Nonspecific shielding of unfavorable electrostatic intramolecular interactions in the erythropoietin native-state increase conformational stability and limit non-native aggregation

Previous equilibrium and kinetic folding studies of the glycoprotein erythropoietin indicate that sodium chloride increases the conformational stability of this therapeutically important cytokine, ostensibly by stabilizing the native-state [Banks DD, (2011) The Effect of Glycosylation on the Folding Kinetics of Erythropoietin. J Mol Biol 412:536–550]. The focus of the current report is to determine the underlying cause of the salt dependent increase in erythropoietin conformational stability and to understand if it has any impact on aggregation, an instability that remains a challenge to the biotech industry in maintaining the efficacy and shelf-life of protein therapeutics. Isothermal urea denaturation experiments conducted at numerous temperatures in the absence and presence of sodium chloride indicated that salt stabilizes erythropoietin primarily by increasing the difference in enthalpy between the native and unfolded sates. This result, and the finding that the salt induced increases in erythropoietin melting temperatures were independent of the identity of the salt cation and anion, indicates that salt likely increases the conformational stability of erythropoietin at neutral pH by nonspecific shielding of unfavorable electrostatic interaction(s) in the native-state. The addition of salt (even low concentrations of the strong chaotrope salt guanidinium hydrochloride) also exponentially decreased the initial rate of soluble erythropoietin non-native aggregation at 37 °C storage.