Impact of Polyethylene Glycol on C-phycocyanin Stability
Résumé
Protein stability depends on both subunit folding and inter-subunit interactions, which are modulated by environmental conditions and cosolutes. This study examined how polyethylene glycols (PEG) of different molecular weights affect the structural stability of the multimeric protein C-phycocyanin (CPC). Using differential scanning calorimetry (DSC), fluorescence spectroscopy, and rheology, we analyzed CPC in the presence of PEG 4 000 g/mol (PEG4) and PEG 35 000 g/mol (PEG35). Fluorescence spectroscopy showed that both PEGs increased the emission intensity of CPC without shifting the emission maximum, indicating changes without major alterations of the global structure. DSC revealed a marked decrease in the enthalpy of unfolding, particularly with PEG35, despite only slight changes in denaturation temperatures. Rheology demonstrated effects of CPC on PEG solution viscosity. These results suggest that the smaller size and lower hydration of PEG4 allow it to intimately penetrate the hydration shell of CPC. In contrast, the larger molecular weight and higher hydration number of PEG35 induce protein-protein association and loss of solubility. Altogether, these results show that PEG molecular weight governs CPC stability: PEG4 may destabilize CPC via crowding and hydration-shell disruption, whereas PEG35 is likely to reduce CPC solubility through depletion-driven aggregation without altering its folded structure.
