The analysis of alphaâ 1â antitrypsin glycosylation with direct LCâ MS/MS

Abstract

A liquid chromatographyâ tandem mass spectrometry (LCâ MS/MS)â based methodology has been developed to differentiate coreâ and antennaryâ fucosylated glycosylation of glycopeptides. Both the glycosylation sites (heterogeneity) and multiple possible glycan occupancy at each site (microheterogeneity) can be resolved via intact glycopeptide analysis. The serum glycoprotein alphaâ 1â antitrypsin (A1AT) which contains both coreâ and antennaryâ fucosylated glycosites was used in this study. Sialidase was used to remove the sialic acids in order to simplify the glycosylation microheterogeneity and to enhance the MS signal of glycopeptides with similar glycan structures. β1â 3,4 galactosidase was used to differentiate coreâ and antennaryâ fucosylation. Inâ source dissociation was found to severely affect the identification and quantification of glycopeptides with low abundance glycan modification. The settings of the mass spectrometer were therefore optimized to minimize the inâ source dissociation. A threeâ step mass spectrometry fragmentation strategy was used for glycopeptide identification, facilitated by pGlyco software annotation and manual checking. The collision energy used for initial glycopeptide fragmentation was found to be crucial for improved detection of oxonium ions and better selection of Y1 ion (peptide+GlcNAc). Structural assignments revealed that all three glycosylation sites of A1AT glycopeptides contain complex Nâ glycan structures: site Asn70 contains biantennary glycans without fucosylation; site Asn107 contains biâ , triâ and tetraâ antennary glycans with both coreâ and antennaryâ fucosylation; site Asn271 contains biâ and triâ antennary glycans with both coreâ and antennaryâ fucosylation. The relative intensity of coreâ and antennaryâ fucosylation on Asn107 was similar to that of the A1AT protein indicating that the glycosylation level of Asn107 is much larger than the other two sites.