Asymmetric glycosylation of soybean seed coat peroxidase

Abstract

Reanalysis of the tryptic digests of soybean seed coat peroxidase (SBP) and its carboxyamidated peptide derivatives in the light of more complete sequence data has thrown light on the diglycosylated tryptic peptides, TP13 (Leu[183–205]Arg) and TP15 (Cys[208–231]Arg). Matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analyses indicate that although all potential sites carry some glycan substituents, not all sites are fully occupied. Tryptic glycopeptide TP13, carrying two N-glycosylation consensus sequons (Asn185 and Asn197), occurs mainly (85–90%) as the diglycosylated species, the remainder (10–15%) being monoglycosylated. In contrast, tryptic peptide TP15, also with two N-glycosylation sites (Asn211 and Asn216), is primarily monoglycosylated (∼90%), with the remainder (10%) being diglycosylated. No non-glycosylated TP13 or TP15 was observed. Some artifacts are noted in the reactions of N-terminal cysteine residues and aspartate/asparagines residues in glycopeptide TP15. Mapping the glycans onto the crystal structure of SBP shows that these are asymmetrically distributed on the molecule, occurring primarily on the substrate-channel face of the enzyme. In contrast, the glycans of HRP, isozyme c, are more uniformly distributed over the enzyme surface.