The distribution of glycan structures in individual N-glycosylation sites in animal and plant glycoproteins

Glycopeptides representing each individual N-glycosylation site in six animal and plant glycoproteins (ovoinhibitor and ovotransferrin, orosomucoid, antitrypsin, phaseolin, and phytohemagglutinin) have been isolated and compared by mass spectrometric analysis. Since the isolation step separates each individual peptide regardless of the nature of the glycan attached to it, it is possible to observe the entire spectrum of glycans associated with each site from the mass spectrum of the corresponding glycopeptide. The three glycosylation sites in ovoinhibitor have very similar but not identical glycans; they are significantly different from those observed in the single site of ovotransferrin. The three sites in serum antitrypsin also have quite similar glycans, whereas the five sites in orosomucoid show considerable variation in both the nature and the relative amount of glycans. The two plant glycoproteins each have two sites with very different glycan structures. Except for the first and third glycosylation sites of antitrypsin which were found to have remarkably homogeneous glycans (97 and 90% of a biantennary complex structure), all the individual glycosylation sites contained heterogeneous mixtures of glycan structures. The results support the proposition that each N-linked glycan in a glycoprotein is affected by its unique protein environment to such an extent that each one may be displayed to the processing enzymes as a unique structural entity. On the basis of a limited number of observations of the glycan interfering with chymotryptic but not tryptic cleavage in the proximity of the glycan attachment site, it is proposed that hydrophobic interactions between the protein and the glycan may be involved in the conformational modulation of the glycans.