Comparative Site-Specific N-Glycosylation Analysis of Lactoperoxidase from Buffalo and Goat Milk Using RP-UHPLC-MS/MS Reveals a Distinct Glycan Pattern.

The N-glycan pattern of lactoperoxidase (LPO) from buffalo and goat milk was analyzed with the corresponding site of attachment. The enzyme was purified from whey on cation exchange chromatography, proteolyzed using chymotrypsin, and the resulting (glyco)peptides were directly analyzed on reverse phase ultrahigh performance liquid chromatography coupled to ESI-Q-TOF MS in tandem mode. N-Glycans such as high mannose, complex, and hybrid types were identified in buffalo and goat LPO. Among sialylated complex and hybrid types, the terminal Neu5Ac linked to either LacNAc/LacdiNAc found exclusively in buffalo, whereas Neu5Gc linked to LacdiNAc was predominant in goat LPO. N-Glycans at Asn6 and Asn349 in buffalo LPO were completely core fucosylated, while these sites in goat LPO showed differential fucosylation. Differential occupancy was observed at Asn112 with or without nonfucosylated complex and hybrid types, whereas mainly high mannose glycans were found in Asn222 in both of the LPOs. The presence of glycan isomers in buffalo and goat LPO was also observed. Despite the presence of distinct complex and hybrid glycans, the common glycosylation features in buffalo and goat LPO were identified and are comparable with those of bovine LPO. This finding could be useful in exploring the beneficial role of these glycans as functional ingredients for food products.

N-Glycosylation analysis of yeast Carboxypeptidase Y reveals the ultimate removal of phosphate from glycans at Asn368.

Carboxypeptidase Y from Saccharomyces cerivisiae was characterized for its site specific N-glycosylation through mass spectrometry. The N-glycopeptides were derived using non specific proteases and are analysed directly on liquid chromatography coupled to ion trap mass spectrometer in tandem mode. The evaluation of glycan fragment ions and the Y1 ions (peptide+HexNAc)+n revealed the glycan sequence and the corresponding site of attachment. We observed the microheterogeneity in N-glycans such as Man11-15GlcNAc2 at Asn13, Man8-12GlcNAc2 at Asn87, Man9-14GlcNAc2 at Asn168 and phosphorylated Man12-17GlcNAc2 as well as Man11-16GlcNAc2 at Asn368. The presence of N-glycans with Man<18GlcNAc2 indicated that in vacuoles the steady release of mannose/phospho mannose residues from glycans occurs initially at Asn13 or Asn168 followed by at Asn368. However, glycans at Asn87 which comprises Man8-12 residues as reported earlier remain intact suggesting its inaccessibility for a similar processing. This in turn indicates the interaction of the glycan at Asn87 with the polypeptide chain implicating it in the folding of the protein.

Comprehensive analysis of α 2-3-linked sialic acid specific Maackia amurensis leukagglutinin reveals differentially occupied N-glycans and C-terminal processing.

Seeds of Maackia amurensis constitutes two sialic acid specific agglutinins known as leukagglutinin and hemagglutinin. Maackia amurensis leukagglutinin (MAL) recognizes α2-3-linked sialic acid present mainly in N-glycans and composed of two disulfide linked monomers. It exhibits potential N-glycosylation sites (four PNGs) which have been assumed to undergo differential occupancy. In this study we have characterized the site specific macro- and microheterogeneity of monomers in detail by analysing N-glycopeptides and peptides through liquid chromatography coupled to ion trap mass spectrometer in MS3 mode (LC-MSn). We observed the presence of mainly paucimannose N-glycans at Asn61, Asn113 and Asn191 whereas a high mannose type with varying Man5-9 occurs at Asn179. Interestingly Asn179 and Asn191 exhibited differential occupancy which was evident by the presence of non-glycosylated peptides. This has contributed to the difference in molecular mass of monomers upon SDS-PAGE. Further the presence of disulfide linked peptides confirmed the covalent linkage of monomers which also undergoes uniform C-terminal processing.

N-glycan analysis of mannose/glucose specific lectin from Dolichos lablab seeds.

An affinity purified mannose/glucose specific lectin from the seeds of Dolichos lablab (Indian bean/lablab bean) resolves into five subunits upon SDS-PAGE in the range of Mr 12-20kDa. Partial de novo sequencing of subunits resulted in 88% and 73% sequence coverage for α and ß subunits of the cDNA derived FRIL (Flt3 receptor interacting lectin) sequence, respectively and suggested that four bands correspond to the α-subunits while the band of lowest molecular mass is designated as ß. It was proposed in an earlier study on FRIL that the difference in molecular mass of α-subunits is due to differences in C-terminal processing and differential N-glycosylation i.e. numbers of N-glycans present (Colucci et al., 1999). Thus, differential N-glycosylation of the purified mannose/glucose specific lectin was unravelled by in-gel trypsin/chymotrypsin digestion of the α-subunits followed by desalting and ZIC-HILIC enrichment of N-glycopeptides. Subsequently, analyses by nano electrospray ionisation quadrupole time of flight mass spectrometry and low-energy collision-induced dissociation experiments revealed the presence of a typical paucimannose type N-glycan (Man2(Xyl)GlcNAc2(Fuc)) in α subunits 2-4.