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. Author manuscript; available in PMC: 2012 Jun 7.
Published in final edited form as: Annu Rev Biochem. 2011 Jun 7;80:825–858. doi: 10.1146/annurev-biochem-060608-102511

Figure 3.

Figure 3

Identification of O-linked N-acetylglucosamine (O-GlcNAc) sites is facilitated by photocleavable biotin tagging, combined with collision-assisted dissociation (CAD) and electron transfer dissociation (ETD) mass spectrometry. O-GlcNAc peptides are enriched by first labeling O-GlcNAc groups with N-azidoacetylgalactosamine (GalNAz) using a mutant galactosyltransferase (GalT1), followed by click chemistry addition of a photocleavable biotin tag. Tagged peptides are purified over an avidin column and subsequently are released from the beads by photochemical cleavage. CAD of tagged peptides generates diagnostic ions at mass/charge (m/z) 503.1 and 300.2. ETD enables peptide sequencing and O-GlcNAc site localization. An ETD spectrum of a sample peptide (SISQSISGQK) indicates that Ser1 is modified with a tagged GlcNAc-GalNAz moiety.