N‐glycosylation at Asn491 in the Asn‐Xaa‐Cys motif of human transferrin

Abstract
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 Asn432and Asn630, both of which are located in a well‐known motif for N‐glycosylation, Asn‐Xaa‐Ser/Thr, but also at Asn491in 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.