Glycan-Protein Interactions Determine Kinetics ofN-Glycan Remodeling

A hallmark of N -linked glycosylation in the secretory compartments of eukaryotic cells is the sequential remodeling of an initially uniform oligosaccharide to a site-specific, heterogeneous ensemble of glycostructures on mature proteins. To understand site-specific processing, we used protein disulfide isomerase (PDI), a model protein with five glycosylation sites, for molecular dynamics (MD) simulations and compared the result to a biochemical in vitro analysis with four different glycan processing enzymes. As predicted by an analysis of the accessibility of the N- glycans for their processing enzymes derived from the MD simulations, N -glycans at different glycosylation sites showed different kinetic properties for the processing enzymes. In addition, altering the tertiary structure context of N -glycan substrates affected N -glycan remodeling in a site-specific way. We propose that differential, tertiary structure context dependent N -glycan reactivities lead to different glycan structures in the same protein through kinetically controlled processing pathways.