Mass Spectrometry of Glycopeptides
The key structural issues of glycoproteomics are protein identification, glycosylation site determination, and the glycan profiling at individual attachment sites (Tajiri et al. 2005). They are elucidated by tandem mass spectrometry of the glycopeptides isolated from tryptic digests of glycoproteins in a rapid and sensitive manner. Electrospray ionization (ESI) or matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is employed (Fig. 1). In case of MALDI, profiling of oligosaccharides is achieved by the linear time-of-flight (TOF) mode more appropriately than the reflectron mode. Robust and reliable identification of proteins and the determination of the attachment sites often require multiple-stage tandem mass spectrometry (Wada et al. 2004).