N-glycosylation at Asn(491) in the Asn-Xaa-Cys motif of human transferrin

Glycopeptides derived from human transferrin were exhaustively analyzed by matrix-assisted laser desorption ionization and electrospray ionization mass spectrometry (MS). Both MS techniques clearly revealed the sequences of and the attachment sites of bi-antennary complex-type oligosaccharides, at both Asn432 and Asn630, both of which are located in a well-known motif for N-glycosylation, Asn-Xaa-Ser/Thr, but also at Asn491 in the Asn-Xaa-Cys motif. The latter has been reported to be a minor N-glycosylation site in several glycoproteins. The relative abundance of this abnormal glycosylation was estimated to be approximately 2 mol% of the transferrin preparation used in this study.