. Four types of known N-glycosylation motifs, prevalent variable modifications and semi-tryptic digestion were considered during searching and the identified intact glycopeptides were validated through target-decoy and motif-specific false discovery rate (FDR) control. The results reveal site-specific glycosylation of serum glycoproteins and provide high-quality tandem mass spectra of 22,677 serum N-linked intact glycopeptides. Graphical Abstract Highlights This study proposed a spectral library search method to accurately identify N-linked glycopeptides in human serum through LC-MS/MS with pMatchGlyco software. The identification depth of serum N-linked intact glycopeptides and glycoproteins was increased by combination of acetonitrile precipitation, HILIC enrichment and high-pH RPLC fractionation. 22,677 unique serum N-linked intact glycopeptides corresponding to 526 N-linked glycoproteins were identified with N-glycosylation motif-specific FDR control. This study revealed the great microheterogeneity of N-linked glycoproteins in serum. Large-scale identification of N-linked intact glycopeptides by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) in human serum is challenging because of the wide dynamic range of serum protein abundances, the lack of a complete serum N-glycan database and the existence of proteoforms. In this regard, a spectral library search method was presented for the identification of N-linked intact glycopeptides from N-linked glycoproteins in human serum with target-decoy and motif-specific false discovery rate (FDR) control. Serum proteins were firstly separated into low-abundance and high-abundance proteins by acetonitrile (ACN) precipitation. After digestion, the N-linked intact glycopeptides were enriched by hydrophilic interaction liquid chromatography (HILIC) and a portion of the enriched N-linked intact glycopeptides were processed by Peptide-N-Glycosidase F (PNGase F) to generate N-linked deglycopeptides. Both N-linked intact glycopeptides and deglycopeptides were analyzed by LC-MS/MS. From N-linked deglycopeptides data sets, 764 N-linked glycoproteins, 1699 N-linked glycosites and 3328 unique N-linked deglycopeptides were identified. Four types of N-linked glycosylation motifs (NXS/T/C/V, X≠P) were used to recognize the N-linked deglycopeptides. The spectra of these N-linked deglycopeptides were utilized for N-linked deglycopeptides library construction and identification of N-linked intact glycopeptides. A database containing 739 N-glycan masses was constructed and utilized during spectral library search for the identification of N-linked intact glycopeptides. In total, 526 N-linked glycoproteins, 1036 N-linked glycosites, 22,677 N-linked intact glycopeptides and 738 N-glycan masses were identified under 1% FDR, representing the most in-depth serum N-glycoproteome identified by LC-MS/MS at N-linked intact glycopeptide level.