Hydration of a glycoprotein: relative water affinity of peptide and glycan moieties

Glycosylation, the most prevalent post-translational modification of proteins, affects a number of physical properties including the interactions with the surrounding aqueous solvent. Such glycan-water interactions have been discussed with respect to the increased solubility generally observed for glycoproteins, but experimental support of this correlation remains sparse. We have applied a two-channel calorimetric method to measure the free energy and enthalpy of hydration at 25 degrees C for the glycoprotein phytase (Phy) and a deglycosylated form (dgPhy) of the same protein. Comparisons of results for Phy and dgPhy show that the polypeptide moiety has a higher affinity for water than the glycans. In fact, at moderate hydration levels (similar to 0.3 g water/g macromolecule) the water uptake appears to be entirely governed by adsorption to the peptide groups. We conclude that strengthened interaction with the solvent is unlikely to be the mechanism underlying the increased solubility and lowered propensity of aggregation often reported to result from the glycosylation of proteins.