BIOMIMETIC CHAPERONES: COFACTOR UPTAKE AND PROTEIN REFOLDING ON NANOSTRUCTURED POLYSTYRENE

Rubredoxins (Rds) are small proteins, containing a tetrahedral Fe(SCys)4 site. Apo-forms of Rds (ApoRds) obtained upon iron removal are almost unable to incorporate metals in their folded form, but undergo quantitative refolding when iron is added to chaotrope-unfolded ApoRds, generating characteristic spectroscopic signals related to individual steps of the refolding processes leading to the holoprotein (HoloRd). HoloRd folding upon addition of iron to a folded and iron-uptake incompetent ApoRd was investigated here in the presence of polystyrene nanoparticles (NP). The rationale was that hydrophobic contacts between ApoRd and the NP surface could expose the ligand cysteines buried in the structured form of ApoRd. The exposure of metal-capturing cysteines is expected to trigger refolding to the native protein structure and the detachment of the folded HoloRd from the NP surface. Indeed, the rate and yield of HoloRd formation increased significantly in the presence of NP, and were influenced by their concentration and size. Refolding rates and yields had an optimum at “catalytic” NP concentrations (≈0.2 g/L NP) when using small NP (46 nm), and decreased at lower or higher NP concentrations. In these conditions, only a fraction of the ApoRd is bound to the NP surface, suggesting the occurrence of turnover events. Lower rates and yields at higher NP concentrations or when using larger NP may indicate the relevance of molecular crowding in favoring specific conformations of the protein on the NP surface. This bio-mimetic chaperone system may be applicable to other proteins requiring some unfolding steps prior to cofactor-triggered refolding.