Glycoforms modify the dynamic stability and functional activity of an enzyme

Abstract

Glycoproteins generally consist of collections of glycosylated variants (glycoforms) in which an ensemble of different oligosaccharides is associated with each glycosylation site. Bovine pancreatic ribonuclease B occurs naturally as a mixture of five glycoforms in which the same polypeptide sequence is associated with a series of oligomannose sugars attached at the single N-glycosylation site. Individual glycoforms were prepared by exoglycosidase digestions of RNase B and analyzed directly at the protein level by capillary electrophoresis. For the first time, electrophoretically pure single glycoforms have been available to explore the possibility that different sugars might specifically modify the structure, dynamics, stability, and functional properties of the protein to which they are attached. Comparisons of the amide proton exchange rates for individual glycoforms of RNase B and unglycosylated RNase A showed that while the 3D structure was unaffected, glycosylation decreased dynamic fluctuations throughout the molecule. There was individual variation in the NH-ND exchange rates of the same protons in different glycoforms, demonstrating the effects of variable glycosylation on dynamic stability. Consistent with the overall decrease in flexibility, and with the possibility that all of the sugars may afford steric protection to susceptible sites, was the finding that each of the glycoforms tested showed increased resistance to Pronase compared with the unglycosylated protein. In a novel sensitive assay using double-stranded RNA substrate, the different glycoforms showed nearly a 4-fold variation in functional activity; molecular modeling suggested that steric factors may also play a role in modulating this interaction.