Abstract
The tryptic glycopeptides of horseradish peroxidase isozyme c (HRPc) were studied by methylation linkage analysis, exoglycosidase degradation, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDITOFMS). Over 90% of the predicted tryptic peptides and glycopeptides of HRPc could be identified in the unfractionated digest. Four glycans, namely (Xyl)Man 3(Fuc)GlcNAc 2 (major species), (Xyl)Man 2(Fuc)GlcNAc 2, (Xyl)Man 3GlcNAc 2, and Man 3(Fuc)GlcNAc 2 (minor species), were observed at all of the N-glycosylation sites and account for greater than 95% of the carbohydrate. Other members of this glycan family, namely (Xyl) x Man m (Fuc) f GlcNAc 2 ( x=0 or 1, f=0 or 1, m= 4, 5, 6, or 7), account for the rest of the glycans. Only traces of high mannose-type glycans were detected in HRPc. Two sites, namely those at Asn-57 and Asn-267, were found to be more heterogeneous than the sites at Asn-13, Asn-158, Asn-186, 198 (doubly glycosylated peptide), Asn-214, and Asn-255. Two of the glycopeptides were observed as part of disulfide-linked species. MALDITOFMS confirmed the N–glycosylation sites previously reported [K.G. Welinder, Eur. J. Biochem., 96 (1979) 483–502] and was used to determine the heterogeneity of the glycan pool at each site.
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